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Principles of Physiological Psychology

by Wilhelm Wundt (1902)

Classics in the History of Psychology

An internet resource developed by

Christopher D. Green

York University, Toronto, Ontario

ISSN 1492-3173

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Principles of Physiological Psychology

by Wilhelm Wundt (1902)

Translated by Edward Bradford Titchener (1904)

Introduction

Part I. The Bodily Substrate of the Mental Life

Chapter 1. The Organic Evolution of Mental Function

Chapter 2. Structural Elements of the Nervous System

Chapter 3. Physiological Mechanics of Nerve Substance

Chapter 4. Morphological Development of the Central Organs

Chapter 5. Course of the Paths of Nervous Conduction

Chapter 6. Physiological Function of the Central Parts

INTRODUCTION

§1. The Problem of Physiological Psychology

THE title of the present work is in itself a sufficiently clear indication of the contents. In it, the

attempt is made to show the connexion between two sciences whose subject-matters are

closely interrelated, but which have, for the most part, followed wholly divergent paths.

Physiology and psychology cover, between them, the field of vital phenomena; they deal with

the facts of life at large, and in particular with the facts of human life. Physiology is concerned

with all those phenomena of life that present them selves to us in sense perception as bodily

processes, and accordingly form part of that total environment which we name the external

world. Psychology, on the other hand, seeks to give account of the interconnexion of processes

which are evinced by our own consciousness, or which we infer from such manifestations of the

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bodily life in other creatures as indicate the presence of a consciousness similar to our own.

This division of vital processes into physical and psychical is useful and even necessary for the

solution of scientific problems. We must, however, remember that the life of an organism is

really one; complex, it is true, but still unitary. We can, therefore, no more separate the

processes of bodily life from conscious processes than we can mark off an outer experience,

mediated by sense perceptions, and oppose it, as something wholly separate and apart, to

what we call 'inner' experience, the events or our own consciousness. On the contrary: just as

one and the same thing, e.g., a tree that I perceive before me, falls as external object within the

scope of natural science, and as conscious contents within that of psychology, so there are

many phenomena of the physical life that are uniformly connected with conscious processes,

while these in turn are always bound up with processes in the living body. It is a matter of

every-day experience that we refer certain bodily movements directly to volitions, which we can

observe as such only in our consciousness. Conversely, we refer the ideas of external objects

that arise in consciousness either to direct affection of the organs of sense, or, in the case of

memory images, to physiological excitations within the sensory centres, which we interpret as

after-effects of foregone sense impressions.

[p. 2] It follows, then, that physiology and psychology have many points of contact. In general

there can of course be no doubt that their problems are distinct. But psychology is called upon

to trace out the relations that obtain between conscious processes and certain phenomena of

the physical life; and physiology, on its side, cannot afford to neglect the conscious contents in

which certain phenomena of this bodily life manifest them selves to us. Indeed, as regards

physiology, the interdependence of the two sciences is plainly in evidence. Practically

everything that the physiologists tell us, by way of fact or of hypothesis, concerning the

processes in the organs of sense and in the brain, is based upon determinate mental

symptoms: so that psychology has long been recognised, explicitly or implicitly, as an

indispensable auxiliary of physiological investigation. Psychologists, it is true, have been apt to

take a different attitude towards physiology. They have tended to regard as superfluous any

reference to the physical organism; they have supposed that nothing more is required for a

science of mind than the direct apprehension of conscious processes themselves. It is in token

of dissent from any such standpoint that the present work is entitled a "physiological

psychology." We take issue, upon this matter, with every treatment of psychology that is based

on simple self-observation or on philosophical presuppositions. We shall, wherever the

occasion seems to demand, employ physiology in the service of psychology. We are thus, as

was indicated above, following the example of physiology itself, which has never been in a

position to disregard facts that properly belong to psychology, - although it has often been

hampered in its use of them by the defects of the empirical or metaphysical psychology which it

has found current.

Physiological psychology is, therefore, first of all psychology. It has in view the same principal

object upon which all other forms of psychological exposition are directed: the investigation of

conscious processes in the modes of connexion peculiar to them. It is not a province of

physiology; nor does it attempt, as has been mistakenly asserted, to derive or explain the

phenomena of the psychical from those of the physical life. We may read this meaning into the

phrase 'physiological psychology,' just as we might interpret the title 'microscopical anatomy' to

mean a discussion, with illustrations from anatomy, of what has been accomplished by the

microscope; but the words should be no more misleading in the one case than they are in the

other. As employed in the present work, the adjective 'physiological' implies simply that our

psychology will avail itself to the full of the means that modern physiology puts at its disposal

for the analysis of conscious processes. It will do this in two ways.

(I) Psychological inquiries have, up to the most recent times, been undertaken solely in the

interest of philosophy ; physiology was enabled, by [p. 3] the character of its problems, to

advance more quickly towards the application of exact experimental methods. Since, however,

the experimental modification of the processes of life, as practised by physiology, oftentimes

effects a concomitant change, direct or indirect, in the processes of consciousness,--which, as

we have seen, form part of vital processes at large,--it is clear that physiology is, in the very

ads:

nature of the case, qualified to assist psychology on the side of method; thus rendering the

same help to psychology that it, itself received from physics. In so far as physiological

psychology receives assistance from physiology in the elaboration of experimental methods, it

may be termed experimental psychology. This name suggests, what should not be forgotten,

that psychology, in adopting the experimental methods of physiology, does not by any means

take them over as they are, and apply them without change to a new material. The methods of

experimental psychology have been transformed--in some instances, actually remodelled--by

psychology itself, to meet the specific requirements of psychological investigation. Psychology

has adapted physiological as physiology adapted physical methods, to its own ends.

(2) An adequate definition of life, taken in the wider sense, must (as we said just now) cover

both the vital processes of the physical organism and the processes of consciousness. Hence,

wherever we meet with vital phenomena that present the two aspects, physical and psychical

there naturally arises a question as to the relations in which these aspects stand to each other.

So we come face to face with a whole series of special problems, which may be occasionally

touched upon by physiology or psychology, but which cannot receive their final solution at the

hands of either, just by reason of that division of labour to which both sciences alike stand

committed. Experimental psychology is no better able to cope with them than is any other form

of psychology, seeing that it differs from its rivals dilly in method, and not in aim or purpose.

Physiological psychology, on the other hand, is competent to investigate the relations that hold

between the processes of the physical and those of the mental life. And in so far as it accepts

this second problem, we may name it a psychophysics.[1] If we free this term from any sort of

metaphysical implication [p. 4] as to the relation of mind and body, and understand by it nothing

more than an investigation of the relations that may be shown empirically to obtain between the

psychical and the physical aspects of vital processes, it is clear at once that psychophysics

becomes for us not, what it is some times taken to be, a science intermediate between

physiology and psychology, but rather a science that is auxiliary to both. It must, however,

render service more especially to psychology, since the relations existing between determinate

conditions of the physical organization, on the one hand, and the processes of consciousness,

on the other, are primarily of interest to the psychologist. In its final purpose, therefore, this

psychophysical problem that we have assigned to physiological psychology proves to be itself

psychological. In execution, it will he predominantly physiological since psychophysics is

concerned to follow up the anatomical and physiological investigation of the bodily substrates of

conscious processes, and to subject its results to critical examination with a view to their

bearing upon our psychical life.

There are thus two problems which are suggested by the title "physiological psychology": the

problem of method, which involves the application of experiment, and the problem of a

psychophysical supplement, which involves a knowledge of the bodily substrates of the mental

life. For psychology itself, the former is the more essential; the second is of importance mainly

for the philosophical question of the unitariness of vital processes at large. As an experimental

science, physiological psychology seeks to accomplish a reform in psychological investigation

comparable with the revolution brought about in the natural sciences by the introduction of the

experimental method. From one point of view, indeed, the change wrought is still more radical:

for while in natural science it is possible under favourable conditions, to male an accurate

observation without recourse to experiment, there is no such possibility in psychology. It is only

with grave reservations that what is called 'pure self-observation' can properly be termed

observation at all and under no circumstances can it lay claim to accuracy. On the other hand,

it is of the essence of experiment that we can vary the conditions of an occurrence at will and, if

we are aiming at exact results; in a quantitatively determinable way Hence, even in the domain

of natural science the aid of the experimental method becomes indispensable whenever the

problem set is the analysis of transient and impermanent phenomena, and not merely the

observation of persistent and relatively constant objects. But conscious contents are at the

opposite pole from permanent objects; they are processes, fleeting occurrences, in continual

flux and change. In their case, therefore, the experimental method is of cardinal importance; it

and it alone males a scientific introspection possible. For all accurate observational implies that

the object [p. 5] of observation (in this case the psychical process) can be held fast by the

attention, and any changes that it undergoes attentively followed. And this fixation by the

attention implies, in its turn, that the observed object is independent of the observer. Now it is

obvious that the required independence does not obtain in any attempt at a direct self-

observation, undertaken without the help of experiment. The endeavour to observe oneself

must inevitably introduce changes into the course of mental events,--changes which could not

have occured without it, and whose usual consequence is that the very process which was to

have been observed disappears from consciousness. The psychological experiment proceeds

very differently. In the first place, it creates external conditions that look towards the

introduction of a determinate mental process at a given moment. In the second place, it makes

the observer so far master of the general situation, that the state of consciousness

accompanying this process remain approximately unchanged. The great importance of the

experimental method, therefore, lies not simply in the fact that, here as in the physical realm, it

enables us arbitrarily to vary the conditions of our observations, but also and essentially in the

further fact that it makes observation itself possible for us. The results of this observation may

then be fruitfully employed in the examination of other mental phenomena, whose nature

prevents their own direct experimental modification.

We may add that, fortunately for the science, there are other sources of objective psychological

knowledge, which become accessible at the very point which the experimental method fails us.

These are certain products of the common mental life, in which we may trace the operation of

determinate psychical motives: chief among them are language, myth and custom. In part

determined by historical conditions, they are also, in part dependent upon universal

psychological laws; and the phenomena that are referable to these laws form the subject-

matter of a special psychological discipline, ethnic psychology. The results of ethnic psychology

constitute, at the same time, our chief source of information regarding the general psychology

of the complex mental processes. In this way, experimental psychology and ethnic psychology

form the two principal departments of scientific psychology at large. They are supplemented by

child and animal psychology, which in conjunction with ethnic psychology attempt to resolve the

problems of psychogenesis. Workers in both these fields may, of course, avail themselves

within certain limits of the advantages of the experimental method. But the results of

experiment are here matters of objective observation only, and the experimental method

accordingly loses the peculiar significance which it possesses as an instrument of introspection.

Finally, child psychology and experimental psychology in the narrower sense may be bracketed

together as individual [p. 6] psychology while animal psychology and ethnic psychology form

the two halves of a generic or comparative psychology. These distinctions within psychology

are, however, by no means to be put on a level with the analogous divisions of the province of

physiology. Child psychology and animal psychology are of relatively slight importance, as

compared with the sciences which deal with the corresponding physiological problems of

ontogeny and phylogeny. On the other hand, ethnic psychology must always come to the

assistance of individual psychology, when the developmental forms of the complex mental

processes are in question.

Kant once declared that psychology was incapable of ever raising itself to the rank of an exact

natural science.[2] The reasons that he gives for this opinion have often been repeated in later

times.[3] In the first place, Kant says, psychology cannot become an exact science because

mathematics is inapplicable to the phenomenon of the internal sense; the pure internal

perception, in which mental phenomena must be constructed,--time,--has but one dimension. In

the second place, however, it cannot even become an experimental science, because in it the

manifold of internal observation cannot be arbitrarily varied,--still less, another thinking subject

be submitted to one's experiments, conformably to the end in view; moreover, the very fact of

observation means alteration of the observed object. The first of these objections is erroneous;

the second is, at the least, one-sided. It is not true that the course of inner events evinces only

one dimension, time. If this were the case, its mathematical representation would, certainly be

impossible; for such representation always requires at least two variables, which can be

subsumed under the concept of magnitude. But, as a matter of fact, our sensations and

feelings are intensive magnitudes, which form temporal series. The course of mental events

has, therefore, at any rate two dimensions; and with this fact is given the general possibility of

its presentation in mathematical form. Otherwise, indeed Herbart could hardly have lighted

upon the idea of applying mathematics to psychology. And his attempt has the indisputable

merit of proving once and for all the possibility of an application of mathematical methods in the

sphere of mind.[4]

If Herbart, nevertheless, failed to accomplish the task which he set himself, the reason of his

failure is very simple; it lay in the overweening confidence with which he regarded the method

of pure self-observation and the hypotheses whereby he filled out the gaps that this observation

leaves. It is Fechner's service to have found and followed the true way; to have shown us how

a 'mathematical psychology' may, within certain limits, be realised in practice. Fechner's

method consists in the experimental modification of consciousness by sensory stimuli; it leads,

under favourable circumstances, to the establishment

of certain quantitative relations between the physical and the psychical.[5]At [p. 7] the present

day, experimental psychology has ceased to regard this formulation of mental measurements

as its exclusive or even as its principal problem. Its aim is now more general; it attempts, by

arbitrary modification of consciousness, to arrive at a causal analysis of mental processes.

Fechner's determinations are also affected, to some extent, by his conception of Psychophysics

as a specific science of the 'interactions of mind and body.' But, in saying this, we do not lessen

the magnitude of his achievement. He was the first to show how Herbart's idea of an 'exact

psychology' might be turned to practical account.

The arguments that Kant adduces in support of his second objection, that the inner experience

is inaccessible to experimental investigation, are all derived from purely internal sources, from

the subjective flow of processes; and there, of course, we cannot challenge its validity. Our

psychical experiences are, primarily, indeterminate magnitudes; they are incapable of exact

treatment until they have been referred to determinate units of measurement, which in turn may

be brought into constant causal relations with other given magnitudes. But we have, in the

experimental modification of consciousness by external stimuli, a means to this very end,--to

the discovery of the units of measurement and the relations required. Modification from without

enables us to subject our mental processes to arbitrarily determined conditions, over which we

have complete control and which we may keep constant or vary as we will. Hence the objection

urged against experimental psychology, that it seeks to do away with introspection, which is the

sine qua non of any psychology, is based upon a misunderstanding. The only form of

introspection which experimental psychology seeks to banish from the science is that

professing self-observation which thinks it can arrive directly, without further assistance, at an

exact characterisation of mental facts, and which is therefore inevitably exposed to the grossest

self-deception. The aim of the experimental procedure is to substitute for this subjective

method, whose sole resource is an inaccurate inner perception, a true and reliable

introspection, and to this end it brings consciousness under accurately adjustable objective

conditions. For the rest, here as elsewhere, we must estimate the value of the method, in the

last resort, by its results. It is certain that the subjective method has no success to boast of; for

there is hardly a single question of fact upon which its representatives do not hold radically

divergent opinions. Whether and how far the experimental method is in better cast, the reader

will be able to decide for himself at the conclusion of this work. He must, however, in all justice

remember that the application of experiment to mental problems is still only a few decades old.

[6]

The omission, in the above list of the various psychological disciplines, of any mention of what

is called rational psychology is not accidental. The term was introduced into mental science by

C. Wolff (1679-1754), to denote a knowledge of the mental life gained, in independence of

experience, simply and solely [p. 8] from metaphysical concepts. The result has proved, that

any such metaphysical treatment of psychology must, if it is to maintain its existence, be

constantly making surreptitious incursions into the realm of experience. Wolff himself found it

necessary to work out an empirical psychology, alongside of the rational: though it must be

confessed that, in fact, the rational contains about as much experience as the empirical and the

empirical about as much metaphysics as the rational. The whole distinction rests upon a

complete misapprehension of the scientific position, not only of psychology, but also of

philosophy. Psychology is, in reality, just as much an experiential science as is physics or

chemistry. But it can never be the business of philosophy to usurp the place of any special

science; philosophy has its beginnings, in every case, in the established results of the special

sciences. Hence the works upon rational psychology stand in approximately the same relation

to the actual progress of psychological science as does the nature-philosophy of Schelling or

Hegel to the development of modern natural science.[7]

There are certain psychological works, still current at the present time, which bear the word

'empirical' upon their title-pages, but make it a matter of principle to confine themselves to what

they term a 'pure' introspection. They are, for the most part, curious mixtures of rational and

empirical psychology. Sometimes the rational part is restricted to a few pages of metaphysical

discussion of the nature of mind; sometimes- as in the great majority of books of the kind

emanating from the Herbartian School --certain hypotheses of metaphysical origin are put

forward as results of self-observation. It has been well said that if a prize were offered to the

discovery by this whole introspective school of one single undisputed fact, it would be offered in

vain.[8] Nevertheless, the assurance of the Herbartians is incredible. Their compendia appear,

one after another; and the memory of the students who use them is burdened with a mixed

medley of purely imaginary processes. On the other side, the supreme advantage of the

experimental method lies in the fact that it and it alone renders a reliable introspection possible,

and that it therefore increases our ability to deal introspectively with processes not directly

accessible to modification from without. This general significance of the experimental method is

being more and more widely recognised in current psychological investigation; and the

definition of experimental psychology has been correspondingly extended beyond its original

limits. We now understand by 'experimental psychology' not simply those portions of

psychology which are directly accessible to experimentation, but the whole of individual

psychology. For all such psychology employs the experimental method: directly, where its

direct use is possible; but in all other cases indirectly, by availing itself of the general results

which the direct employment of the method has yielded, and of the refinement of psychological

observation which this employment induces.

Experimental psychology itself has, it is true, now and again suffered relapse into a

metaphysical treatment of its problems. We recognise the symptoms whenever we find

'physiological psychology' defined, from the outset, in such a way as to give it a determinate

metaphysical implication. The task now assigned to the science is that of the interpretation of

conscious phenomena by [p. 9] their reference to physiological conditions. Usually, the infection

spreads still farther, and the same view is taken of the problem of psychology at large. As

regards sensations, the elements out of which they are compounded, conscious processes (we

are told) have their specific character, their peculiar constitution; but it is impossible by

psychological means to discover uniformities of connexion among these elements. Hence the

only road to a scientific description or explanation of complex mental experiences lies through

the knowledge of the physiological connexions obtaining among the physiological processes

with which the psychical elements are correlated.[9] On this conception, there is no such thing

as psychical, but only psychical causation, and every causal explanation of mental occurrence

must consequently be couched in physiological terms. It is accordingly termed the theory of

'psychophysical materialism.' The theory as such is by no means a new thing in the history of

philosophy. All through the eighteenth century it was struggling for mastery with the rival theory

of mechanical materialism, which explained the psychical elements themselves as confused

apprehensions of molecular motions. But it presents a novel feature in its endeavour to press

physiological psychology into the service of the metaphysical hypothesis and thus apparently to

remove this hypothesis from the metaphysical sphere,--so that psychological materialism

becomes for its representatives compatible even with a philosophical idealism of the order of

Kant or Fichte. Since psychology, from this point of view, forms a supplement to physiology,

and therefore takes its place among the natural sciences, it need, as a matter of fact, pay no

further regard either to philosophy or to the mental sciences. That the mental life itself is the

problem of psychology,--this is mere dogma, handed down to us by past ages.[10]Yet after all,

the assertion that there is no such thing as psychical causation, and that all psychical

connexions must be referred back to physical, is at the present day this: it is an assumption

which, on its negative side, comes into conflict with a large number of actually demonstrable

psychical connexions, and, on the positive, raises a comparatively very limited group of

experiences to the rank of an universal principle. It is, we must suppose, a realisation of the

inadequacy of the arguments offered in support of these two fundamental implications that has

led certain psychologists, who would otherwise take the same theoretical position, to divide the

problem of psychology, and to recognise the interconnexions of mental processes as a

legitimate object of inquiry, alongside of the investigation of their dependence upon determinate

physiological process within the brain. In the psychological portion of their works, these writers

usually adopt the theory of the 'association of ideas,' elaborated in the English psychology of

the eighteenth century.[11] They adopt it for the good and sufficient reason that the doctrine of

association, from David Hartley (1705-1757) [p. 10] down to Herbert Spencer (1820-1904), has

itself for the most part attempted merely a physiological interpretation of the associative

processes.

The materialistic point of view in psychology can claim, at best, only the value of an heuristic

hypothesis. Its justification must, therefore, be sought first of all in its results. But it is apparent

that the diversion of the work of psychology from its proper object, the related manifold of

conscious processes, is precisely calculated to make the experimental method comparatively

barren, so far as concerns psychology itself. And, as a matter of fact, the books upon

physiological psychology that are written from the standpoint of materialism confine themselves

almost entirely, when they are not borrowing from the physiology of brain and sense organs, to

the beaten track of the traditional doctrine of association. Ideas are treated, after as before, as

if they were immutable objects, that come and go, form connexions of sequence with one

another, obey in these connexions the well-known laws of habit and practice, and finally, when

arranged in certain groups, yield the not very startling result that they can be brought under the

same logical categories that have proved generally serviceable for the classification of all sorts

of concepts.[12]

Now physiology and psychology, as we said just now, are auxiliary disciplines, and neither can

advance without assistance from the other. Physiology, in its analysis of the physiological

functions of the sense organs, must use the results of subjective observation of sensations; and

psychology, in its turn, needs to know the physiological aspects of sensory function, in order

rightly to appreciate the psychological. Such instances might easily be multiplied. Moreover, in

view of the gaps in our knowledge, physiological and psychological alike, it is inevitable that the

one science will be called upon, time and again, to do duty for the other. Thus, all our current

theories of the physical processes of light excitation are inferences from the psychological

course and character of visual sensations; and we might very well attempt, conversely, to

explain the conditions of practice and habituation, in the mental sphere, from the properties of

nervous substance, as shown in the changes of excitability due to the continued effect of

previous excitations. But one cannot assert, without wilfully closing one's eyes to the actual

state of affairs or taking theories for facts, that the gaps in our knowledge which demand this

sort of extraneous filling are to be found only on the one side, the side of psychology. In which

of the two sciences our knowledge of processes and of the interconnexion of processes is more

or less perfect or imperfect is a question that, we may safely say, hardly admits of an answer.

But however this may be, the assertion that the mental life lacks all causal connexion, and that

the real and primary object of psychology is therefore not the mental life itself but the physical

substrate of that life,--this assertion stands self-condemned. The effects of such teaching upon

psychology cannot but be detrimental. In the first place, it conceals the proper object of

psychological investigation behind facts and hypotheses that are borrowed from physiology.

Secondly and more especially, it recommends the employment of the experimental methods

without the least regard to the psychological point of view, so that for psychology as such their

results are generally valueless. Hence the gravest danger that besets the path of our science

today comes not from the speculative and empirical dogmas of the older schools, but [p. 11]

rather from this materialistic pseudo-science. Antipsychological tendencies can hardly find

clearer expression than in the statement that the psychological interpretation of the mental life

has no relation whatever to the mental life itself, as manifested in history and in society.

Besides this application of the term 'experimental psychology' in the interests of psychological

materialism, we find it used in still another sense, which is widely different from that of our own

definition. It has become customary, more especially in France, to employ the name principally,

if not exclusively, for experiments upon hypnotism and suggestion. At its best, however, this

usage narrows the definition of 'experimental psychology' in a wholly inacceptable way. If we

are to give the title of 'psychological experiment' to each and every operation upon

consciousness that brings about a change of conscious contents, then, naturally, hypnotisation

and the suggestion of ideas must be accounted experiments. The inducing of a morphine

narcosis, and any purposed interference with the course of a dream consciousness, would fall

under the same category. But if the principal value of the psychological experiment lies in the

fact that it makes an exact introspection possible, very few of these modifications of

consciousness can be termed true psychological experiments. This does not mean of course,

that experiments with suggestion may not, under favourable circumstances,--in the hands of an

experimenter who is guided by correct psychological principles, and who has at his command

reliable and introspectively trained observers,--yield results of high importance to psychology:

so much, indeed, is proved by Vogt's observations on the analysis of the feelings in the

hypnotic state.[13] But in such cases the conditions necessary to the performance of accurate

experiments are, it is plain, peculiarly difficult of fulfilment; and the great majority of what are

called 'hypnotic experiments' either possess, accordingly, no scientific value at all or lead to the

observation of interesting but isolated facts, whose place in the psychological system is still

uncertain.[14]

§2. Survey of the Subject

Physiological psychology is primarily psychology and therefore has for its subject the manifold

of conscious processes, whether as directly experienced by ourselves, or as inferred on the

analogy of our own experiences from objective observation. Hence the order in which it takes

up particular problems will be determined primarily by psychological considerations; the

phenomena of consciousness fall into distinct groups, according to the points of view from

which they are successively regarded. At the same time, any detailed treatment of the relation

between the psychical and physical aspects of vital processes presupposes a digression into

anatomy and physiology such as would naturally be out of place in a purely psychological

exposition. While, then, the following Chapters of this work are arranged in general upon a

systematic plan, the author has not always observed the rule that the reader should be

adequately prepared, at each stage [p. 12] of the discussion, by table contents of preceding

Chapters. Its disregard has enabled him to avoid repetition; and he has acted with the less

scruple, in view of the general understanding of psychology which the rending of a book like the

present implies. Thus a critical review of the results of brain anatomy and brain physiology, with

reference to their value for psychology, presupposes much and various psychological

knowledge. Nevertheless, it is necessary, for other reasons, that the anatomical and

physiological considerations should precede the properly psychological portion of the work. And

similar conditions recur, now and again, even in Chapters that are pre-eminently psychological.

[l5]

Combining in this way the demands of theory and the precepts of practical method, we shall in

what follows (1) devote a first Part to the bodily substrate of the mental life. A wealth of new

knowledge is here placed at our disposal by the anatomy and physiology of the central nervous

system, reinforced at various points by pathology and general biology. This mass of material

calls imperatively for examination from the psychological side: more especially since it has

become customary for the sciences concerned in its acquisition to offer all varieties of

psychological interpretation of their facts. Nay, so far have things gone, that we actually find

proposals made for a complete reconstruction of psychology itself, upon an anatomical and

physiological basis! But, if we are seriously to examine these conjectures and hypotheses, we

must, naturally, acquaint ourselves with the present status of the sciences in question. Even

here, however, our presentation of the facts will depart in some measure from the beaten path.

Our aim is psychological: so that we may restrict ourselves, on the one hand, to matters of

general importance, while on the other we must lay special emphasis upon whatever is

significant for psychology. Thus it cannot be our task to follow brain anatomy into all the details

which it has brought to light concerning the connexions of fibres within the brain,--into all those

minute points whose interpretation is still altogether uncertain, and whose truth is often and

again called in question. It will only be necessary for us to obtain a general view of he structure

of the central organs and of such principal connexions of these with one another and with the

peripheral organs as have been made out with sufficient certainty. We may then in the light of

reasonably secure principles of nerve physiology and of our psychological knowledge, proceed

to discuss the probable relations of physiological structure and function to the processes of

consciousness.

(2) We shall then, in a second Part, begin our work upon the problem [p. 13] of psychology

proper, with the doctrine of the elements of the mental life. Psychological analysis leaves us

with two such elements, of specifically different character: with sensations, which as the

ultimate and irreducible elements of ideas we may term the objective elements of the mental

life, and with feelings, which accompany these objective elements as their subjective

complements, and are referred not to external things but to the state of consciousness itself. In

this sense, therefore, we call blue, yellow, warm, cold, etc., sensations; pleasantness,

unpleasantness, excitement, depression, etc., feelings. It is important that the terms he kept

sharply distinct, in these assigned meanings, and not used indiscriminately, as they often are in

the language of everyday life, and even in certain psychologies. It is also important that they be

reserved strictly for the psychical elements, and not applied at random both to simple and to

complex contents,--a confusion that is regrettably current in physiology. Thus in what follows

we shall not speak of a manifold of several tones or of a coloured extent as a 'sensation', but as

an 'idea'; and when we come to deal with the formations resulting from a combination of

feelings we shall term them expressly 'complex feelings' or (if the special words that language

offers us are in place) 'emotions,' 'volitions,' etc. This terminological distinction cannot of

course, tell us of itself anything whatsoever regarding the mode of origin of such complex

formations from the psychical elements. It does, however, satisfy the imperative requirement

that the results of psychological analysis of complex conscious contents be rendered

permanent, when that analysis is completed, by fitting designations. As for these results

themselves, it need hardly be said that the mental elements are never given directly as

contents of consciousness in the uncompounded state. We may learn here from physiology,

which has long recognised the necessity of abstracting, in its investigations of these products of

analysis, from the connexions in which they occur. Sensations like red, yellow, warm, cold, etc.,

are considered by physiologists in this their abstract character, i.e., without regard to the

connexions in which, in the concrete case, they invariably present themselves. To employ the

single term 'sensation' as well for these ultimate and irreducible elements of our ideas as for

the surfaces and objects that we perceive about us is a confusion of thought which works

sufficient harm in physiology, and which the psychologist must once and for all put behind him.

But there is another and a still worse terminological obscurity, common both to physiology and

to psychology, which has its source in the confusion of conscious processes themselves with

the outcome of a inner reflection upon their objective conditions. It is all too common to find

sensations so named only when they are directly aroused by external sensory stimuli; while the

sensations dependent upon any sort of internal condition are termed ideas, [p. 14] and the word

idea itself is at the same time restricted to the contents known as memory images. This

confusion is psychologically inexcusable. There is absolutely no reason why a sensation --

blue, green, yellow, or what not -- should be one thing when it is accompanied simply by an

excitation in the 'visual centre' of the cortex, and another and quite a different thing where this

excitation is itself set up by the operation of some external stimulus. As conscious contents,

blue is and remains blue, and the idea of an object is always a thing ideated in the outside

world, whether the external stimulus or the thing outside of us be really present or not. It is true

that the memory image is, oftentimes, weaker and more transient then the image of direct

perception. But this difference is by no means constant; we may sense in dreams, or in the

state of hallucination, as intensively as we sense under the operation of actual sensory stimuli.

[16] Such distinctions are, therefore, survivals from the older psychology of reflection, in which

the various contents of consciousness acquired significance only as the reflective thought of

the philosopher read a meaning into them. It was an accepted tenet of this psychology that

ideas enjoy an immaterial existence in the mind, while sensation was regarded as something

that makes its way into mind from the outside. Now all this may be right or wrong; but, whether

right or wrong, it evidently has no bearing whatever upon the conscious process as such.

The attitude of physiological psychology to sensations and feelings, considered as psychical

elements, is, naturally, the attitude of psychology at large. At the same time, physiological

psychology has to face a number of problems which do not arise for general psychology:

problems that originate in the peculiar interest which attaches to the relations sustained by

these ultimate elements of the mental life to the physical processes in the nervous system and

its appended organs. Physiology tells us, with ever increasing conviction, that these relations,

especially in the case of sensations, are absolutely uniform; and with an improved

understanding of bodily expression, of affective symptomatology, we are gradually coming to

see that the feelings too have their laws of correlation, no less uniform, if of an entirely different

nature. But this growth of knowledge lays all the heavier charge upon psychology to determine

the significance of the various psychophysical relations. A pure psychology could afford, if

needs must, to pass them by, and might confine itself to a description of the elements and of

their direct interrelations. A physiological psychology, on the other hand, is bound to regard this

psychophysical aspect of the problems of mind as one of its most important objects of

investigation.

(3) The course of our inquiry proceeds naturally from the mental ele-[p. 15] ments to the

complex psychical processes that take shape in consciousness from the connexion of the

elements. These mental formations must be treated in order; and our third Part will be occupied

with that type of complex process to which all others are referred as concomitant processes:

with the items that arise from the connexion of sensations. Since physiological psychology

stands committed to the experimental method, it will here pay most regard to the sense ideas

aroused by external stimuli, these being most easily brought under experimental control. We

may accordingly designate the contents of this section a study of the composition of sense

ideas. Our conclusions will however, apply equally well to ideas that are not aroused by

external sensory stimuli; the two classes of ideas agree in all essential characters, and are no

more to be separated than are the corresponding sensations.

The task of physiological psychology remains the same in the analysis of ideas that it was in

the investigation of sensations: to act as mediator between the neighbouring sciences of

physiology and psychology. At the same time, the end in view all through the doctrine of ideas

is pre-eminently psychological; the specifically psychophysical problems, that are of such

cardinal importance for the theory of sensation, now retire modestly into the background.

Physiological psychology still takes account of the physical aspect of the sensory functions

involved, but it hardly does more in this regard than it is bound to do in any psychological

inquiry in which it avails itself of the experimental means placed at its disposal.

(4) The doctrine of sense ideas is followed by a fourth Part, dealing with the analysis of mental

processes that, as complex products of the interconnexion of simple feelings, stand in a relation

to the affective elements analogous to that sustained by ideas to the sensations of which they

are compounded. It must not, of course, be understood that the two sets of formations can, in

reality, be kept altogether separate and distinct. Sensations and feelings are, always and

everywhere, complementary constituents of our mental experiences. Hence the conscious

contents that are compounded of feelings can never occur except together with ideational

contents, and in many cases the affective elements are as powerful to influence sensations and

ideas as these are to influence the feelings. This whole group of subjective experiences; in

which feelings are the determining factors, maybe brought under the title of

Gemuthsbewegungen und Willenshandlungun. Of these, Gemuthsbewegungen is the wider

term, since it covers volitional as well as affective processes. Nevertheless, in view of the

peculiar importance of the phenomena of will, and of the relation which external voluntary

actions bear to other organic movements,--a relation whose psychophysical implications

constitute it a special problem of physiological psychology,--we retain the two words side by

side in the title of our section, and limit the meaning [p. 16] of Gemuthsbewegungen on the one

hand to the emotions, and on the other to a class of affective processes that are frequently

bound up with or pass into emotions, the intellectual feelings.[17]

(5) Having thus investigated sense ideas, emotions and voluntary actions, the complex

processes of the mental life, we pass in a fifth Part to the doctrine of consciousness and of the

interconnexion of mental processes. The results of the two preceding sections now form the

basis of an analysis of consciousness and of the connexions of conscious contents. For all

these conscious connexions contain, as their proximate constituents, ideas and emotion and

consciousness itself is nothing else than a general name for the total sum of processes and

their connexions. So far as our analysis of these connexions is experimental we shall he chiefly

concerned with the arbitrary modification of sense ideas and of their course in consciousness.

When, on the other hand, we come to consider the interconnexions of emotions and voluntary

actions, our principal dependence will be upon the results of analysis of the processes of

consciousness at large.

In these five Parts, then, we confine ourselves to a purely empirical examination of the facts. (6)

A sixth and final Part will treat of the origin and principles of mental development. Here we shall

endeavour to set forth, in brief, the general conclusions that may be drawn from these facts for

a comprehensive theory of the mental life and of its relation to our physical existence. So far,

we have set conscious processes and the processes of the bodily life over against each other,

without attempting any exact definition of either. Now at last, when our survey of their

interrelations is completed, we shall be able to ascribe a definitive meaning to the terms

physical and psychical. And this will help us towards a solution of the well worn problem of 'the

interaction of mind and body,' a solution that shall do justice to the present status of our

physiological and psychological knowledge, and shall also meet the requirements of a

philosophical criticism of knowledge itself. Physiological psychology thus ends with those

questions with which the philosophical psychology of an older day was wont to begin,--the

questions of the nature of the mind, and of the relation of consciousness to an external world;

and with a characterisation of the general attitude which psychology is to take up, when it seeks

to trace the laws of the mental life as manifested in history and in society.

§3. Prepsychological Concepts[18]

The human mind is so constituted, that it cannot gather experiences [p. 17] without at the same

time supplying an admixture of its own speculation. The first result of this naive reflection is the

system of concepts which language embodies. Hence, in all departments of human experience,

there are certain concepts that science finds ready made, before it proceeds upon its own

proper business,--results of that primitive reflection which have left its permanent record in the

concept-system of language. 'Heat' and 'light,' e.g., are concepts from the world of external

experience, which had their immediate origin in sense-perception. Modern physics subsumes

them both under the general concept of motion. But it would not be able to do this, if the

physicist had not been willing provisionally to accept the concepts of the common

consciousness, and to begin his inquiries with their investigation. 'Mind,' 'intellect,' 'reason,'

'understanding,' etc., are concepts of just the same kind, concepts that existed before the

advent of any scientific psychology. The fact that the naive consciousness always and

everywhere points to internal experience as a special source of knowledge, may, therefore, be

accepted for the moment as sufficient testimony to the rights of psychology as science. And this

acceptance implies the adoption of the concept of 'mind,' to cover the whole field of internal

experience. ' Mind,' will accordingly be the subject, to which we attribute all the separate facts

of internal observation as predicates. The subject itself is determined wholly and exclusively by

its predicates; and the reference of these to a common substrate must be taken as nothing

more than an expression of their reciprocal connexion. In saying this, we are declining once

and for all to read into the concept of 'mind' a meaning that the naive linguistic consciousness

always attaches to it. Mind, in popular thought, is not simply a subject in the logical sense, but a

substance, a real being; and the various 'activities of mind' as they are termed, are its modes of

expression or action. But there is here involved a metaphysical presupposition, which

psychology may possibly be led to honour at the conclusion of her work, but which she cannot

on any account accept, untested, before she has entered upon it. Moreover, it is not true of this

assumption as it was of the discrimination of internal experience at large, that it is necessary for

the starting of the investigation. The words coined by language to symbolise certain groups of

experiences still bear upon them marks which show that, in their primitive meanings, their stood

not merely [p. 18] for separate modes of existence, for 'substances,' in general but actually for

personal beings. This personification of substances has left its most indelible trace in the

concept of genus. Now the word-symbols of conceptual ideas have passed so long from hand

to hand in the service of the understanding, that they have gradually lost all such fanciful

reference. There are many cases in which we have seen the end, not only of the personification

of substances, but even of the substantialising of concepts. But we are not called upon, on that

account, to dispense with the use whether of the concepts themselves or of the words that

designate them. We speak of virtue, honour, reason; but our thought does not translate any

one of these concepts into a substance. They have ceased to be metaphysical substances, and

have become logical subjects. In the same way, then, we shall consider mind, for the time

being, simply as the logical subject of internal experience. Such a view follows directly from the

mode of concept-formation employed by language, except that it is freed of all those accretions

of crude metaphysics which invariably attach to concepts in their making by the naive

consciousness.

We must take up a precisely similar attitude to other ready-made concepts that denote special

departments or special relations of the internal experience. Thus our language makes a

distinction between 'mind' and 'spirit.' The two concepts carry the same meaning, hut carry it in

different contexts: their correlates in the domain of external experience are 'body' and 'matter.'

The name 'matter' is applied to any object of external experience as it presents itself directly to

our senses, without reference to an inner existence of its own. 'Body' is matter thought of with

reference to such an inner existence. 'Spirit,' in the same way, denotes the internal existence as

considered out of all connexion With an external existence; whereas 'mind,' especially where it

is explicitly opposed to spirit, presupposes this connexion with a corporeal existence, given in

external experience.[19]

While the terms 'mind' and 'spirit' cover the whole field of internal experience, the various

'mental faculties,' as they are called, designate the special provinces of mind as distinguished

by a direct introspection. Language brings against us an array of concepts like 'sensibility,'

'feeling,' 'reason,' 'understanding,'--a classification of the processes given in internal perception

against which, bound down as we are to the use of these words, we are practically powerless.

What we call do, however, and what science is obliged to do, is to reach an exact definition of

the concepts; and to arrange them upon a systematic plan. It is probable that the mental

faculties stood originally not merely for different parts of the field of internal [p. 19] experience,

but for as many different beings; though the relation of these to the total being, the mind or

spirit, was not conceived of in any very definite way. But the hypostatization of these concepts

lies so far back in the remote past, and the mythological interpretation of nature is so alien to

our modes of thought, that there is no need here to warn the reader against a too great

credulity in the matter of metaphysical substances. Nevertheless, there is one legacy which has

come down to modern science from the mythopoeic age. All the concepts that we mentioned

just now have retained a trace of the mythological concept of force; they are not regarded

simply as--what they really are--class-designations of certain departments of the inner

experience, but are oftentimes taken to be forces, by whose means the various phenomena are

produced. Understanding is looked upon as the force that enables us to perceive truth; memory

as the force which stores up ideas for future use; and so on. On the other hand, the effects of

these different 'forces' manifest themselves so irregularly that they hardly seem to be forces in

the proper sense of the word; and so the phrase 'mental faculties' came in to remove all

objections. A faculty, as its derivation indicates, is not a force that must operate, necessarily

and immutably, but only a force that may operate. The influence of the mythological concept of

force is here as plain as it could well be; for the prototype of the operation of force as faculty is,

obviously, to be found in human action. The original significance of faculty is that of a being

which acts. Here, therefore, in the first formation of psychological concepts, we have the germ

of that confusion of classification with explanation which is one of the besetting sins of empirical

psychology. The general statement that the mental faculties are class concepts, belonging to

descriptive psychology, relieves us of the necessity of discussing them and their significance at

the present stage of our inquiry. As a matter of fact, one can quite well conceive of a natural

science of the internal experience in which sensibility, memory, reason and understanding

should be conspicuous by their absence. For the only things that we are directly cognisant of in

internal perception are individual ideas, feelings, impulses, etc.; and the subsumption of these

individual facts under certain general concepts contributes absolutely nothing toward their

explanation.

At the present day, the uselessness of the faculty-concepts is almost universally conceded.

Again, however, there is one point in which they still exercise a widespread influence. Not the

general class-concepts, but the individual facts that, in the old order of things, were subsumed

under them, are now regarded in many quarters as independent phenomena, existing in

isolation. On this view there is, to be sure, no special faculty of ideation or feeling or volition; put

the individual idea, the individual affective process, and the individual voluntary act are looked

upon as inde-[p. 20] pendent processes, connecting with one another and separating from one

another as circumstances determine. Now introspection declares that all these professedly

independent processes through and through interconnected and interdependent. It is evident,

therefore, that their separation involves just the same translation of the products of abstraction

into real things as we have charged to the account of the old doctrine of faculties, - only that in

this case the abstractions come a little nearer to the concrete phenomena. An isolated idea, an

idea that is separable from the processes of feeling and volition, no more exists than does all

isolated mental force of 'understanding.' Necessary as these distinctions are, then, we must still

never forget that they are based upon abstractions--that they do not carry with them any real

separation of objects. Objectively, we can regard the individual mental processes only as

inseparable elements of interconnected wholes.

The argument of the text may be supplemented here by some further critical remarks upon the

two parallel concepts of 'mind' and 'spirit,' and upon the doctrine of mental faculties.

The English language distinguishes spirit from mind; is a second substance-concept, with the

differentia that it is not, as mind is, necessarily bound up, by the mediation of the senses, with a

corporeal existence, but either stands in a merely external connexion with body or is entirely

free of bodily relations The concept of spirit is accordingly used in a two-fold meaning. On the

one hand, it stands for the substrate of all inner experiences which are supposed to be

independent of the activity of the senses; on the other, it denotes a being which has no part or

lot at all in corporeal existence. It is, of course, only in the former of these two meanings that

the concept of spirit comes into psychology. We can, however, see at once that the first

signification must logically pass over into the second. If the connexion of spirit with body is

merely external and as it were accidental there is no reason why spirit should not occur in the

form of pure undivided substance.

Philosophical reflection could not leave the relation of mind and spirit in the obscurity which had

satisfied the needs of the naive consciousness. Are mind and spirit different beings? Is mind a

part of spirit, or spirit a part of mind? The earliest philosophical speculation shows clearly

enough into what perplexity these questions plunged its authors. On the one hand, they are

forced by the interconnexion of the inner experience to postulate a single substance as its

substrate; on the other, they can see no way to escape a separation of the more abstract

spiritual activities from the bodily entanglements of sense-perception. Alongside of the universal

dualism of matter and spirit there remains the more restricted antithesis of spirit and mind. And

ancient philosophy never succeeded in wholly overcoming this antithesis,--whether, with Plato,

it tries to get rid of the substantiality of mind by regarding mind as a mixture of matter and spirit,

[20] or whether, with Aristotle, it transfers to spirit the notion that it has abstracted from mind

and so substitutes a coincident [p. 21] form of definition for unity of substance.[21] Modern

spiritualistic philosophy has, in general, followed the path laid down by PLATO, though it affirms

more decidedly than PLATO did the unity of substance in mind and spirit. The result is that all

real discrimination of the two concepts disappears from the scientific vocabulary. If a difference

is made, it is made in one of to ways, Either spirit is taken as the general concept, within which

the individual mind is contained;[22] or spirit is confused with the mental faculties, of which we

shall speak presently, and retained as a general designation for the 'higher' mental faculties or,

specifically, for intelligence or the faculty of knowledge. The second usage is often

accompanied, in the later works, by the inclusion of feeling and desire in the common concept

of 'disposition'; so that the mind as a whole divides into intellect and disposition,[23] without any

implication of a separation into distinct substances. Sometimes, again, a mere difference of

degree is made between the two terms mind and spirit, and spirit ascribed to man, while mind

alone is assigned to the animals. Thus the distinction becomes less and less definite, while at

the same time the concept of spirit loses its substantial character. So that, if we are to give the

worth a meaning that shall not anticipate the results of later investigation, we can do no more

than say that spirit, like mind, is the subject of the inner experience, but that in it abstraction is

made from the relations of this subject to a corporeal being. Mind is the subject of the inner

experience as conditioned by its connexion with an external existence; spirit is the same subject

without reference to such connexion. We shall accordingly, speak of spirit and of spiritual

phenomena only when we can afford to neglect those moments of the inner experience which

render it dependent upon our sensuous existence, i.e., upon that side of our existence which is

accessible to external experience. This definition leaves entirely open the question whether

spirit really is independent of sensibility. We can abstract from one or more of the aspects of a

phenomenon without denying that these aspects are actually presented.

It has long been an object with philosophers to reduce the various mental faculties

distinguished by language--sensation, feeling, reason, understanding, desire, imagination,

memory, etc.--to certain more general forms. As early as Plato's Timaeus we find an indication

of a tripartite division of the mind, in accordance with the later discrimination of the three

faculties of knowledge, feeling and desire. Parallel with this threefold division runs another, into

the higher and lower faculties. The former, the immortal reason, corresponds to knowledge; the

latter, sensibility or the perishable part of mind, embraces feeling and desire. Feeling or

emotion is here looked upon as mediating between reason and appetite, just as the true idea

mediates between sensuous appearance and knowledge. But while sensation is expressly

referred to the same part of the mind as desire,[24] the mediating thought (dianoia) and the

emotion [p. 22] appear to stand in similar relation only to the faculty of reason. Hence these

attempts at classification give us the impression two principles of division independently of each

other,--the one based upon observation of a fundamental difference between the phenomena

of cognition, feeling and appetition, and the other upon the recognition or stages in the process

of knowledge; and that his not altogether successful attempt to reduce the two to one came

only as an afterthought. In Aristotle the mind, regarded as the principle of life, divides into

nutrition, sensation, and faculty of thought, corresponding to the inner most important stages in

the succession of vital phenomena. It is true that he occasionally introduces other mental

faculties in the course of his discussion; but it is quite clear that he considers these three

phenomena as the most general. Desire, in particular, is subordinated to sensation.[25]

PLATO obtains his tripartite division by ranking the properties of mind in the order of ethical

value; Aristotle obtains his, conformably with his definition of mind, from the three principal

classes of living beings. The plant mind is nutritive only; the animal mind is nutritive and

sensitive; the human mind is nutritive, sensitive and rational. We can hardly doubt that the

classification, with its three separable faculties, was originally suggested by the observation of

the three kinds of living things in nature. But, however different the source from which it springs,

we have only to omit the distinction of nutrition as a specific mental faculty, and we find it

coinciding outright with the Platonic division into sensibility and reason. Hence it cannot itself,

any more than the various later attempts at classification, be regarded as a really new system.

The most influential psychological systematist of modern times, WOLFF, employs both of the

Platonic divisions, side by side, but makes the faculty of feeling subordinate to that of desire.

The consequent dichotomy runs through his whole system. He first of all separates cognition

and desire, and then subdivides each of these into a lower and a higher part. The further

progress is shown in the following table.

I. FACULTY OF KNOWLEDGE

1. Lower Faculty of knowledge.--Sense, Imagination, Poetic faculty, Memory (remembering and

forgetting).

2. Higher Faculty of Knowledge.--Attention and reflection. Understanding.

II. FACULTY OF DESIRE

1. Lower Faculty of Desire.--Pleasantness and unpleasantness, Sensuous desire and sensuous

aversion. Emotions.

2. Higher Faculty of Desire.--Volition (affirmation and negation).Freedom.

This classification has its proximate source in the Leibnizian distinction of ideation and

appetition as the fundamental forces of the monads. It shows a great advance upon previous

systems in not confining the faculty of feeling and desire to emotion and sensuous desire, but

giving it the same range as the faculty of knowledge, so that the old difference in ethical value

disappears. On the other hand, it is obvious that the special faculties grouped under the four

main rubrics are not distinguished upon any systematic principle; their arrangement is purely

empirical. The classification underwent many changes at the hands of WOLFF'S disciples. We

frequently find knowledge and feeling taken as the [p. 23] two principle faculties, or feeling

added as intermediary to knowledge and desire. This last scheme is that adopted by KANT.

WOLFF'S thought, even in the empirical psychology, is guided by his endeavour to reduce all

the various faculties to a single fundamental force, the faculty of ideation; and his rational

psychology is largely devoted to this task. KANT disapproved of any such attempt to obliterate

given differences in the mere effort after unification. Nevertheless, he too allows knowledge to

encroach upon the domains of the other two mental forces, in correlating each of them with a

special faculty within the sphere of cognition. But he maintains the original diversity of cognition,

feeling and desire. The faculty of knowledge comprehends the other two only in the sense that

it is the legislative faculty of mind at large. It is the source both of the concepts of nature and of

the concept of freedom which contains the ground of the practical precepts of the will. It also

produces the intermediate teleological judgments and judgments of taste. So we find KANT

saying that understanding, in the narrower sense, legislates for the faculty of knowledge,

reason for the faculty of desire, and judgment for feeling;[26] while understanding, judgment

and reason are elsewhere bracketed together as understanding in the wider sense.[27] On the

other side, KANT accepts the distinction of a lower and a higher faculty of knowledge,--the

former embracing sensibility and the latter understanding,--but rejects the hypothesis that they

are separated by a mere difference of degree. Sensibility is, for him, the receptive,

understanding the active side of knowledge.[28] Hence in his great Critique he opposes

sensibility to understanding. When connected with sensibility, understanding mediates empirical

concepts; alone, it gives us pure notions.[29]

It is evident that there are three principal points to be emphasised in the course of this whole

development. The first is the distinction of the three mental faculties; the second, the tripartite

division of the higher faculty of knowledge; and the third, the relation of this to the three

principal faculties. Th first is, in all essentials, a legacy from the Wolffian psychology: the other

two are peculiar to KANT. Previous philosophy had, in general, defended reason (logoV) as

that activity of mind which by inference (ratiocinatio) gives account of the grounds of things. The

definition was, however, compatible with various views of the position of reason. Sometimes,

just as in Neoplatonism, reason was subordinated to understanding (nouV, intellectus); the

latter is a source of immediate knowledge, while the activity of inference implies commerce with

the world of sense. Sometimes, it was ranked above understanding, as the means whereby we

penetrate to the ultimate grounds of things. Sometimes, again it was considered as a special

mode of manifestation of understanding. Illustrations of all three views may be found in the

scholastic philosophy. The cause of this varying estimate of the place of reason is to be sought

in the fact that the term ratio was used in two distinct senses. On the one hand, it meant the

ground of a given consequence of individual truths, the 'reason for'; on the other, the capacity

of ratiocinatio, of inferring individual truths from their grounds of 'reasoning'. First of all, ratio

makes its appearance among the mental faculties, in this latter significance, as faculty of

inference; later on, it appears [p. 24] also as a faculty of insight into the grounds of things. And

wherever the emphasis fell upon this second meaning, reason shone forth as the very organ

and instrument of religious and moral truths, or as a purely metaphysical faculty

contradistingished from understanding, whose concepts could never pass the bounds of outer

or inner sense-experience. A definition which includes both meanings of 'reason' makes it the

faculty whereby we penetrate the interconnexion of universal truths.[30] Now KANT set out

from the first of the three views above mentioned, the view which regards understanding as the

faculty of concepts and reason as the faculty of inference. And he might well be encouraged to

attempt, by the help of logic, to carry out to its conclusion the division of the higher faculty of

knowledge which this view adumbrates, seeing that he had already achieved entire success in

a similar undertaking, his deduction of the categories. He accordingly assumed that, since

judgment stands midway between concept and inference (conclusion), the faculty of judgment

stands midway between the faculties of understanding and reason. He had however, in his

great Critique, sought to bring the two aspects of the concept of reason into a more vital

relation by his doctrine of the unconditioned. In the conclusion, reason subsumes a judgment

under its general rule. Now it must proceed, in the same way, to subordinate this rule to a

higher condition; and so on, until in the last resort it arrives at the idea of an unconditioned. This

idea, then, in its various forms as mind, world and God, remained the peculiar property of

reason in the narrower sense; while all concepts and principles a priori, from which reason as

faculty of inference derives individual judgments, became the exclusive property of

understanding. So we find reason playing a curious double part in the Kantian philosophy. As

faculty of inference, it is the handmaid of understanding, charged with the and principles which

understanding propounds. As faculty of transcendent ideas, it ranks high above understanding.

Understanding is directed merely upon the empirical interconnexion of phenomena. If it follow

the idea of reason at all, it follows it only as a regulative principle, which prescribes the course

that shall lead to a comprehension of phenomena into an absolute whole,--something of which

understanding itself has no conception. It is, however, this regulative office of the ideas of

reason that gives them their practical value. For the moral law, in KANT, is not constitutive, but

regulative; it does not say how we really act, but how we ought to act. At the same time, by the

imperative form in which it demands obedience, it proves the truth of the idea of unconditioned

freedom of the will.[31] In fine, then, reason legislates for the faculty of desire, just as

understanding legislates for the faculty of knowledge. For feeling, which stands midway

between cognition and desire, there then remains only the faculty of judgment, which in like

manner stands midway between the faculty of concepts and the faculty of inference.[32] The

three fundamental faculties of mind are thus referred to the three modes of manifestation of the

faculty of knowledge distinguished by formal logic. And we see at once how largely this

reference is the product of an artificial schematisation suggested by the logical forms. This

intellectualism has also had its reactive influence upon the treatment of the mental faculties;

KANT pays attention only to the higher expressions of his three principal faculties. Now it may

[p. 25] be doubted whether the totality of phenomena embraced by the first faculty can properly

be summed up in the word 'knowledge.' But, at all events, it is obvious that the limitation of

pleasant and unpleasant feeling to the judgment of aesthetic taste, and the reference of the

faculty of desire to the ideal of the good, are not suited to serve as the starting-point of a

psychological consideration.

HERBART'S criticism of the faculty-theory is principally directed against the form which it had

assumed in the systems of WOLFF and KANT. The heart of his argumentation lies in the two

following objections. (1) The mental faculties are mere possibilities, which add nothing to the

facts of the inner experience. Only the individual facts of this experience, the individual idea and

feeling and what not, can really be predicated of the mind. There is no sensibility before

sensation, no memory before the stock of ideas which it lays up. Hence these concepts, notions

of possibility, cannot be employed for the derivation of the facts.[33] (2) The mental faculties

are class-concepts, obtained by a provisional abstraction from the inner experience, and then

raised to the rank of fundamental forces of the mind and used for the explanation of our internal

processes.[34] Both objections seem to shoot beyond the mark at which they are primarily

aimed; they tell against methods of scientific explanation which have found application in

practically all the natural sciences. The forces of physics, e.g., do not exist apart, by

themselves, but only in the phenomena which we term their effects; and the functional

capacities of physiology--nutrition, contractility, irritability, etc. --are one and all 'empty

possibilities.' Again, gravity, heat, assimilation, reproduction, etc., are class-concepts,

abstracted from a certain number of similar phenomena, which have been transformed on just

the same analogy as the class-concepts of the inner experience into forces or faculties, to be

employed for the explanation of the phenomena themselves. Indeed, if we term sensation,

thought, etc., 'manifestations' of mind, the proposition that the mind possesses the 'faculties' of

sensing, thinking, etc., seems to give direct expression to a conceptual construction which

comes naturally to us wherever an object evinces effects that must be ascribed to causes lying

within and not outside of the object. Nor has HERBART any objection to raise against the use

of the concept of force at large. But he makes a distinction between force and faculty. We

assume the action of a force, in all cases where We have learned to look upon a result as

inevitable under given conditions. We speak of a faculty, when the result may just as well not

occur as occur.[35]

Objection has been taken to this distinction, on the ground that it presupposes a concept of

faculty which is found only in the most unscientific form of the psychological faculty-theory.[36]

Nevertheless, it must be conceded that the discrimination of the terms is not without

significance. With the development of modern natural science, the concept of force has

gradually assumed the character of a concept of relation. The conditions which it implies are

always reciprocally determinant; it is on their co-operation that the manifestation of force

depends; and the removal of either side of the conditions renders it null and void. Thus the

concept of force is correctly used when, e.g., the tendency to movement, that has its source in

the interrelations of physical bodies, is derived from a force of gravitation, whereby these

bodies determine each the [p. 26] other's position in space. On the other hand, it is an over-

hasty generalisation to refer the phenomena of falling bodies to a force of falling natively

inherent in every physical body. If we thus translate the conditions of a certain set of

phenomena, resident in a given object, into a force of which the object is possessed, and ignore

the external conditions of the observation, we evidently have no criterion for deciding whether a

variation in the effects of this object depends upon a variation in intrinsic or in extrinsic

conditions. The result is confusion: disparate phenomena are brought together, and (what is of

more frequent occurrence) related phenomena rested apart. Many of the forces distinguished

by the older physiology--the forces of procreation, of growth, of regeneration, etc.--are, beyond

all question, nothing more than manifestations of a single force operating under different

circumstances. And the same thing is pretty generally admitted of the final ramifications of the

doctrine of mental faculties,--of the distinction, e.g., between space-memory number-memory,

word-memory, etc. Similarly, the older physics explained the phenomena of gravitation by

appeal to a number of forces: fall by the force of falling, the barometric vacuum by the 'horror

vacui,' the motions of the planets by invisible arms from the sun or by vortices. But, further, the

habit of abstraction from the external conditions of phenomena may easily lead to the

erroneous conception of faculty, of a force that awaits an opportunity to produce its effect: force

becomes incarnate in a mythological being. It would, therefore, be unjust to psychology, were

we to accuse her and her alone of this aberration. Only, she has the one great advantage over

the sciences of inorganic nature, that their work has paved the way for her advance. In their

hands, the general concepts that belong at once to the outer and the inner experience have

been purged of the errors natural to the earlier stages of the development of thought. And

along with this advantage goes the obligation to make us of it to the full.

HERBART not only realised the untenability of the faculty-theory; he arrived at the positive

conviction that mental processes must be considered as unitary processes. But he sought to

satisfy the requirement of unity by raising one of the products of current psychological

abstraction above all the rest. He regarded the idea as the real and only contents of the mind.

Nay, he went so far as to declare that the idea, when once it has arisen, is imperishable, while

all the other elements of mind--feelings, emotions, impulses--are merely the resultants of the

momentary interactions of ideas. These opinions, as we shall see later, rest upon no better

foundation than hypothesis, and bring their author, at every point, into conflict with an exact

analysis of experience.[37] For the rest, it is obvious that the reduction of all mental processes

to processes of ideation is a survival from the intellectualism of previous psychological systems.

Nevertheless, HERBART had taken the right path in his endeavor to avoid that atomic

conception of mental processes which simply repeats the mistakes of the old faculty-theory in

less glaring form. Unfortunately. in escaping the one error, he was fated to fall into another. The

fault of the older view is, not that it confuses unreality with reality, but that it substitutes for

reality the products of our own discriminative abstraction.[38]

Footnotes

[1] The word was coined by Fechner; see his Elements der Psychophysik, 1860, i. 8. In this

passage, Fechner defines psychophysics as an "exact science of the functional relations or

relations of dependency between body and mind, or, in more general terms, between the bodily

and mental the physical and psychical worlds"; and his main object in the Elemente is,

accordingly, to establish the laws that govern the interaction of mental and bodily phenomena.

It is clear that we have implied here the metaphysical assumption of a substantial difference

between body and mind; we can hardly conceive, in any other way, of the existence of such a

borderland, with facts and laws of its own. Fechner himself, however, rejected this substantial

difference, for theoretical reasons: so that in strictness he could hardly have raised objection to

such a purely empirical formulation of the problem of psychophysics as is given in the text. Cf.

the concluding Chapter of this work.

[2] KANT, Metaphysische Aufangsgründe d. Naturwissenschaft. In Sämmtliche Werke, ed. by

ROSESKRANZ, V. 310.

[3] Cf. esp. E. ZELLER, Abh. d. Berliner Akad. 1881, Phil.-hist. Cl., Abh. iii.; Sitzungsher. of the

same, 1882, 295 ff.; and my remarks upon the question, Philos. Studien, i. 250, 463 ff.

[4] HEBERT, Psychologie als Wissenschaft neu gegründet auf Erfahrung, Metaphysik u.

Mathematik. In Ges. Werke, ed. by HARTENSTEIN, vols. v., vi.

[5] FECHNER, El. d. Psychophysik, ii. 9ff. An interesting light is thrown upon origination of the

idea of 'mental measurement' in Fechner's mind, and also upon the inspiration that he derived

from Herbart, by the "Kurze Darstellung eines neuen Princips mathematischer Psychologie" in

his Zendavesta, 1851, ii. 373 ff. For a detailed treatment of mental measurement, see Ch. ix.

below.

[6] On the question of method in general, cf. my Beiträge zur Theorie der

Sinneswahrnehmungen, 1862, Einleitung: Ueber die Methoden in der Psychologie; Logik, 2nd

ed., ii. 2, 151 ff.; the essay on the problems of experimental psychology in my Essays, Leipzig,

1885, 127 ff.; the article Selbstbeobachtung u. innere Wahrnehmung, in the Phils. Studien, iv.

292 ff.; and Volkerpsychologie, i. I, 1900, Einleitung.

[7] Cf. with this the essay Philosophie u. Wissenschaft, in my Essays, I ff.; and the article Ueber

d. Eintheilung d. Wissenschaften, in the Philos. Studien, v. I ff.

[8] F. A. LANGE, Geschichte des Materialismus, 2te Aufl., ii. 383; History of Materialism, iii.,

1892, 171.

[9] H. MUNSTERBERG. Ueber Aufgaben und Methoden der Psychologie, in Schriften der

Gesellschaft f. psychol. Forshung, i. 111 ff. Practically the same position, though with minor

changes of expression, is taken by the author in his Grundzüge der Psychologie, i., 1900, 382

ff.

[10] MUNSTERBERG. Grundzüge der Psychologie, Vorwort, viii. C. the same author's

Psychology and Life, 1899. This view, of the irrelevancy of psychology to the mental sciences,

is further shared by certain modern philosophers: see the criticism of it in my Einleitung in die

Philosophie, 1901, § 4.

[11] Cf. e.g., T. ZIEHEN, Leitfaden der physiologischen Psychologie, 5te Aufl. 1900, 3 ff.;

Introduction to Physiological Psychology, 1895, 3 ff.

[12] On the doctrine of association. see Part v., below. For a general criticism of psychological

materialism, cf. the articles Ueber psychische Causalität and Ueber die Definition der

Psychologie, in the Philos. Studien, x. 47 ff., xii. 1 ff.

[13] O. Vogt, Die directe psychologie Experimentalmethode in hypnotischen I[unknown

character] wussteseinszuständen. In the Zeitschr. fur Hypnotismus, v., 1897, 7, 180 ff.

[14] For a general discussion of hypnotism, see Part v., below.

[15] In my Grundriss der Psychologie (4te Aufl. 1901; Outlines of Psychology, 1897), in which I

have attempted to give an elementary exposition of psychology so far as possible under the

exclusive guidance of psychological principles, I have adhered more strictly to the systematic

point of view. Hence the Grundriss may be regarded in this connexion both as a supplement

and as introduction to the present work.

[16] For a more extended discussion of these terminological questions see Ch. vii. § I,

below.

[17] Gemüthsbewegungen, as first used above, means "complex affective, affective-volitional

and volitional processes." There is no exact English equivalent. See BALDWIN'S Dict. of Phil.

and Psych. ii. 1902, 680. Willenshandlungen means, of course, voluntary actions, internal and

external.--TRANSLATOR.

[18] In the first four editions of the Physiologische Psychologie, the Introduction consists of two

sections, entitled respectively Aufgabe der physiologischen Psychologie, and Psychologische

Vorbegriffe. In the present, fifth edition, the second of these sections is replaced by an

Uebersicht des Gegenstandes. I here reprint the section on Psychologische Vorbegriffe as it

appeared in 1893. It was, in all probability, omitted mainly for reasons of space. Cf. Preface to

this fifth edition. It will, I think, be found useful by English readers in its present form, although a

good deal of its criticism is implicit in the constructions of the final chapter of the work. I print it

only after much hesitation, and with the express reminder to the reader that the author, for

whatever reason, has not included it in the current edition of his book.--TRANSLATOR.

[19] The German terms for 'body' and 'matter' are Leib and Körper; for 'mind' and 'spirit,' Seele

and Geist. See BALDWIN'S Dict. Of Phil. and Psych., ii. 680.--TRANSLATOR.

[20] Timaeus, 35. JOWETT'S Plato, iii. 453-4.

[21] The Aristotelian definition of mind in general as 'earlier or implicit entelechy (i.e. perfect

realisation) of a natural body possessed potentially of life,' holds also of the (logoV

nouVpoihtikoV), the spirit as independent of sensibility. Spirit is, however, the reality of the mind

itself, and so can be conceived of as separated from the body; which is not the case with the

other parts of mind. De anima, ii. sub fin. WALLACE's trans,. 65; HAMMOND's trans. 44 f.

[22] SO WOLFF, Psychologia rationalis, §§ 643 ff.

[23] Geist and Gemüth.-TRANSLATOR.

[24] Timaeus, 77. Jowett's Plato, iii. 449-50.

[25] De anima, ii. 2, 3. WALLACE'S trans,. 65-77; HAMMOND'S trans., 48-56.

[26] Kritik d. Urtheilskraft, ROSENKRANZ' ed., iv. 14 ff. BERRNARD'S trans., 1892, 13 ff.

[27] Anthropologie, vii. 2, 100 and 104.

[28] Anthropologie, 28.

[29] Kritik d. reinen Vernunft, ii. 31, 55. MULLER'S trans., 1896, 15, 40.

[30] WOLFF, Psychologia empirica, § 483.

[31] Kritik d. prakt. Vernunft, viii. 106.

[32] Kritik d. Urtheilskraft, iv. 15. BERNARD'S trans., 16.

[33] HERBART, Werke, vii. 611.

[34] Werke, v. 214.

[35] Werke, vii. 601.

[36] J. B. MEYER, Kant's Psychologie, 116.

[37] Cf. ch. xix. [of the present edition].

[38] Cf. with this the essay on feeling and idea, in my Essays, 199 ff.

Part I

The Bodily Substrate of the Mental Life

CHAPTER I

The Organic Evolution of Mental Function

§1. The Criteria of Mind and the Range of the Mental Life

THE mental functions form a part of the phenomena of life. Wherever we observe them, they

are accompanied by the processes of nutrition and reproduction. On the other hand, the

general phenomena of life may be manifested in cases where we have no reason for supposing

the presence of a mind. Hence the first question that arises, in an inquiry concerning the bodily

substrate of mentality, is this: What are the characteristics that justify our attributing mental

functions to a living body, an object in the domain of animate nature?

Here, upon the very threshold of physiological psychology, we are confronted with unusual

difficulties. The distinguishing characteristics of mind are of a subjective sort; we know them

only from the contents of our own consciousness. But the question calls for objective criteria,

from which we shall be able to argue to the presence of a consciousness. Now the only

possible criteria of the kind consist in certain bodily movements, which carry with them an

indication of their origin in psychical processes. But where are we justified in referring the

movements of a living creature to conscious conditions? How uncertain the answer to this

question is, especially when metaphysical prejudice has a part to play in it, may be seen at

once by an appeal to history. Hylozoism inclines to regard every movement, even the fall of a

stone, as a mental action; Cartesian spiritualism recognises no expression of mental life

beyond the voluntary movements of man. These are extreme views. The first is beyond all

verification; the second is correct only upon the one point that the manifestations of our own

conscious life must always furnish the standard of reference in our judgments of similar

indications in other creatures. Hence we must not begin our search for mental function among

the lower types of organised nature, where its modes of expression are least perfect. It is only

by working our way downwards, from man to the animals, that we shall find the point at which

mental life begins.

[p. 28] Now, there are a very large number of bodily movements, having their source in our

nervous system, that do not possess the character of conscious actions. Not only are the

normal movements of heart, respiratory muscles, blood vessels and intestines for the most part

unaccompanied by any sort of conscious affection; we find also that the muscles subserving

change of position at the periphery of the body often react to stimuli in a purely mechanical and

automatic way. To regard these movement-processes as mental functions would be every whit

as arbitrary as to ascribe sensation to the falling stone. When, however, we rule out all the

movements that may possibly go on without the participation of consciousness, there remains

but one class that bears upon it the constant and unmistakable signs of an expression of the

mental life, -- the class of external voluntary actions. The subjective criterion of the external

voluntary action, as directly given in introspection, is that it is preceded by feelings and ideas

which we take to be the conditions of the movement. Hence a movement that we observe

objectively may also be regarded as dependent on the will if it points to similar mental

processes as its conditions.

But the discovery of this criterion does not by any means remove the practical difficulties of our

diagnosis of mind. It is not possible to distinguish certainly in every case between a purely

mechanical reflex -- or even, in the lowest organisms, a movement due to external physical

causes, such as the imbibition of tumescent bodies, the change of volume from fluctuations of

temperature, etc. -- and a voluntary action. We have to note; in particular, that while there are

characters by which we can argue with absolute confidence to the existence of a voluntary

action, the absence of these characters does not always necessarily imply the absence of such

action, still less the absence of psychical functions at large. Hence all that our inquiry can hope

to accomplish is the determination of the lower limit at which a mental life is demonstrably

present. Whether it does not, in actual fact, begin at a still lower level must remain a matter of

speculation only.

The generally accepted objective criterion of an external voluntary action is the reference of

movement to the universal animal impulses, the nutritive and the sexual. It is only as a result of

sensory excitations that these impulses can lead the animal to a change of place that shows

the marks of a voluntary action; and the special character that prompts us to refer such

sensorily stimulated movements to a process in consciousness is their variability. They do not

appear with mechanical regularity in response to a given external stimulus, but are varied to

suit varying conditions, and brought into connexion with sense-impressions previously secured.

Judgment on the ground of these criteria may, in the individual case, remain doubtful; since all

vital processes, even those that are entirely [p. 29] automatic and unconscious, evince a certain

adaptation to ends, and a certain consequence in their successive stages. But sustained and

attentive observation of living creatures will as a general rule, enable us to decide with certainty

whether any particular manifestation of life is intelligible only from that continuity of internal

states which we name consciousness, or whether it may possibly have arisen in the absence of

mind. That consciousness, in this sense, is an universal possession of living organisms, from

man down to the protozoa, is beyond the reach of doubt. At the lowest levels of this

developmental series the processes of consciousness are, of course, confined within extremely

narrow limits, and the will is determined by the universal organic impulses only in the very

simplest way. Nevertheless, the manifestations of life, even among the lowest protozoa, are

explicable only upon the hypothesis that they possess a mind. Thus the amoeba, which is to be

regarded morphologically as a naked cell (see Fig. 2, p. 33) will sometimes return after a short

interval to the starch grains that it has come upon in the course of its wanderings, and will

incept a new portion as nutritive material in the soft protoplasm of its body.[1] Many of the

ciliated infusoria pursue others, which they kill and devour.[2] These are all phenomena that

point towards continuity of mental processes, though in all probability to a continuity that

extends only over a very short space of time. They point also, at all events in the case of the

Ciliata, to a variation in the choice of means, for the satisfaction of the organic impulses, that

would be unintelligible as a merely mechanical result of external influences.

We enter, of course, upon much less certain ground when we ask, further, whether the mental

life really makes its first appearance at that point upon the scale of organised existence at

which we notice the external voluntary action, or whether its beginnings do not reach back to a

still lower level of life. Wherever living protoplasm occurs, it possesses the property of

contractility. Contractile movements arise, sometimes at the instigation of external stimuli but

sometimes also in the absence of any apparent external influence. They resemble the voluntary

actions of the lowest protozoa, and are not explicable in terms of external physical affection, but

only as the results of forces resident in the contractile substance itself.[p. 30] They cease at

once with the cessation of life. We find them evinced both by the protoplasmic contents of

young plant-cells and by the free protoplasm occurring throughout the animal and vegetable

kingdoms. Indeed, it is probably that all elementary organisms, whether they enjoy an

independent existence or form part of a compound organism, possess the property of

contractility at least during a certain period of their development. Consider, e.g., the lymph

corpuscles, which are found in the blood and lymph of animals, and in pus, and which occur as

migratory elements in the tissues. They are not only entirely similar in bodily

configuration to certain of the lowest protozoa, but they also undergo changes of form which, in

outward appearance, are indistinguishable from the movements of these unicellular organisms

(Fig. 1). Only, the voluntary character of these movements is beyond the reach of

demonstration. It is true that similar structures -- particularly the colourless blood-corpuscles of

invertebrates -- have been seen to take up solid substances, and that this action may be

interpreted as an inception of food.[3] It is true, also, that movements in response to stimulus

accompany the exercise of the digestive functions in

certain plants. But in neither case is there any definite

indication of a true impulse, i.e. an impulse determined

by sensation, toward the food-stuff, or of any sort of

psychological middle term between stimulus and

movement.[4] The same thing holds of the movements of

the lower forms of algae, fungi and swarm-spores,

produced by a variable distribution of water and carbon

dioxide, or by different kinds of light rays. On the other

hand, the movements of certain bacteria are so suddenly

affected by light ad by the gases of respiration, that they

at once suggest an origin in sensations, But, here again,

we cannot he sure that the changes are not simply

physical effects, as is undoubtedly the case with the

movements evolved by hygrometric changes in the

environment.[5][p. 31]

We must, however, always remember, in passing

judgment upon this whole group of observations, that the demonstration of physical conditions,

to which the phenomena of protoplasmic contraction and of the movement of elementary

organisms may be referred, is by no means incompatible with the hypothesis of concomitant

psychical processes. Physiology seeks to derive the processes in our own nervous system from

general physical forces, without considering whether these processes are or are not

accompanied by processes of consciousness. We are bidden to believe, both by theory of

knowledge and by the philosophy of nature, that all manifestations of life, on the physical side,

are referable to natural laws of universal validity. And physiology, acting in accordance with this

requirement, has found it justified in every instance in which she has succeeded in reaching a

solution of her problems. It follows, then, that the existence of mental functions can never be

inferred from the physical nature of organic movements, but only from certain special conditions

attending their performance. On the other hand, observation shows that the chemical and

physiological properties of living protoplasm are essentially the same, whether we can prove

that it manifests a mental life or whether we cannot. This holds, in particular, of the attributes of

contractility and irritability. In physical regard, therefore, protoplasm maintains its identity

throughout. If we add to this the fact that it is impossible to draw a hard and fast line at the point

where protoplasmic movements first begin to take on a psychological character, -- that there is

a gradual transition from the walled-in protoplasm of the plant-cell on through the migratory

lymph-corpuscles of animals and the free-living monera and rhizopods, to the more motile

ciliated and mouth-bearing infusoria -- we cannot resist the conjecture that psychical life and

the capacity of giving expression to it are universally represented in contractile substance.

From the standpoint of observation, then, we must regard it as a highly probable hypothesis

that the beginnings of the mental life date from as far back as the beginnings of life at large.

The question of the origin of mental development thus resolves itself into the question of the

origin of life. Further, if physiology is obliged, by the uniformity of interaction of physical forces

throughout the universe, to accept the postulate that the processes of life have their ultimate

basis in the general properties of matter, psychology finds it no less obligatory to assume, in

this same matter, the universal substrate of natural phenomena, the presence of conditions

which attain to expression as the psychical aspect of vital phenomena. But this latter statement

must not mislead us. The latent life of inorganic matter must not be confused, as hylozoism

confuses it, with real life and actual conscious-[p. 32] ness; nor must it be considered, with

materialism, as a function of matter. The former interpretation is wrong, because it assumes the

existence of vital phenomena at a point where not these phenomena themselves are given, but

only the common ground upon which they rest and whereby they become possible; the second

is wrong, because it posits a one-sided dependence, where in reality we find an interrelation of

simultaneously presented but incommensurable processes. We employ the concept of material

substance to denote the ground of all objective phenomena. Hence it is the office of this

concept to make intelligible all the various form of physical occurrence, including the physical

manifestations of life. Now among these manifestations we find movements which indicate the

presence of a consciousness. Our postulates concerning matter will then, explain the physical

causation of such movements, but can never account for the concomitant psychical functions.

To explain these, we must make appeal to our own consciousness.

We cannot, of course, here at the very outset of our psychology, return any final answer to the

question of the ultimate objective criteria of the mental life. All that we can do, at the present

stage, is to indicate in brief the position to be taken up in psychological practice. It is however,

easy to see that the wide divergence of opinion on the subject is mainly due to the intermixture

of science with philosophy, or to a fixity of judgment that has its source in philosophical theory.

Only in this way can we account for the fact that there may still be found, in works upon the

scope of the mental life, views that range between the two extremes current in DESCARTES'

day. One author will assert that the animals, if not without exception, at least as far up the scale

as the higher invertebrates and the lower vertebrates, are mere reflex machines;[6] another

looks upon life and mind as convertible terms, and accordingly endows plants as well as

animals with consciousness.[7] The former view is evidently influenced, to some extent, by the

idea that psychical and physical are antithetical terms. The alternative (physical or psychical) is

often presented as if the one concept necessarily excluded the other, -- as, indeed it did, in the

metaphysical dualism of DESCARTES. But this is misleading. The close interconnection of the

phenomena of the physical life and the processes of consciousness makes the relation

'physical and psychical' on the face of it, much more probable. We should, as a matter of fact,

admit at once that, e.g., a sensation is a psychical quality, without meaning to deny that it is

accompanied by a physical process in the sense-organ and the sense-centre. And such a

coexistence of the two kinds of vital processes is, in many crises, beyond all dispute. How far it

extends, over the phenomena of life at large, is again a question that, naturally, cannot be

answered at the outset of our psychological investigations. But, at all events, we should be

merely obscuring the facts, if we made our first approach to them with the alternative 'physical

or psychical' in our hands. And the danger of misinter- [p. 33] pretation is, at best, grave

enough. Many movements, that may in all probability be regarded as purely automatic, are, as

we said above, purposive in character; and many of them, again, are self-regulating. It is,

therefore, very difficult to draw the line of division in the concrete case.[8]

We may say, then, that the mechanistic explanation of the movements of the lower animals is

not the outcome of impartial and unprejudiced observation. But the rival theory, which ascribes

mind and consciousness to the plant-world, is in no better case. Fechner, the chief

representative of this theory, himself expressly declares that be derived it from considerations

of general philosophy: he further attributes consciousness to the earth and the other heavenly

bodies, making this cosmic consciousness the whole, of which the individual forms of

consciousness in plant and animal are parts.[9] Hypotheses of this sort have, no doubt, a

certain justification. They emphasise the intrinsic impossibility of the view that mental life may

suddenly appear, at some point of time and space, as a new thing; that we need not seek for its

general conditions in the universal substrate of the vital processes. When, however, we ask

how we should conceive of these conditions, we raise a metaphysical question, -- a question

that lies well beyond the reach of psychology and its empirical problems.

§ 2. The Differentiation of Mental Functions and of their Physical Substrate

The organic cell in the earliest stages of its development, consists either of a naked mass of

protoplasm, contractile throughout its substance, or of a denser and immotile cortex within

which motile protoplasm is contained.

And the same two forms are evinced by the lowest independent organisms in which we can

observe movement-processes indicative of psychical conditions (Fig. 2). The substrate of the

elementary mental functions is here entirely homogeneous, and coextensive with the whole

mass of the [p. 34] body. The only sense that is plainly functioning is the sense of touch. An

impression made upon any portion of the contractile protoplasm first of all releases a

movement at the place of direct impact, which may then extend to purposively co-ordinated

motion of the entire body. The beginnings of a differentiation of mental function can, however,

be found even in the protozoa, wherever the cortical layer surrounding be contractile body-

substance has developed special organs of movement, cilia and flagella (Fig. 3). Oftentimes

this development

goes hand in hand

with a

differentiation of

the nutritive

functions, An [sic]

oral aperture and

digestive cavity

are found, and in

many instances a

system of open

canals appears,

whose fluid

contents are kept

in motion by a

contractile vessicle. The cilia with which these infusoria are furnished render them

incomparably more motile than the organisms lying at the very lowest point of the organic

scale, the monera and rhizopods, which consist merely of a viscous body-mass. They are,

however, more than organs of locomotion; they function as organs of touch, and sometimes

appear to be sensitive to light as well. The spot of red pigment noticed in many of the infusoria

may also have some connexion with light-sensation; but we have as yet no certain ground for

regarding it as a primitive organ of vision.

In the compound organisms we observe a more radical differentiation of mental function and its

bodily substrate. The metazoan germ-cell divides into a number of cells. These seem to be

originally of the same kind, so that not infrequently all like manifest the primitive contractility of

protoplasm in course of time, however, they become modified in matter and form; the tissues of

the plant and animal body are derived from them and from the products of their growth, and the

structural changes are accompanied by a more and more complete specialisation of function.

The conditions which govern this process of differentiation, to which the whole of organic nature

is subject, are still wrapped in obscurity. Our knowledge halts abruptly at the changes of

outward form in which the internal development finds its expression.

In the plant-world we see the nutritive functions attain such a degree of elaboration that the

organism (and this is true more especially of the higher plants) has, so to say, , no other

concern than to increase its present stock of organic substance. In the animal world, on the

other hand, the process of evolution is characterised by the progressive discrimination of the

animal and vegetative functions, and a consequent differentiation of these two great provinces

into their separate departments. The cell-mass of the yolk, originally homogeneous, divides up

first of all into a peripheral and a central layer of different structural character (Figg. 4 and 5);

while the cleavage cavity gradually widens out to form the future body-cavity.[10][p. 35] At this

stage, sensation and movement appear to reside exclusively in the outer cell-layer, the

ectoderm, while the nutritive functions are discharged by the inner layer or entoderm. At a

higher level of evolution a third

layer of cells, the

mesoderm, forms

between the two.

The initial stages

of development

are thus identical

over the whole

series of forms

from coelenterates

to vertebrates, the

differentiation of

organs beginning

always with the

distinction of three germinal layers. The outermost layer is the source of the nervous system

and sense-organs, as well as of the muscular system; the innermost furnishes the organs of

nutrition; and the intermediate layer, the vascular system. In the vertebrates, the skeleton is

also derived from the ectoderm.[11]

This discrimination of organs is accompanied by a differentiation of the elementary constituents

of the tissues. When the separation of ectoderm and entoderm is first accomplished, the cells

of the former discharge the combined function of sensation and movement. The initial step

toward a separation of these two cardinal functions is apparently taken in the hydridae and

medusae, where the ectoderm cells send out contractile processes into the interior of the body.

The sensory and motor functions are here still united in a single cell, but are distributed over

different portions of it (Fig. 6).[12] In the next stage, the properties of sensation and contractility

pass to special and spatially separated cells, while connective elements [p.36] develop, to

mediate the functional interconnexion of the different structures. There thus arises a third class

of cells, lying in the paths of connexion between sensory and muscular cells, and acting

probably as organs for the reception and transmission of stimuli. The sensory cells now

become external organs, devoted to the reception of physical stimuli. At the same time, they

undergo a differentiation, which fits them for excitation by various forms of movement-process

in the outside world. Similarly, the contractile cells become organs for receiving and converting

into external movements the excitations transmitted to them. But the psychical functions par

excellence are discharged by the cells of the third class, the nerve-cells, which are connected

by their processes with both the sensory and the muscular cells, and, as we have said, mediate

the functional interconnexion of the two group's of organs. Hence the simplest scheme of a

nervous system is given with a centrally situated nerve-cell connected on the one hand with a

sense-cell and on the other hand with a contractile muscle-cell both directed towards the

external world, but mediating the one the reception of sense-stimuli and the other the motor

reaction upon them.

It is, however, quite certain that this simplest scheme

never actually occurs. As soon as special nerve-cells are formed at all they are formed in

numbers, joined together in longitudinal and transverse series, so that a great many of them

are connected only by way of others of their kind with the peripheral structures. This

multiplication of the central elements means, of course, that the process of differentiation

extends to the nerve-cells themselves. They assume various functions, according to the

connexions in which they stand with one another and with the peripheral organs. Those lying in

the neighbourhood of the terminal organs are employed in functions, auxiliary to the strictly

psycho-physical processes, which run their after course without the participation of

consciousness. Others enter into intimate relation with the mechanisms of nutrition; they

sustain and regulate the physiological processes of secretion and circulation. They thus lose

their place among the immediate bodily conditions of the mental [p. 37] life, and exert only an

indirect influence upon mind. This progressive differentiation of functions and of their substrate

within the nervous system finds its expression in the relative increase of the mass of the

nervous elements, and in the elaboration of special nerve-centres, compact bodies of nerve-

cells and their fibrillar processes. We have an instance of such centres in the ganglia of the

invertebrates, which appear at the most various stages of development, from the comparatively

simple nerve-rings of the coelenterates and the lower worms and molluscs, up to the brain-like

ganglionic masses of the anthropods and higher molluscs (Fig. 7).

Finally, among the vertebrates, the importance of the nerve-centres for the whole organisation

of the animal is shown, from the first, in their relation to the external bodily form and to the

development of the various systems of organs.

Immediately after

the separation of

the formative

materials into the

two layers of the

germ-primule,

there appears in

the ectoderm a

groove, open

above, at the

bottom of which is

a streak of darker

tissue. This is the

primitive streak,

whose direction

corresponds with

the future

longitudinal axis of

the embryo (Fig.

8). Presently, the

groove closes and

becomes the

neural tube, the

primule of the myel (spinal cord) and its sheaths.[13] The

anterior portion of this; tube gives rise, by expansion, to

the primule of the brain. Concomitantly with the closure

of the neural tube begins the differentiation of the germinating cells into nerve-cells. They

increase in size, and send out runners, which become transformed into the various cell-

processes (Fig. 9).[14]

At this point there begins a serial differentiation of function and its physical substrate, whose

investigation will form the subject of the following Chapters. We shall set out with a

consideration of the structural elements of the nervous system in their morphological and

chemical characters. We shall next raise the question of the nature of the processes at work

within those elements; in other words, we shall attack the problem of a physiological mechanics

of nervous substance. This discussion will be followed by a brief description of the structural

development of the nervous centres, with especial reference to the morphology of the human

brain. We shall then be prepared to approach the two main problems that are presented by the

co-ordination of functions in the nervous system. The first of these is the determination of the

course of the paths of nervous conduction, as conditioned by the individual connexions of the

nervous elements; and the second is the problem of the physiological functions of the central

parts, -- the last and most important question for the relation of nervous process to the

processes of the psychical life.

Notes

[1] ROMANES, Animal Intelligence, 4th ed., 1886, 18 ff.; Mental Evolution in Animals, 1885, 18,

55; MAX VERWORN, Psychophysiologische Protisten-Studien,1889, 146 ff. VERWORN'S

statement that voluntary actions appear for the first time in the Ciliata, and that all movements

made in response to stimulus by the non-ciliated protozoa, so far as they are not of purely

mechanical or chemical origin, should be interpreted as reflexes, is evidently a result not of

observation, but rather of a forgone theoretical conviction that voluntary actions must have

developed out of reflexes. On this theory see Part iv. below.

[2] FAMINZYN, The Mental Life of the Simplest Organisms, 1890 (Russian). Quoted by

BECHTEREW, Bewusstsein und Hirnlocalisation, 1898, 6.

[3] M. SCHULTZE, Das Protoplasma der Rhizopoden, 1863. ENGELMANN, Beiträge zur

Physiologie des Protoplasmas, ii., 1869. VERWORN, Die Bewegung der lebendigen Substanz,

1892, 51 ff.; Allgemeine Physiologie, 1901, 363 ff. (General Physiology, 1899, 146 ff., 527)

[4] DARWIN, Insectivorous Plants, 1875, esp. ch. x. PFEFFER, Pflanzenphysiologie, 2te Aufl.,

1897, 364 ff.

[5] T. W. ENGELMANN, in PFLÜGER'S Archiv. f. d. ges. Physiologie, xxvi. 537; xxix. 415; xxx.

95. PFEFFER, Untersuchungen aus d. botan. Institut zu Tübingen, i. 363, 483; ii. 582. For

further details, see Ch. vii. § 3, below. On the physical causes of proto-plasmic movement,

cf. BÜTSCHLI, Untersuchungen über mikroskopische Schäume und das Protoplasma. 1892,

172.

[6] A. BETHE, Dürfen wir den Ameisen und Bienen psychische Qualitäten zuschreiben? In

PFLÜGER'S Arch. f. d. ges. Physiol., 1xx. 1898, 15 ff. Cf. the critical remarks of WASSMANN,

Die psychischen Fähigkeiten der Ameisen, 1899, and Biol. Centralblatt, xviii. 1898, 578.

[7] FECHNER, Nanna oder über das Seelenleben der Pflanzen, 1848; 2nd ed., 1899.

[8] Cf. with this the later discussions of impulsive movement (Part iv.) and of consciousness

(Part v.).

[9] FECHNER, Zendavesta oder über die Dinge des Himmels und des Jenseits, i., 851, 2nd ed.,

1901.

[10] The relations of the various cavities, three or four in number, are in reality much more

complicated. It would be more nearly true to say that, where the change indicated in the text

takes place, the body-cavity gradually replaces the cleavage-cavity. Cf. MINOT, Embryology,

1897, ch. ix. -- TRANSLATOR.

[11] The author gives no references here. The mesoderm is now divided, by the best writers,

into mesothlium, the source of the muscles and mesodermic glands, and mesenchyma, the

source of connective and skeletal tissue. The derivation of the mesenchyma itself is still an

open question. -- TRANSLATOR.

[12] KLEINENBERG, Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung 1872,

21 ff. O and R. HERTWIG, Das Nervensystem und die Sinnesorgane der Medusen, 1878, 157.

[The cells from the epithelial layer of Hydra shown in Fig. 6 (KLEINENBERG'S 'neuromuscular'

cells) are now to be regarded as muscle-cells. Later Note, by AUTHOR.]

[13] The myelic furrow is now known to be entirely distinct from the primitive groove. See O.

HERTWIG, Embryology, 79 ff., 125, 416 ff. -- TRANSLATOR.

[14] HIS, Archiv für Anatomie u. Physiologie, Anat. Abth. 1890, 95.

CHAPTER II

Structural Elements of the Nervous System

§1. Morphological Elements

THE nervous system is made up of three kinds of morphological elements: (1) cells of peculiar

form and structure, the nerve-cells or ganglion cells; (2) fibrous structures, originating as

outgrowths from the cells, -- the nerve-fibres; and (3) a ground-reticulum, which in places is

finely granular and in places fibrillar, and which consists of the terminal ramifications of the

nerve-fibres and processes of the nerve-cells. To these must be added (4) a sustentacular

substance, fibrous or amorphous in structure, which is regarded as a form of connective tissue.

[1] The nerve-cells, with the fibrillar ground-reticulum that surrounds them, are essential

constituents of all the central parts. In the higher nervous centres, however, they are restricted

to definite areas, which, partly from their rich supply of capillary blood-vessels and partly from

the presence of pigment-granules, collected both in the protoplasm of the cell-bodies and in the

ground-reticulum, possess a darker coloration than the surrounding tissue. This grey substance

contrasts so sharply with the white or myelinic substance that the distribution of cell-groups

through the central organs may readily be followed by the naked eye. The myelinic substance

itself owes its peculiar character mainly to the myelinic sheaths which enclose the nerve-fibres

issuing from the grey substance. The connective tissue cement-substance occurs in three

principal forms. As a soft and for the most part amorphous mass, the neuroglia, it serves to

support the central nerves and cells. In the form of endoneurium and perineurium,[2] a denser

tissue, showing tendon-like fibrillation, it extends among and surrounds the peripheral nerves.

As the primitive sheath of Schwann, a membrane of glassy transparency and great elasticity,

nucleated at intervals, it encases nearly all peripheral and a portion of the central nerve-fibres.

These cement-substances form a sustentacular framework for the nervous elements. They

serve, further, to carry the blood-vessels. And the perineurium [2][p. 40] imparts to the

peripheral nerves, which have no solid wall of bone to protect them, the necessary power of

resistance to mechanical injury.

(a) -- The Nerve-Cells

It is probable that the nerve-cells

(Figg. 10-14) are everywhere

devoid of a true cell-cortex. They

vary in form from spherical to

irregularly angular, and differ so

extraordinarily in size that some

can hardly be distinguished with

certainty from the minute

corpuscles of the connective

tissue, while others are visible to

the naked eye. A clear nucleus,

plainly vesicular in form, and

provided with a large nucleolus,

stands out in sharp contrast to the

dully pigmented protoplasm. In

the central organs the cells are

embedded directly in the soft

substance of the supporting

tissue; in the ganglia, they are

usually surrounded with an elastic

sheath of connective tissue, often

directly continuous with the primitive sheath of a nerve-fibre proceeding from them. The nerve-

cells are characterised by their processes, one of which usually passes over directly into a

nerve fibre, while the others ramify, if not immediately, after running a brief course, into fine

fibrils. The former is called the axis-cylinder, nerve-[p. 41] process or neurite; the latter are

termed protoplasmic processes or dendrites. Secondary dendritic processes may also arise,

not from the cell itself, but from its neurite (Fig. 14, c). They are then named collaterals. The

two types of process are shown with special clearness in many of the larger cells of the myel

(spinal cord) and brain of vertebrates.

The nerve-fibres do not form independent elements of the nervous system. They originate, as

embryology teaches us (Fig. 9), in outgrowths from nerve-cells, and they remain throughout in

connexion with the cells whose processes they are. We may accordingly consider the nervous

system in its entirety as a vast conglomerate of nerve-cells, all woven together by fibrillar

runners. Under these conditions, the only processes of the central cells that attain to any

measure of apparent independence, as fibrillar elements, are those entering into connexion

with the peripheral organs. But even the fibrils of the muscular and cutaneous nerves, which in

many cases extend without break over large distances, are really nothing more than cell

processes long drawn out. It is, therefore, the nerve-cell that is the main variable in the nervous

system. Both in number and nature of its processes and in its own internal structure, the cell

evinces characteristic differences, often strongly marked, from one part of the nervous system

to another.[3] When highly magnified, most nerve-cells show, even without treatment by

selective reagents, a fibrillated structure; clusters of granules are set, in scattered masses,

between the meshes of this fibrillar network, and a special network of granules and fibrillae

encloses the nucleus (Fig. 10). The granular deposits are named, from their discoverer, the

corpuscles of Nissl; they are also known as tigroid bodies, or as chromophilous substance.

Colour-staining brings them out with greater clearness, since they have an affinity for the dyes

of the histologist, while the fibrillae and the amorphous ground-substance remain unaffected

(Fig. 11). It appears, further, that these bodies stand in a peculiar relation to the different forms

of cell-process; they are assembled in greater numbers at the points of origin of the dendrites,

but are entirely absent from the part of the cell that gives off the neurite or axis-cylinder (Fig.

12, lower right-hand portion). Finally, besides this network of fibrillae which run their course

within the substance of the cell, and whose continuity with the cell-processes evidences their

nervous character, there is sometimes found a pericellular reticulum, which, basket-like,

encloses the whole outer wall of the cell. Its fibrillae can, in most cases, be traced [p. 42] into

the dendrites, so that they too are, in all probability, to be looked upon as nervous structures

(Fig. 13).

Nerve-cells are classified, according to the number of processes they send out, as unipolar,

bipolar and multipolar.

Unipolar cells are, however, always of

rare occurrence; and, where they occur,

have probably arisen secondarily, in

course of growth from the originally

bipolar form, by a fusion of its two

processes, -- which, we may note, divide

again immediately after their emergence

from the cell (see Fig. 21, z, p. 50). The

bipolar cell is found more especially in the

peripheral regions, e.g. in the spinal

ganglia, in the retina, and (to some

extent) in the ganglia of the sympathetic

system. The great majority of nerve-cells

are, however, multipolar. As a rule, every

such cell gives off a single neurite, and

an indeterminate number of dendrites.

The divergent characters not only of the

processes themselves, but also of the

portions of the cell with which they are

connected (Fig. 12) render it, in the

present case, an exceedingly probable hypothesis, that the difference of structure is paralleled

by a corresponding difference of function. As a matter of fact; the fibrils of the large cells of the

ventral cornua of the myel that pass over into the motor nerves, are without exception neuritic;

while the processes that tend from the same cells towards the higher regions of the myel are

dendritic in nature. RAMON Y CAJAL has accordingly suggested that the dendrites are devoted

exclusively to cellipetal, the neurites to cellifugal conduction.[4] This scheme can, however,

hardly he applied to all nerve-cells, without exception, since there are many cases in which no

clear difference between the various cell-processes can be made out.

For the rest, over and above their different manner of origination from the cell body, their

shorter course, and their greater wealth of branches, the dendrites are morphologically

distinguishable from the neurites by their character as 'protoplasmic' processes; their irregular

[p. 43] nodosity (Fig. 14) suggests the pseudopodial processes of the Rhizopoda (Fig. 2). They

have also been observed, under the action of mechanical, chemical or electrical stimulation, to

make amoeboid movements; though it is doubtful whether these changes are to be interpreted

as vital phenomena, on the

analogy of the contraction of

protoplasm and of muscular

tissue, or whether they are not

rather simply the direct physical

and chemical effects of the stimuli

applied.[5]

These differences between the

two kinds of cell-processes are,

however, as we said above, not

equally well marked in all cases.

In particular, the difference in

length and character of course

may be comparatively slight, or

may even disappear altogether,

the neurite, like the dendrite,

dividing after a brief period into a

large number of delicate

branches. It is also not

uncommon to find cells,

especially cells of small size,

whose processes show no distinct

sign of difference, of whatever sort. The cells [p. 44] with processes of markedly different form

are usually termed, from their discoverer, the cells of DEITERS (Fig. 12); cells with quickly

dividing neurites are known as cells of GOLGI'S type; and the cells without marked distinction

of the processes are called intermediary or intercalary cells.[6]

Finally, the dendrites, like the neurite, evince certain structural differences. Sometimes, as in

the pyramidal cells of the cerebral cortex (Fig. 14) they divide without much complication, their

branches tending in definite directions. Sometimes, again, as in the large PURKINJE cells of

the cerebellar cortex (Fig. 15), their ramifications are exceedingly complex and widely

extended.

(b) -- The Nerve-Fibres

We have seen that the nerve-process issuing from the nerve-cell forms the basis of the nerve-

fibre. The main differences in the structure of the nerve-fibres depend upon differences in the

character of the investing [p. 45] substances, which envelope the original neurite as it proceeds

on its way. The constant constituent of a nerve-fibre, as follows at once from its mode of origin,

is the neurite or axis-cylinder that forms the direct continuation of the nerve-process of a cell.

The neurite is enclosed, first of all in the myelinic sheath, a substance which after death breaks

up by a process of decomposition into bulbous masses; later in its course, it becomes

surrounded by a structureless membrane, supplied at intervals with nuclei, -- the primitive

sheath of SCHWANN (Fig. 17). Most of the central nerve-fibres possess a myelinic sheath, but

no primitive sheath; and within the grey substance the myelinic sheath itself not seldom

disappears. In the sympathetic

system, on the other hand, the

neurite is, as a rule, enveloped

directly by the nucleated primitive

sheath, and lacks the intervening

myelinic substance (Fig. 16, c).

(With few exceptions,) the nerve-

fibres of invertebrates evince this

constitution throughout. Lastly,

the terminal branches of the

nerves in the peripheral end-

organs often break up into

arborisation, consisting simply of

fine neuritic fibrils.

The two innermost of the three

principal constituents of the

nerve-fibre, myelinic sheath and

neurite, possess a composite

structure. If we trace a fibre throughout any considerable portion of its extent, we find that the

myelinic substance does not afford a continuous investment of the neurite. The primitive sheath

undergoes constriction at more or less regularly recurring intervals (nodes of RANVIER), and

the [p. 46] myelinic sheath is thus divided up into cylindrical sections, separated by transverse

partitions (Fig. 17). Since each section carries but a single cell-nucleus, we may suppose that it

represents one of the cells of which the sheath is ultimately composed (Fig, 12). Within this

internodal space (bounded by r r in the Fig.) there is, further, according to some observers,

another double sheath, composed of a substance akin to epithelial tissue, and separating the

neuritic thread from the myelinic sheath (hi).[7] While the myelinic sheath is thus subdivided,

the neurite itself runs uninterruptedly front its point of origin to the conclusion of its course. It is

made up, as was first observed by MAX SCHULTZE, of numerous primitive fibrils, which in

many places, and especially where it issues from the nerve-cell, give it a finely striated

appearance.[8] It is probable that these primitive fibrils pass, in the peripheral nerve

terminations, into the dendritic arborisation into which many nerve-fibres are ultimately

resolved.

Putting all this together, we may infer that the neuritic thread is the constituent of the nerve-

fibre essential to the conduction of nervous processes; that the myelinic sheath discharges not

a nervous but a nutritive function; and that the remaining investments are merely protecting

structures.[9] The inference is borne out by the fact that the formation of the myelinic sheath

follows at a comparatively long interval, in the development of the nervous system, upon the

appearance of the neuritic thread. At the same time, there can be no doubt of its great

importance. The fibres that are to become myelinic give no clear indication of irritability, or of

functional capacity at large, until myelinisation is complete.[10]

The nerve-processes and the nerve-fibres that proceed from them are, then, extremely

important for the connexion of the nerve-cells with the peripheral appendages of the nervous

system, the sense-organs, glands, muscles, etc. But they never mediate a direct connexion

between cell and cell. Wherever such connexion occurs, it appears to be mediated solely by

the contact into which dendrites and collaterals are brought with one another throughout the

grey substance. This view finds support in observations made upon the peripheral terminations

of the nerve-fibres.[p. 47]

The termination of a nerve in the

peripheral organs may take one of two

forms. Either the ends of the neuritic

threads divide up into a fascicle or

network of finest dendritic fibrils, that

terminate freely along the elements of

other, non-nervous tissues; or the neuritic

thread passes directly over into a terminal

cell situated within or between the

organs. The terminal cell may be an

original nerve-cell pushed out towards the

periphery of the body; or it may have

acquired this character later on in the

course of development, by the

penetration of a nerve fibril into an

epithelial cell. The two forms of nerve-

termination occur side by side, in these

their characteristic differences, in the

different sense-organs, where they are

evidently connected with essential

differences in the mode of sensory

excitation. The first form shows most

plainly in the terminations of sensory

nerves in the skin. The neurite, as soon as it enters the lowermost epithelial layer of the cutis,

breaks up, into a reticulum of delicate fibrils, whose dendritic branches surround the separate

epithelial cells (Fig. 18, A). In some cases, it is true, this arrangement is so modified as to

approximate more or less closely to the second form: there are cutaneous nerve-fibres whose

fibrils penetrate the cells of the epidermis, or pass into or between the cells of the deeper lying

connective tissue, and thus transform these originally non-nervous elements into peculiar

sense-organs (touch-cells, end bulbs, touch-corpuscles, etc.). The nerve-terminations in the

organ of hearing also follow, in the main, this cutaneous type.

The second form of nerve-termination is best

illustrated from the organ of smell. Every olfactory

nerve-fibre enters, in the olfactory mucous

membrane, into a nerve-cell. This cell which lies

between epithelial cells, [p. 48] is drawn out at its

opposite pole, i.e. at the end turned towards the

free sensory surface, into a thread-like continuation

(Fig. 18, B). The nerve-terminations in the tongue

and in the retina of the eye follow this second type.

In both organs, the terminal fibrils are connected

with sensory cells. In their case, however, the

sensory cells (taste-cells, retinal rods and cones)

appear to be not true nerve-cells, but epithelial

cells, which have been transformed into sense-

cells by their connexion with nerve-fibres.[11]

The nerve-endings in muscle conform in all essentials to the first of these types. Here too we

observe, in the first place, a more or less elaborate division of the nerve-fibres that run to the

separate elements of the muscular tissue.[12] In the muscles of reptiles, birds and mammals,

the terminal fibrils finally branch out in a peculiar flattened prominence, the end-plate. Most

observers place this structure within the transparent elastic sheath of the muscle-fibre, the

sarcolemma, though some describe it as attached to the outer surface (Fig. 19).[13]

(d) -- The Neurone Theory

The facts which we have now passed in review as regards the nerve-cells, their processes, and

the continuations of these processes into the peripheral organs appended to the nervous

system, have led in recent years to the hypothesis that the conduction of nervous processes is

mediated, in many cases, not -- as was formerly supposed -- by an unbroken continuity of the

fibrillar elements, but rather by contact between the arborisations of the fibres of different

nerve-cells. This hypothesis, it is needless to say, ascribes a greatly added importance to the

nerve-cell. According to it, the functions of the nervous system are conditioned upon the

spheres of function of the individual cells, -- the 'cell' in this sense including as an essential

constituent the fibrillar elements issuing from the cell-body. We may therefore regard the nerve-

cell together with its processes as the morphological, and presumably also as the functional

unit, to which we [p. 49] are in the last resort referred for an understanding of the entire nervous

system. This unit of nerve-cell with its dependent territory of fibrillar processes and

arborisations, has been designated, on WALDEYER'S suggestion, a neurone. In the light of the

neurone theory, the whole of the central nervous system, reaching with its appended organs to

the extreme periphery of the body, appears as a system of such units, set side by side or

arranged in ascending series: each unit maintaining a relative independence, from the

unbroken continuity of its parts, and each connected with other similar units only contact-wise,

by way of the terminal arborisations of the fibrils of the individual neurones.[14] Figg. 20 and 21

illustrate this conception, schematically, for two trains of neurones, a motor and a sensory,

which may be taken as typical of the systems of conduction realised in the nervous system at

large. The hypothetical scheme of the motor neurone train, given in Fig. 20, consists of two

neurones, the one of which (NI), as motor cell (ZI) in the ventral cornu of the myel, is attached

directly to a peripheral muscle-fibre (M), while the second (NII) belongs to a higher nervous

centre. The neurite proceeding from the cell ZII gives off a certain number of collaterals, and

finally resolves into fibrils that come into contact with the dendrites of the cell ZI. This cell in turn

sends out a neurite, whose ramose fibrillar termination stands in contact with the motor end-

plate of a cross-striated muscle-fibre. The hypothetical schema of the sensory neurone train, in

Fig. 21, also shows two neurones: a peripheral, NI, that has its centre in a bipolar spinal-

ganglion cell ZI, and a central neurone, NII, that belongs to a nerve-cell, ZII, lying somewhere in

the higher regions of myel or brain. The neurone NI is connected by contact on the one side,

through the terminal arborisation of its longer, peripherally directed fibre, with the cutaneous

region H (cf. p. 47, Fig. 18 A), and on the other, through the dendrites of its second, upward

trending process, with the neurone NII. These bimembral chains are, naturally, to be

considered only as the very simplest schemata of neurone connexion. We must suppose in

general that several neurones, now all lying at the same level and now arranged in ascending

order, are united in the nervous centres to form neurone chains Where nerve-cells have been

forced outward, as 'sensory cells,' into the peripheral organs, it is possible that there, too,

similar arrangements may prevail. Indeed, as we shall see later on, the morphological

conditions often point unequivocally to such peripheral neurone connexions (cf. below, Chs. V.,

VIII.).

Whether the individual cell territories are, always and everywhere, related to one another in the

manner indicated by these diagrams is, we must [p. 50] admit, still an open question. So far,

the neurone theory must be regarded simply as an hypothesis that brings together, in a very

happy way, a large number of the data of current histology. Whether the definition of the

neurone in general, and whether in particular the views of the interconnexion of the neurones

promulgated especially by RAMON Y CAJAL, will prove to be tenable in all cases, cannot now

be decided. Even at the present day, the theory does not want for opponents. Fortunately, the

settlement of these controversies among the morphologists is not of decisive importance for a

physiological understanding of nervous functions. Physiological interpretation must be based,

first of all, upon the manifestations of

function, and these can be brought, later

on, into relation to the anatomical facts.

The opposite plan, of erecting elaborate

physiological -- not to say psychological --

hypotheses upon purely anatomical

foundations, is, of course, to be rejected

without further argument. From this point

of view, however, it must be conceded

that the idea of neurones, and the view

that this idea [p. 51] suggests of a

connexion between the central elements

which is relatively variable, and in certain

circumstances perhaps determinable by

the exercise of the functions themselves,

accords better with the facts than the

older view of an uninterrupted continuity

of the nerve-fibres, and its dogmatic

corollary of isolated conduction, were

able to do. We need appeal only to the

observations on the possibility of

vicarious functioning, and on the

substitution of new conduction-paths for

others that have for some reason become

impracticable. The anatomical plan of

neurone connexions is evidently more adequate than this older view to the physiological results

which prove that there exists, along with a certain localisation of functions, a very considerable

capacity for adaptation to changed conditions. More than this, -- more than an ex post facto

representation of the course of events, -- the neurone theory, naturally, cannot give us. Should

that theory fall, the acts of vicarious function and of new adaptation would still all remain as

they were, and would still have to be brought somehow into agreement with the properties of

the anatomical substrate of the functions involved.

The morphological differences between the processes of the nerve-cells, that have formed the

point of departure for the development of the neurone theory, were first pointed out by

DEITERS, in his work upon the large cells of the ventral cornua of the myel. GERLACH

discovered the fibrillar structure of the intercellular substance, and HIS the embryological

connexion of nerve-fibres with nerve-cells. GOLGI, KÖLLIKER, NANSEN, W. HIS, G.

RETZIUS, RAMON Y CAJAL and many others have made the nerve-cell a subject of special

investigation.[15]

It is but natural that the results obtained should not be always in agreement. GOLGI and

NANSEN supposed that the dendrites are merely nutritive elements; and GOLGI held, further,

that the interlacing fibres of the ground-reticulum anastomose to form a closed system. The

other observers declared for the nervous character of the dendrites, and were unable to confirm

the occurrence of anastomosis in the ground-reticulum. On the side of function, GOLGI

propounded the hypothesis that the neurites pass exclusively into motor nerve-fibres, while the

sensory nerves take their origin from the ground-reticulum. It would follow from this, since

GOLGI did not recognise the nervous nature ot the dendrites, that the connexion between

sensory and motor fibres is mediated not by any sort of nerve-cell, but only by the fibrillar

substance of the ground-reticulum, and there, in all probability, by mere mechanical contact of

the fibres. If on the other hand we admit, as the great majority of observers are now ready to

do, that the dendrites are nervous in character, then we must suppose, as has been shown in

particular by RAMON Y CAJAL, that while all centripetally conducting nerve-fibres first of all

arborise into fibrils in the ground-reticulum, they afterwards avail themselves of the

protoplasmic processes to discharge into nerve-cells. If this hypothesis be sound, the terms

'centripetal' and 'centrifugal' cannot be regarded as identical with 'sensory' and 'motor'; they are

referable, in every case, only to the cells with which the fibres are con-[p. 52]nected.

Centripetal, in this sense, are all conduction-paths that convey excitations to determinate

nerve-cells; centrifugal, all conduction-paths that carry excitations from them. In general,

therefore, the peripheral sensory nerves will belong to a centripetal, and the motor nerves to a

centrifugal system. But within the central conduction-paths, i.e. those that run between different

ganglionic systems, there may be fibres, centrifugal in respect of proximate cell-origin, that

possibly possess a sensory character, and others, centripetal in origin, whose functions may

possibly be motor. This view of the functions of the cell-processes evidently carries with it a

relative independence of the territories of the individual nerve-cells, -- a phase of the subject to

which WALDEYER especially has called attention, and which has led him to introduce the idea

of the neurone. Most recent investigators adopt the neurone theory. At the same time, there

has alas been a certain amount of dissent, based especially upon the oft repeated observation

of the continuity of the fibrils within the nerve-cells.[16] It has even been maintained that the

fibrils pursue an unbroken course throughout the entire nervous system, the nerve-cells

included: an hypothesis first put forward by MAX SCHULTZE, the discoverer of fibrillar cell-

structure,[17] and now revived on the ground of further work upon the same morphological

phenomena.[18]

The structural schema, of RAMON Y CAJAL, and the neurone theory that is based upon it,

stand in the forefront of recent neurological investigation. Anatomists have also devoted much

attention to the finer structure of the nerve-cell itself. There have been two remarkable

discoveries in this field, that have aroused especial interest: NISSL'S announcement of the

tussock-like accumulations of granules (Figg. 10, 14),[19] and the observations made in many

quarters on the fibrillar structure of the nerve-cells.[20] Neither of these, it is true, has passed

unchallenged; both the granular masses and the fibrils have been explained as precipitates

from the cell-substance, due to microchemical treatment or to post-mortem coagulation.[21]

Nevertheless, the hypothesis that these structures exist in the living tissue is confirmed br the

fact that they have been observed in fresh preparations, untreated by staining reagents (Fig.

10).[22]

NISSL'S corpuscles have further been observed to undergo noteworthy changes under the

action of poisons, like arsenic, or as the effect of intense fatigue or other trophic disturbances.

The tussocks decrease, both in size and in number, so that in many cases they can still be

observed only at certain parts of the cell-body, while the nucleus becomes farther and farther

dis-[p. 53]placed towards the cell-periphery, and finally disappears altogether. These changes

correspond exactly to those observed in inflammatory conditions of the grey substance in the

human brain, and termed homogeneous turgescence of the cells (Fig. 22 A). They suggest the

idea that the tussocks discharge a specific function, intimately related to cell-nutrition. These

structures are, perhaps, to be explained as accumulations of reserve material, to be drawn

upon for functional purposes if this be true, we must probably attribute to them the trophic

influence which the nerve-cell exercises upon the fibres proceedings from it, and which

apparently makes the cell their nutritive as well as their functional centre.[23] This influence is

shown by the fact that those fibres of a transsected nerve which remain connected with the

central organ persist for a long time without change, whereas the fibres of the peripheral portion

of the nerve, the part that is separated from the centre, very soon show signs of degeneration.

First of all, the myelinic contents of the fibre divides into clots (Fig. 23 a). Then, these clots,

together with the neuritic fibrils,

break up into granules (b). These

in turn are slowly resorbed (c)

until they altogether disappear: so

that, finally, nothing is left of the

nerve but its connective tissue

investments.[24] It is, however,

probable that the appearance of

these degenerative processes is

further hastened by the arrest of

function which naturally follows

from the sectioning of the nerve.

This view is confirmed, on the one

hand, by the fact that, after a very

long time, the central end of the

transsected nerve also becomes atrophied, and on the other by the observation that nerve-

cells, which have been thrown out of function by sectioning of a nerve-trunk or by injury to the

peripheral region supplied by them, gradually shrink up (Fig. 22 B). In the case of young

animals especially, this cell-shrinkage sets in comparatively quickly, after extirpation of the

region of nervous diffusion. It has also been observed in man, as a secondary atrophy of the

nerve-centres.[25][p. 54]

Such are the phenomena that occur as after-effects of enhancement or abolition of function in

the nerve-cells and nerve-fibres. The changes observed as the results of stimulation in the

dendritic processes, and interpreted by many observers as immediate manifestations of life, are

of a very much more questionable nature. Amoeboid movements of the dendrites were first

described by RABL-RÜCKHARD. They may possibly be explained as phenomena of imbibition

and coagulation. At any rate, the psychophysical theories of sleep and waking, dissociation of

consciousness, and what not, that certain authors have created upon them, are purely

imaginary psychological constructions, based on an extremely scant and more than doubtful

foundation of physiological observation.[26]

§2. Chemical Constituents

The chemical substances of which the morphological elements of the nervous system are

composed are as yet but imperfectly known. The greater portion of the investing and

sustentacular tissues -- the endoneurium and perineurium, the primitive sheath, and in part the

neuroglia of the nerve-centres -- belong to the class of collogenic and elastic substances. The

only exception is the corneal sheath surrounding the myelin, which is said to consist of a

corneal substance allied to epithelial tissue, and termed neurokeratin.[27] The nerve-mass

proper is a mixture of various substances, several of which resemble the fats in their

solubilities, while they differ widely in chemical constitution. They have been found, not only in

nerve-substance, but also in the corpuscles of blood and lymph, in egg-yolk, in sperma, and to

a less degree in many other tissues. The most important of them is protagon, a highly complex

body, to which LIEBREICH has assigned the empirical formula C116H241N4PO22. This

formula is, naturally, intended merely to give an approximate idea of the extreme complexity of

the chemical molecule of this compound.[28] From protagon are derived lecithin and cerebrin,

decomposition-products which probably occur along side of it in the nerve-substance, and

together with it form the myelin of the myelinic envelope. Lecithin, it is supposed, is not a single

body of stable constitution, but consists of a series of compounds that resemble the compound

ethers: substances which in physical and chemical con-[p. 55]stitution are closely allied to the

fats, and in which the radicals of certain fatty acids, of phosphoric acid and of glycerin (a

component of most of the animal fats) are combined with one another and with a strong amine

base, cholin.[29] Lecithin has two characteristic properties. The large proportion of carbon and

hydrogen which it contains gives it a high heat of combustion; and its complex nature renders it

easily decomposable. Cerebrin, if boiled with acids, yields a sugar and other, unknown,

decomposition products, and has accordingly been referred to the nitrogenous glucosides. Like

lecithin, it is in all probability not a single body, but a mixture of several substances, which have

been distinguished as cerebrin, homocerebrin and encephalin.[30] Lastly, cholesterin, a solid

alcohol rich in carbon, which occurs in almost all the tissues and fluids of the body, plays a not

inconsiderable part in the composition of nervous tissue. Besides these substances, which are

all characterised by their high heat of combustion. Nervous tissue contains substances which

are classed with the proteins, but of whose composition and chemical conduct very little is

understood. Finally. it must be mentioned, as a characteristic difference between the grey

substance of the nerve-centres and the white myelinic substance, that the former gives a

weakly acid, the latter an alkaline or neutral reaction. The acid reaction appears, like that of the

muscles, to be due to the presence of free lactic acid.[31] Some observers have, in fact,

maintained that this free acid increases, as a result of activity, just as it does in muscle.[32]

Apart from these differences of reaction, little is known of the distribution of the various

constituents in the various elementary divisions of nervous tissue. Only so much is certain, that

in the peripheral nerve-fibres the neurite has all the general characteristics of a proteid, while

the myelinic sheath evinces those of the myelins. In the ganglion cells, too, the nucleus would

seem, from its microchemical conduct, to consist of a complex albumin-like substance, while in

the protoplasm there is a mixture of albuminoid materials with protagon and its associates. The

same constituents appear, further, to penetrate in part into the intercellular reticulum.

These facts render it probable that nervous substance is the seat of a chemical synthesis,

whereby the complex nutritive substances carried by the blood are ultimately transformed into

compounds of still greater com-[p. 56]plexity, representing (as their high heats of combustion

show) a very considerable amount of potential energy. This view of the chemism of nerve-

substance is attested, first of all, by the appearance of protagon and the lecithins in such

quantity that their production in situ is evidently far more probable than their deposition by the

blood. The parent substances of protagon itself and of the bodies associated with it are to be

sought, we must suppose, in the albumin-like substances of ganglion cell and neurite. There

can, for that matter, be no doubt that the elementary structures of the animal body have the

power of converting simpler proteids into more complex. Apart from the undisputed observation

of synthetic processes within the body,[33] we have further evidence in the fact that substances

containing phosphorus, which closely resemble the albuminates in their composition and

chemical conduct, appear under conditions that definitely suggest their formation within the

organic cell. A compound of this kind, nuclein, appears in particular to form the principal

constituent of the cell-nuclei.[34] Hence we may say, tentatively, that the most important

physiological result of the attempts so far made to penetrate the chemical constitution of the

constituents of the nervous system is this and this only: that the chemism of nerve-substance is

very particularly directed upon the formation of compounds possessing a higher heat of

combustion or a larger store of potential energy, At the same time, the differences in the

properties of the grey and white substance, scanty as they are, point to the conclusion that the

central elements are the principal seat of the chemical processes which mediate the functions

of the nervous system. These results, then, are practically all that we need bear in mind, as the

outcome of chemical investigation of nervous substance up to the present time, when we

approach the problems of the physiological mechanics of the nervous system.

Notes

[1] Connective tissue forms a part of the sustentacular tissue of the nervous system. But the

neuroglia, which forms its larger part, is an ectodermic structure, with close relations to the

neurogenetic tract. See G. A. PIERSOL, Normal Histology, 1893, 79. -- TRANSLATOR.

[2] The text has 'neurilemma' in both instances. This is now a synonym for the primitive sheath.

-- TRANSLATOR.

[3] DEITERS, Untersuchungen über Gehirn u. Rückenmark des Menschen u. der Säugethiere,

1865, 53 f. HIS, Arch. f. Anatomie, Supplementband, 1890, 95 ff. VON LENHOSSEK, Der

feinere Bau des Nervensystems, 2te Aufl. 1895, 36 ff. HELD, Arch. f. Anatomie, 1897, 204;

Suppl. 273. BETHE, Arch. f. mikroskip. Anatomie, 1900, lv., 513. GOLGI, Verhandl. d. anatom.

Gesellschaft auf d. 14 Vers. zu Pavia, 1900; Anat. Anzeiger, xviii., Ergänzungsheft.

[4] RAMON Y CAJAL, Les nouvelles idées sur la structure de systéme nerveux ches l'homme

et chez les vertébrés, 1894.

[5] RABL-RÜCKHARD, Neurol. Centralblatt, 1890, 199. DUVAL, Soc. de Biologie, 1895. Cf.

KÖLLIKER, Verh. d. Würzburger phys-med. Gesellschaft, 1895.

[6] These intermediate cells (intermediäre oder Schaltzellen) appear to correspond to what are

sometimes termed the GOLGI cells of the first type, and the GOLGI cells of the text to the

GOLGI cells of the second type. -- TRANSLATOR.

[7] EWALD and KÜHNE, Verhandl. d. naturhist.-med. Vereins zu Heidelberg, N. F. i. 5. The

presence of this intermediate membrane in the living nerve-fibre is denied by T. W.

ENGELMANN, in PFLÜGER'S Arch. f. d. ges. Physiol., xxii., 1880, 1 ff.; KÖLLIKER,

Gewebelehre, 6te Aufl., ii., 13.

[8] MAX SCHULTZE, in STRICKER'S Gewebelehre, 1871, 108. POWER'S trs., i., 1870, 150.

[9] PIERSOL, Normal Histology. 63 f. -- TRANSLATOR.

[10] See below, Ch. v. § 2.

[11] For a detailed account of the central and peripheral terminations of the sensory nerves,

see Chs. v. vii., below.

[12] PIERSOL, Normal Hisology, 90. -- TRANSLATOR.

[13] KÜHNE, in STRICKER'S Gewebelehre, ii., 1871, 682. POWER'S trs., i., 1870, 210.

SZYMONOWICZ, Lehrbuch d. Histologie, 1901, 306. MACCALLUM'S trs., 1902, 312 ff.

[14] WALDEYER Ueber einige neuere Forschungen im Gebiet der Anatomie des

Centralnervensystems, in the Deutsche med. Wochenschrift, 1891, nos. 44-48. For the history

of the theory, see M. von LENHOSSEK, Der feinere Bau des Nervensystems, 2te Aufl., 1895,

103 ff.; M. VERWORN, Das Neuron in Anatomie und Physiologie, 1900.

[15] Cf. the bibliographies in M. VON LENHOSSEK, op. cit. 36 ff.; KÖLLIKER, Gewebelehre,

6te Aufl. ii. 5 ff.

[16] NISSL, Kritische Fragen der Nervenzellenanatomie, in the Biol. Centralblatt, 1896, 1898.

HELD, Arch. f. Anatomie, 1897, 204; Suppl., 273. BETHE, Biol. Centralblatt, 1898, 10. 18.

These authors believe, in general that the neurone theory affords an adequate idea of the

earlier stages in the development of the nervous system; but that, at a later period, the

processes of the individual cells oftentimes grow together, so that the original independence of

the cell territories is not maintained.

[17] MAX SCHULTZE in STRICKER'S Gewebelehre, 1871, 108 ff. POWER'S trs., i. 172.

[18] APATHY, Biol. Centralblatt, 1889 and 1898 (vols, ix. and xviii.). Mittheilungen aus der Zool.

Station zu Neapel, xii. 1897; also in Amer. Journ. of Insanity, lv., 1898, 51 ff.

[19] NISSL, Allg. Zeitschr. f. Psychiatrie, l., 1894.

[20] FLEMING, Arch. f. Mikrosk. Anatomie, xlvi., 1895, 373. LENHOSSEK, ibid., 345.

MÖNCKEBERG and BETHE, ibid., liv., 1899, 135. <{>[21] HELD, op. cit. Sometimes, as was

discovered by BÜTSCHLI (Untersuchungen über mikroskopische Schäume und Protoplasma,

1892) and confirmed by HELD, a honey-combed appearance is presented both by the cell itself

and by its nerve-process. HELD, however, regards this too as a result of coagulation.

[22] J. ARNOLD, Arch. f. mikrosk. Anatomie, lii., 1898, 542.

[23] NISSL, Allg. Zeitschr. f. Psychiatrie, xlviii., 1892. MARINESCO, Arch. f. Physiol., 1899, 89.

VON WENDT, Skandin. Arch. f. Physio., xi., 1902, 372. M. FRIEDMANN, Neurolog.

Centralblatt, 1891, 1.

[24] MÖNCKEBERG and BETHE, Arch. f. Mikrosk. Anatomie, liv., 1899, 135.

[25] GUDDEN, Arch. f. Psychiatrie, ii., 693.

[26] DUVAL, Hypothése sur la physiol. des centres nerveux, in the Comptes rendus de la

société de biologie, 1895. SOUKHANOFF, La théorie des neurones, etc., in the Arch. de

neurologie, 1897. QUERTON, Le sommeil hibernal et les modifications des neurones, Institut

Solvay, Bruxelles, 1898.

[27] W. KÜHNE and CHITTENDEN, Zeitschr. f. Biologie, N. F. viii., 1890, 291.

[28] LIEBREICH, Ann. der Chemie u. Pharmacie, cxxxiv., 1865, 29. According to KESSEL and

FREYTAG (Zeitschr. f. physiol. Chemie, xvii., 1893, 431), protagon further contains sulphur in

its molecule. The views of these chemists, with the protagon theory at large, are sharply

controverted by J. L. W. THUDICHUM (Die chemische Constitution des Gehirns des Menschen

und der Thiere, 1901, 44 ff.). We cannot enter here into these differences of opinion. We can

pass them over with the less scruple since they are, at present, without significance for the

general relations of the chemism of nerve-substance to the physiological processes.

[29] The constitution of ordinary lecithin, according to DIAKONOW, is C44H90NPO9=distearyl-

glycerin-phosphoric acid + trimethyl-oxethyl-ammonium-hydroxide. According to STRECKER,

other lecithins may be formed in which the radical of stearic acid is replaced by some other fatty

acid radical. See NEUMEISTER, Lehrbuch der physiol. Chemie, 2te Aufl., 1897, 91 ff.

[30] W. MÜLLER (Ann. d. Chem. u. Pharm. cv., 1858, 361) has worked oat for cerebrin the

empirical formula, C37H33NO3. On the cerebrin series, cf. PARCUS, Journ. f. prakt. Chemie,

1881, 310; NEUMEISTER, Physiol. Chemie, 2te Aufl. 472.

[31] GSCHEIDLEN, in PFLÜGER'S Arch. f. d. ges. Physiol., viii., 1874, 71.

[32] MOLESCHOTT and BATTESTINI, Arch. de biologie ital., viii., 1887, 90.

[33] E. BAUMANN, Die synthetischen Processe im Thierkörper. Inaugural lecture. Berlin, 1878.

[34] MIESCHER, in HOPPE-SEYLER'S Physiologisch-chemische Untersuchungen, 4, 452;

LUBAVIN, ibid., 463.

CHAPTER III

Physiological Mechanics of Nerve-Substance

§1. General Principles and Problems of a Mechanics of Innervation

(a) -- Methods of a Mechanics of Innervation

THE processes that run their course within the elements of the nervous system, the nerve-cells

and nerve-fibres described above, have been studied in two different ways. By the one of

these, investigators have sought to gain a knowledge of the internal by the other of the external

molecular mechanics of nervous substance. The former sets out from an examination of the

physical and chemical properties of the nervous elements, and inquires into the changes which

these properties evince as a result of physiological function, attempting in this manner to

discover the internal forces at work in the nerves and nerve-centres. Inviting as this path may

appear, in its promise directly to reveal the intimate nature of the nervous functions, it still takes

us so short a distance towards its goal that we cannot venture to trust ourselves upon it. Apart

from the scanty results of morphological investigation, mentioned above (p. 53), the study of

the functional changes of the central elements is, as yet, hardly more than a programme. And

our knowledge of the internal processes in the peripheral nerves is also severely limited. We

know that their functioning is attended by electrical and chemical changes, -- the meaning of

which is still obscure: we know little more. The only road that remains open to us, therefore, is

the second, that of an external molecular mechanics. In taking this, we avoid altogether the

question of the special nature of the nervous forces: we set out simply from the proposition that

the processes in the elementary divisions of the nervous system are movement-processes, of

some sort or other, and that their relations to one another and to the forces of external nature

are determined by the mechanical principles valid for motion at large. We thus take up a

position akin, let us say, to that of the general theory of heat in modern physics, where the

investigator is satisfied to begin with the proposition that heat is a mode of motion, from which --

with the aid of the laws of mechanics -- he derives all the phenomena with systematic

completeness. If the molecular mechanics of the nervous system [p. 58] is to accomplish a like

result, it must first of all reduce the phenomena that form the subject-matter of its inquiries to

their lowest terms: it must investigate the physiological function of the nervous elements, first,

under the simplest possible conditions, and, secondly, under conditions that can be

experimentally varied and controlled. Now any outside affection of the nervous elements, that

serves in some way to arouse or modify their functions, is termed in physiology a stimulus. In

using this term, we must, of course, abstract entirely from the ideas which HALLER'S theory of

irritability and other modes of thought current in the older vitalistic physiology read into it. If we

do this, the term retains its usefulness not only in our modern physiology of the nervous system

and its auxiliary organs, but also by extension of meaning in psychology, seeing that all the

multiplicity of outside affections that are embraced by it depend primarily upon a peculiar

character of living substance itself, and may therefore produce identical results.

Stimuli are classified, in terms of the source from which their activity proceeds, as internal and

external. Under internal stimuli are included all stimulatory influences that have their seat in the

tissues and organs surrounding the nervous elements: we may instance, especially, rapid

changes in the quality of the blood and of the fluids of the tissues. Under external stimuli are

included, on the other hand, all the physical and chemical influences exerted upon the

organism by the external world in which it lives. As regards nerve-substance, therefore, all

stimuli whatsoever are to be classed as external. Whether, for instance, a chemical stimulus

arises primarily in the blood in which the nerve-elements are bathed, or makes its way to them

from the environment, is indifferent for the intrinsic character of the process. When, however,

we desire to apply to nervous substance stimuli of a predetermined intensity and duration, we

find, as a rule, that the internal stimuli (in the technical sense) are not available, since they are

almost entirely beyond the range of experimental control. We accordingly have recourse to

external stimuli, and most frequently to electric shocks and currents, which recommend

themselves particularly both by the ease with which they destroy the molecular equilibrium of

the nerve-elements, and by the extreme accuracy with which their mode of application may be

regulated. In attempting an analysis of the processes in the nerve-fibres, we then begin with

that peripheral effect of nervous excitation which is most open to investigation, -- the muscle

contraction that follows upon stimulation of the motor nerves, -- and make this our measure of

the internal processes. Similarly, for an understanding of the changes in the nerve-cells, we

employ the simplest process, amenable to external measurement, that is released in the central

organ by the stimulation of a centrally directed nerve-fibre, -- the reflex contraction. In neither of

these cases, [p. 59] however, does the muscle-contraction afford a direct measure of the

processes that run their course in the corresponding nerve-fibres and at their points of central

origin, or of the changes induced in these processes by any determinate outside influence; of

itself, it can never furnish more than a certain measure of the processes operative in the

substance of the muscle which contracts. As a rule, therefore, every change in the irritability of

the nervous elements, to which we have applied artificial stimulation, may be expected to

produce a change in the phenomena exhibited by the muscle: thus, if the irritability of the motor

nerves is diminished, the muscular contraction will be weaker; if enhanced, it will be stronger.

But we shall not be justified in arguing, conversely, that every change in contraction implies a

corresponding change in nervous excitability. On the contrary, since the contractile substance

has its own intrinsic irritability, which it maintains in face of stimulation whether directly applied

or transmitted to it by the motor nerves, very different stimuli may possibly act upon the nerve,

or upon the central structures connected with it, to release precisely the same processes in the

nervous substance itself, and nevertheless, if the irritability of the contractile substance has

changed in the meantime, may produce quite different effects in muscle: or conversely, may set

up different processes in the nervous substance, while the contractile substance shows the

same reaction. We must, therefore, never lose sight of the fact that the muscular contraction

furnishes only an indirect measure of the processes of nervous excitation. If we are to argue

immediately from the symptoms of altered contractility to the nervous processes, we must be

sure that the observations are made under conditions which guarantee a sufficient constancy in

the properties of the muscle experimented upon, or at least make such constancy highly

probable. For the rest, the properties of the contractile substance itself, and the related

phenomena of the course of the muscular contraction, may here be left out of consideration, as

their interest is purely physiological. In no case are we concerned with the muscular contraction

save as the changes which it undergoes possess a symptomatic importance for the nervous

processes with which they are connected.[1]

It is the task of a physiological mechanics of the nervous substance to reduce the phenomena

of nerve-stimulation, so far as they can be traced in the related mechanical phenomena

evinced by muscular tissue, to the universal laws of mechanics. In essaying this problem, it

must at the outset bring its subject-matter into relation with one, especially, of the great laws of

mechanics, -- a law which has proved pre-eminently serviceable [p. 60] in explaining the

interrelations of various forms of movement-process. This is the law of the conservation of

work.

(b) -- The Principle of the Conservation of Work

We understand by work, in the most general meaning of the term, any operation that changes

the position of ponderable masses in space. The amount of work done, in a given case, is

accordingly measured by the change of position which it can produce in a weight of determinate

magnitude. Ponderable bodies can be moved from their place by light, heat, electricity,

magnetism. But all these 'natural forces,' as they are called, are simply forms of molecular

motion. It follows, then, that the different modes of molecular motion can do work. The heat of

steam, e.g., consists in movements for the most part rectilinear, but oftentimes interferential of

the steam particles. As soon as the steam does work, -- let us say, by moving the piston of an

engine, -- a corresponding quantum of these movements disappears. This result is commonly

expressed in the phrase, 'A certain quantity of heat has been transformed into an equivalent

quantity of mechanical work.' It would be more accurate to say that a part of the irregular

movements of the steam-particles has been used up, in order to set a larger ponderable mass

in motion. We have, then, merely the transformation of the one form of motion into the other;

and the work done, measured by the product of the moved weight into the distance through

which it is moved, is exactly equal to a sum of lesser amounts of work, which could be

measured by the products of the weights of a number of steam-particles into the distances

traversed by them, and which now, during the performance of the external work, have

disappeared. Conversely, when mechanical work disappears and heat arises in its place, by the

friction or compression of physical bodies, we have the opposite transformation of mechanical

work into its equivalent amount of molecular work. Not that mechanical work (in the ordinary

sense of the term) appears in all cases where heat is latent: the heat is, very commonly,

employed simply for the transposition of the particles of the heated body itself. It is a familiar

fact that all bodies -- gases most of all liquids and solid bodies in less degree -- expand under

the influence of heat. Here, again, molecular work disappears. Just as it is used in the steam-

engine to move the piston, so it is used in this case to alter the distance that separates the

molecules.

__________________________________________________________________________

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[p. 104] CHAPTER IV

Morphological Development of the Central Organs

§1. General Survey

(a) -- Object of the Following Exposition

In the preceding chapter we have attempted to analyse, in their elementary phenomena, the

vital processes conditioned upon the constitution of the nervous substance. Now in every

organism which stands high enough in the scale of organic life to possess a nervous system at

all, the elementary parts connect to form complicated structures, or organs; and the processes

which we have been studying manifest themselves, accordingly, in co-ordinated activities of

greater or less complexity. As a rule, it is far from easy to refer these complex phenomena to

the relatively simple conditions laid down by a mechanics of nerve-substance that has been

worked out from individual, isolated structures. We must he content to do this in the rough, and

as a matter of general direction. At the same time, the more complicated and difficult the

problem, the more strictly necessary is it, if our analysis of the complex physiological functions

is not to go wrong from the very beginning, to keep constantly in view the general principles

which our study of the simple nerve processes has brought to light. In considering, more

especially, those developmental forms of the nervous system which are of chief importance for

psychology, and which will therefore form the main subject of the following Chapters, -- the

nerve-centres of the higher vertebrates and of man, -- we must remember that the elementary

nerve-forces are still at work, though the manifold connexions of the elementary parts place

their effects under conditions of almost inconceivable complexity. The properties of compound

organs can be understood only in so far as we are able to refer them, at any rate as regards the

general point of view from which they are appraised, to the properties of the elementary

structures. The more firmly the physiology of the nervous system holds to this principle, which

is surely beyond the need of argument, the sooner will it be competent to render service to

psychology. On the other hand, neglect of the rule, combined with the adoption of a haphazard

popular psychology, as manufactured on occasion by anatomists and physiologists for their

own [p. 105] private use, has wrought havoc from the days of GALL and phrenology down to

the present time. Great advances have been made in our knowledge of the morphology of the

nervous system and of its complex physiological functions; GALL himself deserves credit for his

investigations in anatomy. But they have failed to bear fruit for an understanding of the relations

of the nervous system to the processes of the psychical life. Nay, more, under the conditions

just mentioned, they have in this regard oftentimes done more to confuse than to further

knowledge.

Now an inquiry concerning the bodily substrate of the mental life evidently presupposes, first of

all and before it turns to the properly physiological aspect of its problem, an adequate

knowledge of the morphology of the organs. This is not a merely logical requirement. Apart

from the experiments on nervous elements, isolated so far as possible from their connexions,

which fall within the sphere of the general mechanics of nerve-substance, both the

physiological experiment and the pathological observation which serves to supplement it in

various directions are conditioned upon this anatomical knowledge. When we remember the

immense complexity of the structures involved, we must admit, at the same time, that as

instruments of analysis they are comparatively crude: a limitation that should be steadily borne

in mind in any estimate of the value of such experiments and observations. The first part of the

following discussion will, then, be devoted to a general sketch of the morphology of the central

nervous system. This will, of course, contain nothing that is new to the anatomist and

physiologist, to whom the subject is familiar. On the contrary, it will often fail to make mention of

special points that, for the time being, possess only an anatomical interest. The primary

purpose of the exposition is to furnish a brief account of the structure and function of the

nervous system that shall appeal especially to the psychologist and shall take account of the

things that interest him. It has, however, over and above this, the secondary purpose of

showing the anatomist and physiologist themselves how the familiar facts of structure and

function appear when viewed, for the nonce, from the standpoint of psychology. It is true that

this standpoint is not altogether neglected in the anatomical and physiological text-books. On

the contrary, one cannot but admire the courage with which the anatomists and physiologists,

when occasion arises, invade a difficult and (for so it really is to them) an unknown country. But

the psychologist is, all the more for this very reason, in duty bound to discuss, from the point of

view of a scientific psychology, the results obtained from the observation of microscopical

structures, or of animals whose mind has in some way been impaired by extirpation of

particular portions of the brain. He is bound to pass the facts themselves in critical review, and

to estimate their psychological [p. 106] importance, quite apart from the more or less accidental

and arbitrary reflections that are usually appended to the observational data. Now it would

obviously be out of place, for the purposes of a survey specially intended, as this is, to

subserve psychological requirements, to give any such detailed account of the topography of

brain structure as is necessary for anatomy and pathology. We must rather lay the chief

emphasis upon the morphological complexes, upon the organs as such; more particularly

where structural connexions point us to co-ordination of functions. And this need seems to be

most adequately met by a genetic consideration, which seeks to explain the complex conditions

found in the fully developed organs from their origination in simpler forms, whether these stand

lower in the scale of organic life or represent earlier stages of individual development. We shall,

therefore, in this opening Chapter, say what is necessary for our present purpose of the general

differentiation of the substrate of psychical functions in the animal kingdom. That done,

however, we may thenceforth confine ourselves to a detailed consideration of the

morphological development of the central organs in the vertebrates. Here, too, we shall make

use of the lower forms of development chiefly to prepare the way for an understanding of the

structural plan of the human brain.

(b) -- The Neural Tube and the Three Main Divisions of the Brain

The first step in the evolution of the central nervous system of vertebrates is taken, as we have

seen, with that primitive differentiation of the germ where a dark streak of tissue marks the

place of the myel, and therefore the longitudinal body-axis of the future organism (Fig. 8, p. 37).

The next stage in the development of this primule of the nervous system occurs when the outer

layer of the germ disc folds on either side of the axis of the primitive streak to form two ridge-

like elevations, containing a groove between them. This, the primitive groove, is the primule of

the future myel. The sides at first grow rapidly upwards, and then bend inwards, so that the

groove closes to form a tube, the neural tube, within which the myel arises by proliferation of

the original formative cells (Fig. 9, p. 37). The primary cavity of the myel persists throughout the

vertebrate series as the longitudinal central canal or myelocele. This is lined with cinerea or

grey matter, which is itself invested with an envelope of alba or white matter; and from this,

again, the roots of the myelic (spinal) nerves issue in a fan-shaped radiation.

The primule of the brain is formed from the anterior end of the neural tube. The rapid growth of

this region leads to the formation of a bladder-like expansion, the primitive brain vesicle, which

soon divides into three compartments, the fore, mid and hind brain-vesicles (Fig. 35). Both the

genetic and the later functional relations of the primary vesicle suggest that this tripartite

division, like the development of the brain at large, is intimately connected with the

development of the three anterior sense-organs. The nervous primule of the olfactory organ

grows out directly from the anterior end of the fore brain; that of the auditory organs from the

lateral walls of the hind brain; while, despite the fact that the eyes would seem to represent

product of the growth of the fore brain, we must, in the light of indisputable physiological facts,

look to the mid brain as the ultimate source of origin of the optic nerves.

Of the three primary brain-divisions: the first and third, the fore and hind brains, undergo the

greatest changes. The anterior

extremity of both soon begins to outstrip the rest in growth,

so that both alike divide into a principal and a secondary

vesicle. The original fore brain now consists of fore brain

and 'tween brain, the original hind brain of hind brain and

after brain (Fig. 36). Of the five brain divisions which have

thus arisen from the three primary vesicles, the fore brain

or prosencephalon corresponds to the future cerebral

hemispheres; the 'tween brain or diencephalon becomes

the thalami; the undivided mid brain or mesencephalon

developes into the quadrigemina of man and the animals,

the bigemina or optic lobes of the lower vertebrates; the

hind brain or epencephalon becomes the cerebellum; and

the after brain, or metencephalon, the oblongata. The

diencephalon is to be considered the anterior, and the

metencephalon the posterior stem-vesicle, from which the

prosencephalon and epencephaon have grown out

respectively as secondary vesicles. The structures

developed from the three stem-vesicles (metencephalon,

mesencephalon and diencephalon), -- i.e., the oblongata,

the quadrigemina and the thalami, -- and the fibre-systems

that ascend among them from the myel, are grouped

together in the nomenclature of the developed brain as the

caudex or brain-stem. The structures of the first and fourth

vesicles, the cerebral hemispheres and cerebellum, are

named, in contradistinction to the brain-stem, the pallium or

brain-mantle, since in the more highly organised brains

they envelope the brain-stem as a mantle-like covering.[p.

108]

The three brain vesicles, then, represent expansions of the

anterior end of the neural tube. With the tube itself, they

form a

closed system, whose parts intercommunicate by way of the continuous central cavity, But the

development of the two secondary vesicles from the first and third primary vesicles brings other

changes with it. The roof of the fore and hind brains divides up longitudinally; so that two slit-

like apertures appear lying exactly in the median line of the body, whereby the cavities of the

fore and hind stem-vesicles are exposed. The anterior roof-slit divides the prosencephalon into

its two hemispheres, and leaves the diencephalon open above. The mesencephalon, which

does not share the advance in organisation characteristic of the rest of the brain, merely divides

by a longitudinal furrow into two halves. The posterior roof-slit appears at the place where the

neural tube passes over into the brain. The cerebellum, which grows out directly forwards from

this point, is at first separated into two entirely distinct halves, but afterwards grows together

again in the median line. The two roof-openings serve to admit blood-vessels into the brain-

cavities. These ensure the food supply requisite for further growth and for the simultaneous

thickening of the brain-walls by deposition

of nerve-substance from within.

The level of development now attained is

practically the level of permanent

organisation in the lowest vertebrates, the

fishes and amphibia (Figg. 37, 38). The

original prosencephalic vesicle is here

divided, in most cases, into two almost

entirely separate halves, the cerebral

hemispheres; the only remaining

connexion occurs over a small area of the

floor of the vesicle. The anterior stem-

vesicle (diencephalon) is split into two

lateral halves, the thalami, which retain

connexion at their base. The cerebellum

forms for the most part a narrow unpaired

lamella, from which [p. 109] all trace of

division has disappeared. In the

metencephalon or oblongata., the

posterior roof-slit has formed a

rhomboidal depression, on the floor of

which the principal mass of the organ

shows in undivided form.

Differentiation of the Parts of the Brain

The division of the brain into five vesicles brings with it a further change: a modification in the

form of the central brain cavities, whose origin as simple expansions of the myelocele we have

already noticed. The brain-cavity divides, in accordance with the

separation of the brain-vesicles, at first into three and

then into five pockets. The division of the

hemispheres subdivides the first of these again into

two symmetrical halves, the paraceles or lateral

brain-ventricles. If, now, we set out from these on a

journey through the brain-cavities, we shall traverse

them in the following order (Fig. 39). The two

paraceles (h), which as a rule are entirely separated

from each other, open into the cavity of their stem-

vesicle, -- a cleft-like space, bounded laterally by the

thalami, and left roofless by the anterior roof-slit (z),

-- the diacele or third ventricle. This leads directly into

the cavity of the mesencephalon (m). In mammals

the mesocele is extraordinarily [p. 110] reduced, so

that it appears only in the form of a narrow canal, the

aqueduct of Sylvius, running below the

quadrigemina, and connecting the diacele with the

cavity of the metencephalon. In the birds, the

mesocele is more extensive, sending lateral

offshoots into the bigemina (mesencephalon); and in

the lower vertebrates

the bigemina

contain quite large

cavities, which

communicate with

the central cavity.

The two

derivatives from

the third primary

vesicle,

epencephalon and

metencephalon,

have originally

each its own

special cavity. But

the cerebellum

(epencephalon) is

now a rounded

vesicle, arching

backward over the

metencephalon

front the point at

which this borders

upon the mesencephalon. The mesocele

accordingly divides at its posterior end into two

branches, the one of which turns upwards, and

leads into the epicele (cavity of the cerebellum),

while the other pursues a straight course into the

cavity of the metencephalon or oblongata (Fig.

40). This latter cavity, the metacele (fourth

ventricle [1]) is termed from its rhomboidal form

the fossa rhomboidalis (r in Fig. 39). The metacele is, therefore, not strictly a cavity, but a

furrow; the posterior roof-slit has completely exposed it. It closes posteriorly to pass over into

the myelocele. In mammals the epicele disappears entirely by the filling up of the epencephalic

vesicle with alba. At this stage, then, the paraceles,[p. 111] diacele, Sylvian aqueduct

(mesocele) and metacele represent the whole system of brain-cavities. In the lower

vertebrates, we have, further, the cavities of the thalami as extensions of the diacele; the

cavities of the bigemina as branches of the aqueduct; and the epicele as a tributary of the

metacele. In the lower orders of vertebrates, primary and secondary cavities alike are, as a

general rule, more extensive in proportion to the mass of the brain, -- i.e. approximate more

nearly to an embryonic state. However, the different classes evince wide differences in this

regard as between the various subdivisions of the brain. In the fishes, the cerebral hemispheres

and cerebellum are filled with alba and become solid structures. Their growth is soon arrested,

and they consequently attain to no considerable size. In the amphibia, the paraceles persist,

but the cerebellum is

usually solid. When we come to the reptiles and birds, we find a cerebellum with a spacious

epicele; but in all the mammals this has again

disappeared. In the mammals, too, the lateral cavities of the

mesencephalon (quadrigemina or bigemina) are lost; in all the

lower vertebrates, from the fishes to the birds, they not only persist

but develop prominences of cinerea upon their floor (Fig. 41).

Similar growths, the striata, appear in the paraceles of the avian

and mammalian brain.

Both in the myel and in the brain (encephalon), the nervous mass is

formed by proliferation of the cells constituting the walls of the

original cavities. Many of these cells evince the character of the

formative cells of tissue, and so mediate the secretion of the

amorphous intercellular substance or neuroglia. Others become

nerve-cells, and send out runners. In the myel, the greater part of

the fibres radiate out towards the periphery, so that the cinerea is

collected about the myelocele and surrounded by an envelope of

alba. In the encephalon this distribution of alba and cinerea

persists, practically unchanged, in the structures developed from

the three stem-vesicles. In the developments from the secondary

vesicles, on the other hand, the nerve-cells retain their position in

the walls of the cavities, and the fibres connected with them trend

towards the interior. Hence in the caudex -- the oblongata, the

quadrigemina and the thalami -- we have a layer of cinerea lining

the continuations of the myelocele and surrounded by an envelope

[p. 112] of alba; in the pallium, a mass of alba invested with a

covering of cinerea. The Linerea thus shows two distinct

formations. The one, the entocinerea (tubular grey matter) belongs

to myel and caudex, the other, the ectocinerea (cortical grey

matter), to the pallium. The entocinerea of the encephalon

undergoes still further transformation. Even in the highest region of

the myel, various bundles of nerve-fibres from the myelic columns have shifted from their

former position at the periphery of the cenerea, so that this is broken up by masses of alba. In

the oblongata, this process has gone so far that only a comparatively small part of the cinerea

holds its original place as floor of the fossa rhomboidalis, by far the greater portion being

separated by the intercurrence of myelinic fibres into distinct masses. These

collections of entocinerea, invested by alba, are termed nidi (nuclei). We see, then, that the

entocinerea of the myel undergoes an essential modification

as it passes into the encephalon; it is broken up by the

interposition of masses of alba, and so gives rise to a third

formation of grey matter, the nidal cinerea (nuclear grey,

ganglionic grey). This nidal cinerea lies midway between the

entocinerea and the ectocinerea. As we travel from the

central cavity towards the periphery, we come first of all

upon entocinerea, then upon the nidal cinerea, then again

upon alba, and finally upon the ectocinerea.

The series of changes that we have been describing hitherto

is accompanied, in all the vertebrates, by changes in the

relative position of the primitive brain-divisions, as a result of

which the whole brain is bent over ventralward. The various

parts of the caudex are thus brought out of the straight line

and set at a certain angle to one another. The bend or

flexure, which in the lowest classes is but slight,

approximates more and more closely to a right angle the

higher we ascend in the vertebrate series (Fig. 36, p. 180).

And the form of the brain is further modified by the disproportionate growth of certain divisions,

especially the prosencephalon and epencephalon, which extend over and conceal the rest.

Three flexures of the central nervous system can thus be distinguished. The first appears at the

junction of the myel and oblongata; the second takes place in the cerebellum; and the third at

the level of the mesencephalon (Fig. 42). The extent of these flexures is principally conditioned

by the growth of the prosencephalon; so that degree of curvature runs practically parallel with

development of the hemispheres.[2][p. 113] In the early stages of the development of the

vertebrate brain, the prosencephalon extends anteriorly beyond the remaining brain-divisions,

without covering them. But the more its growth outstrips the growth of the rest of the brain, the

more opposition does the rigid attachment of the embryo to the germinative vesicle offer to its

forward expansion. It must, therefore, grow out backwards, arching first over the diencephalon,

then over the mesencephalon, and finally over the cerebellum itself. At the same time, it follows

the curve of the mesencephalic flexure; its most posterior portion -- that which covers

mesencephalon and epencephalon -- bends upon itself at an angle. The more vigorous the

growth of the hemisphere, the farther does the bent portion of it extend back again towards its

point of departure, or, in other words, the more nearly does the curve described

about the diencephalon approach to a perfect circle. In this way a depression (the Sylvian

fossa; s Fig. 42) is formed at the place where the hemisphere rests upon its stem-vesicle

(diencephalon). In the most highly developed mammalian brains, the concavity of the curve of

growth is almost entirely obliterated, and the walls of the fossa draw together, leaving between

them a deep and narrow fissure.

The growth of the prosencephalon over the

caudex is necessarily followed by a

modification of the form of the paraceles.

These are originally spherical cavities, lying

within the hemispheric vesicles. As the

prosencephalon grows, they extend out at

first posteriorly, and then, when the curve of

the hemispheric arch begins to close in upon

itself, inferiorly and anteriorly as well. The

central portion of the resulting cavity is

termed the cella; the extensions are the

postcornu, precornu and medicornu of the

paracele. While this transformation is in

progress, the outer walls of the prosocele are

growing more rapidly than the inner or

median wall which surrounds the caudex. In

this there is a narrow, [p. 114] originally

vertical slit, the aula or foramen of Monro (a

Fig. 23) whereby the paraceles communicate

with the diacele (third ventricle). Anteriorly to

the aula, the hemispheres are held together

by a lamella of alba (b-d). As the

prosencephalon arches over the rest of the

brain, the aula and its anterior limiting lamella

naturally arch with it. They have, therefore, in

the developed brain, the appearance of a

vault laid over the diencephalon. The

posterior part of the aula soon closes, and

only the extreme anterior portion of the

original cleft remains open; this serves as a

channel for vascular processes passing from

the diacele into the paraceles. The lower end of the lamella of white matter, which forms the

anterior boundary of the aula, becomes the precommissure (k); the remaining portion follows

the curve of the hemispheric arch and is the primule of the fornix. Directly above this the

hemispheres are united by a strong transverse band of alba, the callosum or great commissure

(g). The portion of the median wall of the hemispheres lying above the callosum forms yet

another arch, running concentrically with the fornix, and separated from the surrounding parts

by a special furrow, f f'. This is the marginal arch, h. Its anterior division becomes the gyrus

fornicatus; the posterior passes into a structure, continuous with the gyrus fornicatus and

extending from the median wall outwards into the paracele, termed the hippocampus (cornu

Ammonis). A more detailed description of these parts, which attain to development only in the

mammalian brain, will be given later.

§2. The Myel in the Higher Vertebrates

The neural tube from which the myel developes is originally a hollow tube, filled with liquid, and

lined along its interior wall with

formative cells. The cells increase and multiply: some taking on the character of connective

tissue-cells, and furnishing an amorphous intercellular substance, while others become nerve-

cells. The processes of these last either pass directly into the fibres of peripheral nerves, or

divide and subdivide to form a terminal reticulum. The main trend of all fibres is to the periphery

of the neural tube, so that the cellular structures are shifted towards the centre of the myelic

cavity (Fig. 44, and Fig. 9, p. 37 above). The nerve-cells, and the nerves issuing from them, are

arranged, from the first, in accordance with the bilateral symmetry of the primule of the

vertebrate body, in symmetrical (right and left) groups. Moreover, the connexion of the nerves

with two different parts of the germ-primule carries with it the further separation of each group

into two subdivisions. All nerves and fibres that enter into connexion with the corneal layer, the

primule of the sense-organs and the sensitive investment of the body, arrange themselves in a

dorsal group, in the near neighbourhood of the

germinal structures dependent upon them.[p.

115] All those nervous elements, on the other

hand, that have relations to the striated muscles

collect in a ventral group, corresponding to the

animal muscle-plate. It results from this that the

cinerea formed by the association of cells

appears to right and left as a dorsal and a

ventral column, surrounded by an envelope of

myelinated fibres or alba. The columns are

termed, from their appearance in transverse

section, the dorsal and ventral cornua (horns) of

the myel: a special branch of the latter is known

as the lateral cornu. The dorsal and ventral

cornua of each side are united at the centre.

The nerve-roots issuing from the cornua are

arranged, in the same way, in two series: the

dorsal or sensory, and the ventral or motor (Fig.

44, e, f; Fig. 45, HW and VW).

Under these conditions of growth, the myelic

cavity at first assumes the form of a rhombus

(Fig. 44, cm), drawn out into a ventral and a

dorsal cleft. The cavity of the dorsal cleft soon

becomes almost entirely filled; that of the

ventral is more plainly

marked, but is closed by nerve-fibres running

from side to side of the myel, and constituting

the white ventral commissure. The commissure,

which originally crosses near the periphery (Fig.

44 h), gradually reaches a deeper level (Fig.

45). Behind it, a remnant of the myelic cavity

persists as an extremely narrow canal, the myelocele or central canal of the myel, around

which the two collections of cinerea enter into cross-connexion (Fig. 45 A). The dorsal and

ventral sulk (fissure median post. and ant.) divide the myel into two symmetrical halves. Each of

these is again subdivided by the outgoing nerve-roots into three columns (Fig. 45 B). The

column of alba lying between the dorsal sulcus and the cell-column of the dorsal roots is

termed the dorsal column (hs); that lying between the ventral sulcus and the cell-column of the

ventral roots the ventral column (vs); and a third column, ascending between the cell-columns

of the dorsal and ventral roots, the lateral column (ss). The greater part of the nerve-fibres of

these white columns run their course vertically, in the direction of the longitudinal axis of the

myel. An ex-[p. 116]ception to the rule is furnished by the area at the central end of the ventral

sulcus, occupied, as we saw just now, by the ventral commissure; here the decussating fibres

follow a horizontal or oblique course; and the same directions are, naturally, taken for a short

distance by the fibres which constitute the direct continuations centralward of the incoming

nerve-roots. The grey cornua are of different shapes: the ventral, particularly in the lumbar

region of the myel, are broader and shorter, the dorsal longer and narrower. The former contain

a quantity of large multipolar ganglion-cells; the latter consist exclusively of smaller cells. A

good portion of the dorsal cornua consists, further, of the nervous ground reticulum and its

interpenetrating fibrils. This gives them a peculiarly bright appearance, more especially as they

approach the periphery of the myel: the region is known as the gelatinosa of Rolando. Passing

centralward from this formation we find on either side of the central canal a compact column of

alba and cinerea, Clarke's column (Fig. 45 B),[p. 117] containing a well-marked group of

roundish ganglion-cells, and extending from the end of the cervical region into the lumbar

enlargement. The immediate points of origin of the dorsal roots, within the myel, seem to be

less richly supplied with nerve-cells than those of the ventral; but the difference is compensated

later on. A cluster of large bipolar ganglion-cells is intercalated in the course of the nerve-fibres

after they have left the myel, and forms with them the spiral ganglion of the dorsal roots (e Fig.

44). The dorsal columns are mot

connected, as the ventral are, by white

myelinated fibres, but by a grey

commissure, composed of fine fibres

running transversely in the mass of

cinerea behind the myelocele (dorsal

comm., Fig. 45 A). Similar grey fibres

surround the whole of the central canal;

its interior is lined by a single layer of

cylindrical epithelium, derived from a

small remnant of the formative cells that

originally invested the cavity of the neural

tube (Fig. 9, p. 37 above).

So long as the development of the central

organs is confined to the formation of the

myel, we find, of course, a certain

uniformity dominating the entire bodily

organisation. The myel, over its whole

length, simply repeats the same

arrangement of elementary parts and the

same mode of origin of nerve-fibres; and

the sensory surfaces and motor

apparatus that depend upon it must

accordingly show a like uniformity of

distribution and structure. Hence we find,

as a matter of fact, that so long as the

central nervous system of the embryo

consists merely of the neural tube, no

one of the higher sense-organs attains to

development. The primules of the

sensory investment of the body and of

the locomotor apparatus are distributed

uniformly about the central axis. A single

exception occurs at the place where the

nerve-mass takes on a stronger growth,

for the supply of the posterior extremities;

this is indicated, at an early stage, by an

extension of the primitive groove, the

sinus rhomboidalis or future lumbar

enlargement. A similar, but less strongly marked thickening of the neural tube occurs, later on,

at the point of departure of the nerves supplying the anterior extremities, -- the cervical

enlargement.[3] This uniformity of organisation is permanently retained in the lowest vertebrate,

Amphioxus lanceolatus, in which the development of a central nervous system halts at the

formation of the neural tube. The organ of vision, in this brainless vertebrate, consists of simple

refracting cells, surrounded by pigment [4]; the organ of smell, of an unpaired cup-shaped

depression at the anterior end of the body; the presence of an auditory organ has not been

demonstrated. We see, then, that the arrest of development in this case affects just those [p.

118] organs which appear to exercise a determining influence upon the formation of the higher

central parts, the vesicular differentiations of the myel.

§3. The Oblongata

In the lower vertebrates, the bundles of nerve-fibres take a course that is, to all appearance,

but little different from their course in the myel. The only changes are that the dorsal columns

split apart, disclosing the fossa rhomboidalis (Fig. 37, 38, p. 109);

and that the grey cornua, as may be seen in section, have been divided off from the central

cinerea and intercalated in the course of the ventral and dorsal columns. The three columns of

the myel, ventral, lateral and dorsal, can still be distinguished,

but receive special names. Their fibres

pursue a more tortuous course, and

ganglionic nidi appear among the fibre-

bundles. They thus differ very

considerably from the correspondingly

situated myelic columns; and, indeed,

for the most part do not represent direct

continuations of the myelic structure.

The ventral columns are termed

pyramids; their fibres decussate, in the

posterior part of their course, so that the

ventral sulcus is entirely obliterated

(Fig. 46 A; Fig. 47 p). This decussation

seems to be a repetition, on a larger

scale, of the decussation of the ventral

columns of the myel in the ventral

commissure. At their upper end, where

they enclose a ribbon-like stria of

cinerea (pyramidal nidus, inner

accessory nucleus of olive; pyram.

nidus, Fig. 46 B), the pyramids are

bounded on either side by the olives

(Fig. 46 B; Fig. 47 o). These are well-

marked prominences, which contain in

their interior a large ganglionic nidus,

dentate in section and therefore termed

the dentatum (nd). The vertically

ascending fibre-bundles which enclose

the dentate nidus are known as the

capsular columns (finiculi siliquae). The

lateral columns (s Figg. 47, 48) grow

smaller and smaller [p. 119] from the

lower end of the oblongata upwards,

until at about the point where the

opening of the fossa rhomboidalis

appears they disappear entirely in the

interior. The dorsal columns, on the

other hand, increase in external

diameter. In the lower portion of the oblongata, they are divided by a shallow sulcus into inner

and outer columns, the slender and the cuneate funiculi (fg and fc Fig. 48), which at the lower

end of the fossa rhomboidalis carry knob-like prominences, produced by the grey nidi of the

interior (nid. grac. and nid. cun., Fig. 46). Farther upwards, the two funiculi appear to continue

their course in the columns which bound the fossa rhomboidalis to right and left. These have

been termed the restes (restiform bodies: pi Fig. 48). They are the largest columns of the

oblongata, and, like the funiculi just mentioned, contain grey nidi. They are characterised by the

intricate, trellis-like interlacing of their constituent fibres. Anteriorly, the restes pass entirely over

into the alba of the cerebellum forming the inferior cerebellar peduncles. Between them, on the

floor of the fossa rhomboidalis and directly covered by entocinereal matter, are two further

tracts, which appear to represent the continuations of the parts of the myel lying ventrally of the

myelocele, i.e. the ventral cornua and the deeper-lying portions of the ventral columns. These

structures, which extend over the whole floor of the fossa rhomboidalis, and are principally [p.

120] composed of cinerea, are known, from the arching convexity of their form, as the

cylindrical columns (eminentiae teretes, et). Their cinerea is in connexion with most of the grey

nidi of the oblongata, though some of these

are forced out from the median line, and

thus isolated, as a result of the splitting up of

the oblongata by tracts of alba. A further and

final consequence of the changes of

structural conditions which we have been

describing is the formation of an entirely new

system of fibre-groups surrounding the

oblongata in a transverse direction. Some of

them enter into the ventral sulcus and the

sulcus separating the pyramids from the

olives; others strike across the fossa

rhomboidalis. The system is known as the

zonal fibre-system (stratum zonale, fibrae

arcuatae, g).

The re-arrangement of parts in the

oblongata leads to a redistribution of the

points of origin of the peripheral nerves.

The simple rule of the myel is no longer

obeyed, the nerve-roots are more or less

displaced from their old positions. It is

true that they still form, roughly, two

longitudinal series, a dorsal and a ventral.

But root-fibres that are exclusively motor

issue only from the ventrolateral sulcus

(twelfth cranial nerve, hypoglossal or

lingual motor). The dorsolateral sulcus

(or, at any rate, its immediate

neighbourhood) gives rise, on the other

hand, both to sensory and to motor fibres.

Here begin all the other cranial nerves,

with the exception of the first and second,

olfactory and optic, and the third and

fourth, the two anterior oculomotor

nerves, whose place of origin lies further forward (cf. Figg. 47, 52).[5][p. 121]

§4. The Cerebellum

At the anterior end of the oblongata, the structural relations of the parts are further, and very

essentially, modified, owing to the outgrowth of the cerebellum from the primule of the third

brain-vesicle (epencephalon). At the lowest stage of its development, the cerebellum varies but

little, in outward form, from its original primule (Figg. 37, 38, p. 109); it consists of a transverse

stria,

bridging over the anterior end of the fossa

rhomboidalis and receiving into its substance the

restis of either side. Anteriorly, it is continued by

a myelinated plate to the mesencephalon (Fig.

40, p. 110); laterally, it gives out tracts of

transverse fibres, which run towards the lower

surface of the oblongata, and decussate with

each other and with the vertically ascending fibre-

tracts of the pyramidal and olivary columns.

These connexions remain the same later on,

when the cerebellum has attained its further

development. The bundles that enter it from the

restes are the inferior cerebellar peduncles (crura

ad medullam oblongatam, pi Fig. 18); the

myelinated fibres issuing from it towards the

mesencephalon are the superior cerebellar

peduncles (crura ad cerebrum, ps). These are

united by a thin myelinated plate, the valvula

(velum medullare superius, vm), which forms the

roof of the fossa rhomboidalis, and effects a

direct connexion of the cerebellum with the

adjoining anterior brain-division, the mesencephalon or quadrigemina. Finally, the white tracts

proceeding from either side of the cerebellum form the middle cerebellar peduncles (crura ad

pontem, pm). The structure that arises at the brain-basis by the junction of the middle peduncles

and their decussations with the longitudinal myelinated [p. 122] fibres ascending from the

oblongata is known as the pons (pons Varolii, br Fig. 47). It constitutes on the one hand a

connecting link in the longitudinal direction between metencephalon and mesencephalon, and on

the other a connecting link in the horizontal direction between the two lateral halves (pilea) of the

cerebellum. The superior and inferior cerebella-peduncles are clearly visible at the very earliest

stage of cerebellar development. The middle peduncles, on the contrary, do not attain to a growth

that enables us to distinguish the pons as a special structure until the development of the

cerebellum, and especially of the pilea, has advanced a considerable distance. Even in the birds,

their place is supplied by little more than the longitudinal continuations of the ventral and lateral

columns of the oblongata (Fig. 49 B). From the points at which the peduncles enter the cerebellar

mass, above, below and

sideward, myelinated fibres

radiate out towards the periphery

of the organ.

The morphological development

of the cerebellum, the posterior

section of the pallium, is

completed at a comparatively

early period. In all vertebrates, it

is covered by ectocinerea, clearly

differentiated from the radiation of

myelinated fibres that occupies

the interior; and even in the

lowest vertebrates, the fishes, the

cerebellar cortex divides into a

number of layers, characterised

by differences of coloration. In the cerebellum of the amphibia, we find groups of nerve-cells

intercalated in the course of the myelinated fibres, -- the first traces of ganglionic nidi. In the

birds, these increase in number, while at the same time the layer-formation of the cortex

becomes plainer, and an increase in the mass of the cortical elements is rendered possible by

superficial folding (Figg. 40, 49).

The cerebellum undergoes its final stage of structural development in the mammals. Here we

find, first, an unpaired median portion, the surface of which is crossed by transverse folds, and

which has received the descriptive name of vermis; and, secondly, more strongly developed

symmetrical lateral portions, the pilea. In the lowest mammals, it is true, the growth of the

vermis exceeds that of the pilea; but in the higher forms, the pilea grow out about it in all

directions (Fig. 50). The development of the pilea is accompanied by a more vigorous growth of

the middle peduncles, which in the lower vertebrates are merely indicated by slender

transverse fibre-tracts to the oblongata. The transverse folds of the ectocinerea increase in

number, and show in cross-section the well known dendritic appearance of the arbor (arbor

vitae, av Fig. 50). At the same time, larger ganglionic nidi appear in the radiation of myelinated

fibres within the cerebellar mass. Thus each pileum contains a dentate nucleus, similar to that

of the olives (nucleus dentatus cerebelli, cn). Other cinereal nidi, analogous in function, are

scattered among the alba of the pons; their cells are intercalated between the decussations of

the various fibre-bundles.[p. 123]

§5. The Mesencephalon

The mesencephalon -- the division of the caudex which corresponds to the bigemina (lobi

optici) of the lower vertebrate; and the quadrigemina of mammals (t, n Fig. 48; d Fig. 37) --

contains two formations of grey matter, entocinerea and nidal cinerea; only the secondary

vesicles develope the ectocinereal pallium. The entocinerea surrounds the Sylvian aqueduct in

a layer of

moderate thickness; the most

anteriorly situated nerve-nidi

(those of the oculomotor and

trochlear nerves and of the upper

root of the trigeminus) are in

connexion with it. Ganglionic nidi

are found scattered through the

bigemina or quadrigemina, and

intercalated in the course of the

myelinated fibre-tracts that pass

below the Sylvian aqueduct.

These are paired tracts of alba,

united in the median line, and

forming in the first place

continuations of the ventral and

lateral columns of the oblongata.

Anteriorly, they are strengthened

by the addition of longitudinal

fibre-tracts, proceeding from the

quadrigemina and thalami. They

are termed, over their whole

extent from oblongata to cerebral

hemispheres, the brain crura (crura cerebri). The mesencephalic portion of the crura in the

mammalian brain contains two well marked ganglionic nidi, the one of which, characterised by its

dark coloration, is known as the intercalatum (substantia nigra of Soemmering: sn Fig. 51). It

divides the crus of either side into a posterior and exterior portion, the crusta (basis pedunculi, pes

cruris cerebri; f Figg. 47, 51), and an anterior and median portion, the tegmentum (tegmentum

pedunculi; hb Figg. 47, 51). The most anterior extremity of the median portion, a band of alba

which curves anteriorly into a fillet, and directly underlies the quadrigemina, is termed the

lemniscus (ribbon; or laqueus, fillet; sl Fig. 51). A second tegmental nidus is named from its colour

the [p. 124] rubrum (nucleus tegmenti; hb Fig. 56). Dorsal of the crura lie the quadrigemina (v Fig.

51). Posteriorly, they are connected with the superior cerebellar peduncles; anteriorly and laterally,

they give out myelinated fibres, which in part mingle with the fibres of the tegmentum, in part pass

into the thalami, and in part form the fibres of origin of the optic nerves. The connexion with

thalami and optic nerves is mediated, in the mammalian brain, by the quadrigeminal brachia (ta

Fig. 48). The pregemina are joined by the prebrachia to the thalami, and the postgemina by the

postbrachia to the postgeniculum. In the space between the pregemina and the posterior

extremity of the thalami lies the epiphysis (conarium, pineal body; z Figg. 48, 51), which

Descartes, in the old days, looked upon as the 'seat of the mind.' It is a highly vascular

structure, which genetically represents, in all probability, a rudimentary sense-organ: it is

supposed to be the central remnant of a median eye, functional in the primitive vertebrates. The

mammalian quadrigemina are, as we have already seen (p. 111), completely solid. They are

connected by a lamina of alba, which posteriorly forms the direct continuation of the valvula,

and anteriorly passes into the postcommissure (cp Fig. 53) running along the boundary-line of

quadrigemina and thalami.

§6. The Diencephalon

The diencephalon, or region of the thalami, is smaller than the mesencephalon in all the lower

vertebrates (f Fig. 37, p. 109); in the mammalian brain their relation is reversed (th Figg. 47, 48,

51). In the fishes, however, we have an indication of the change: a paired continuation of the

diencephalon extends posteriorly to the base of the brain, and there appears in the form of two

hemispherical prominences, lying ventrally and somewhat anteriorly of the bigemina (lobi

optici). These are the inferior lobes of the fish-brain (li Fig. 41, p. 111). They enclose a cavity,

which stands in connexion with the diacele, the cleft-like aperture

resulting from the anterior roof-slit and

dividing the diencephalon into the two

thalami. At the place where the inferior lobes

meet in the median line, they are continued

into an unpaired structure, the hypophysis

(h Fig. 41), whose dorsal portion is an

evagination of the diencephalon, while its

ventral half is a remnant of embryonic tissue

that originally belonged to the anterior

extremity of the gullet, and remained in

conjunction with the diencephalon when the

base of the cranium developed. The

hypophysis persists in the higher

vertebrates, after the inferior lobes have

entirely disappeared in consequence of the

more vigorous development of the crura (h

Fig. 52). The only point at which the

ganglionic substance of the diencephalon

appears, in these [p. 125] forms, is between

the divergent crura, where we find a grey

prominence, the tuber cinereum. This is

continued anteriorly in the direction of the

hypophysis, in a funnel-shaped prolongation,

the infundibulum (i Fig. 38; t Fig. 47). The

infundibulum contains a small cavity,

communicating dorsally with the diacele.

The entrance of small blood-vessels into the

brain-mass gives the cinerea between the

crura a perforated, sieve-like appearance;

the region is termed the postcribrum (lamina

perforata posterior; pp Figg. 47, 52). In the

mammalian brain, two myelinated

prominences, the albicantia (corpora candicantia or mammillaria; cc), issue ventrally from the

floor of the diencephalon. Situated immediately in front of the anterior line of the pons, they bound

the tuber posteriorly, as infundibulum and hypophysis bound it anteriorly. Their genetic

signification is still uncertain.

The diencephalon, like the mesencephalon, contains two formations of grey matter, entocinerea

and nidal cinerea. In the first place, the interior of the diacele is lined with a cinereal layer,

which at the same time invests a thin band of alba, joining the two thalami, and termed the

medicommissure (cm Fig. 51). The entocinerea of the diacele extends ventrally to the brain-[p.

126]base, where it is directly continuous with the tuber and infundibulum. Secondly, however, a

number of ganglionic nidi, separated by masses of alba, are scattered throughout the interior of

the thalami (th Fig. 56). Similar nidi may be found in two smaller rounded prominences, which

in mammals form the posterior boundary of the thalami and externally are in connexion with

them, the pregeniculum and postgeniculum (k k' Fig. 48, p. 120). The fibres of origin of the optic

nerve interlace with the fibres of both genicula, and the postgeniculum alsohreceives [sic] the

quadrigeminal postbrachium. The anterior and lateral portions of the thalami show a gentle

roof-slope; posteriorly, the dorsal surface is separated from the ventral by a marginal swelling,

the pulvinar (pv Fig. 47, p. 119)

§7. The Prosencephalon

(a) -- The Brain Cavities and the Surrounding Parts

In the earlier stages of its development, the prosencephalon is a vesicular structure, overlying

the diencephalon. Originally simple, it is later divided by the anterior roof-slit into symmetrical

halves, entirely separate save for the continuity of their floor. At the place where the roof-slit of

the diencephalon is continued into the intercerebral fissure, the diencephalic cavity was

primitively in open communication with the two paraceles. In all the vertebrates (except the

fishes, whose hemispheres are solid structures: p. 111), the vascular trunk that penetrates the

cavity of the diencephalon sends out a large number of branches from it into the hemicerebral

vesicles. When the diencephalon becomes so far solid, by growth of the constituent nerve-

mass, that only the diacele (third ventricle) is left, the earlier doors of communication are almost

entirely closed; only two very narrow apertures are left, at the anterior end of the diacele, which

permit of the entrance of blood-vessels into the hemicerebral cavities. These are the portae

(foramina of Monro: mo Fig. 53), the remnants of the original aula (Fig. 43, p. 113). They are

separated anteriorly by a septum of alba, which represents the posterior line of junction of the

two prosencephalic vesicles. The floor of the septum is usually formed of large bundles of

myelinated fibres, transverse in direction, termed the precommissure (ca). In the reptilian, and

still more in the avian and mammalian brains, the hemispheres come to such a growth as to

arch more or less completely over the diencephalon. As a result of this, the paraceles run out

posteriorly, and the thalami, instead of lying behind the hemispheres, as they did at first, form

prominences that project with the greater part of their surface into the paraceles, and show only

their internal faces to the diacele.[p. 127]

The grey matter of the prosencephalon occurs in all three possible formations. As entocinerea,

it covers the walls of the diacele, and therefore, more especially, the inner faces of the thalami

and the cavity of the infundibulum, as well as the whole of the infundibular region; as ganglionic

cinerea, it forms considerable masses, intercalated in the course of the continuations of the

crura below the thalami; and as ectocinerea, it invests the hemicerebral pallium at large. The

position of these collections of grey matter, and their relation to the radiations of myelinated

fibres, are the essential conditions of the structure of the prosencephalon. In all vertebrates,

except the fishes and amphibia, the ganglionic nidi are placed upon the floor of the paraceles.

They there form rounded prominences,[p. 128] from which myelinated fibres radiate out

towards the periphery of the hemispheres.

The lowermost stratum of the floor of the paraceles is, therefore, composed of the

continuations of the ascending and diverging crura. Upon these rest, first of all, the thalami.

New tracts of alba issue from the thalami, and join and reinforce the crural bundles that run

forwards and outwards below them. These terminal radiations of the crus, at the anterior and

external border

of each thalamus,

are, again,

intermixed with

large ganglionic

nidi. The result is

that the floor of the

paracele rises in a

rounded

prominence of

considerable

extent, which forms

the anterior and

exterior boundary

of the thalamus. It

is termed the

striatum (st Figg.

54, 55). The club-

like extremity, lying

anteriorly of the

thalamus, is called

the caput; the

narrower portion,

surrounding the thalamus exteriorly, the cauda. Striatum and thalamus together cover the entire

floor of the paracele. The surface of the striatum is invested with a tolerably thick layer of cinerea,

whereas the thalamus (i.e., that portion of its surface which projects into the paracele) is covered

by a lamina of alba. Along the border of thalamus and striatum lies a narrow hand of alba, the

tenia (stria terminalis, strin cornea: sc Fig. 55). The ganglionic nidi of the striatum appear in the

mammalian brain, as three characteristically shaped masses. The first is directly connected with

the grey investment of the striatum; it follows the arch of the peripheral surface, and so acquires a

curving [p. 129] form, which has given it the name of caudatum (nucleus caudatus: st Fig, 56). It

constitutes, with the myelinated masses that begin their course below it, the striatum in the

narrower sense. A second and very considerable nidus, the lenticula (nucleus lentiformis: lk), lies

to the outside of the caudatum. In vertical section it appears as a triangle, whose apex points

towards the internal edge of the striatum, while its base extends far out into the alba of the

hemisphere. The lenticular

cinerea is divided up by intervening myelinated fibres

into three groups, two external and ribbon-shaped,

and one internal and triangular. The third and last

nidus of the striatum lies outward from the lenticula.

It, too, has the form of a narrow ribbon of tissue, and

surrounds the third subdivision of the lenticula. It is

named, from the closeness of its approach to the

brain surface, the rampart or claustrum (nucleus

taeniaeformis: cl). Ventrally from the claustrum, and

near the cortex of the brain-base, lies yet another

small nidus, the amygdala (mk).[6] These ganglionic

nidi of the hemispheres take up many of the

myelinated fibres that spring from the quadrigemina

and thalami; others pass under the striatum and

forwards, without coming into connexion with its

cinerea. Above the nidi, the myelinated bundles

coming up from below radiate out from the whole

extent of the striatum, in the most various directions,

toward the cerebral cortex. The terminal division of

the great longitudinal fibre-tract, that begins in the

columns of [p. 130] the myel, then passes over into

the columns of the oblongata, and thereafter takes its

place among the bundles of the crura, is the corona

(corona radiata; m). The factors which most largely

determine the arrangement of its fibres are those

discussed just now, as concerned in the formation of

the paraceles. Since the vascular plexuses that find

their way into the cavities spread over the entire

floor, the coronal fibres which are to continue the

crura cortexward must curve out around the vessels

at the periphery, in order to attain their goal.

The terminal division of the prosencephalon consists

of the two olfactory bulbs or olfactory gyres

(rhinencephalon). In most fishes, the

rhinencephalonis so strongly developed that it not

seldom surpasses in extent all the remaining portions

of the prosencephalon. In the higher classes of

vertebrates, and especially in the birds, it decreases

in importance; but in the lower mammals it appears

again as a structure of relatively considerable size (cf. Figg. 37, 38, 49, 54). In the mammalian

brain it forms special gyri, which issue from the brain-base and project to a greater or less

degree beyond the frontal portion of the prosencephalon. The olfactory bulbs contain cavities,

the rhinoceles, which communicate with the prosocele (paraceles). In some of the mammalian

orders, viz., in the cetacea and (to a less extent) in the apes and in man, the rhinencephalon

degenerates.[p. 131] The olfactory bulbs lie far back under the frontal regions of the

hemispheres and are connected by a narrow stalk, the olfactory tract, to the middle part of the

brain-base (Fig. 52, p. 125). The area which serves as point of departure for the tract, the

olfactory area, presents a sieve-like appearance due to the incoming of numerous small

vessels, and is consequently termed the precribrum (lamina perforata anterior: sp Figg. 47, 52).

The fuller development of the prosencephalon brings with it a radical transformation of the two

lateral ventricles, the paraceles. This is due, in part, to the growth of the hemicerebral masses

which enclose them, but in part also to the appearance of special

structures which project into the

cavities. As the hemispheric

vesicle of the mammalian brain

arches back over the

diencephalon and

mesencephalon, the portion that

lies behind the Sylvian fossa

takes a downward turn (Figg. 36,

42, p. 108, 112). The result is that

the paracele possesses two

branches, or cornua, as they are

termed: a precornu, bounded on

the outside by the arched wall of

the hemisphere, and a medicornu

(cornu inferius) whose extremity

is drawn out to a point. The

growth of the hemispheric vesicle

over the caudex is accompanied

throughout its progress (as we

have already seen: p. 114 above)

by a parallel growth of the aula

(foramen of Monro), the original

means of communication between

prosocele and diacele. As the

aula, then, curves over the

caudex, at first posteriorly and

then ventrally, what was originally

its dorsal extremity coincides with

the pointed end of the medicornu.

The part of the aperture that now

lies in the anterior wall of the

medicornu forms a fissure (the

hippocampal fissure, of which

more presently), which is occluded by a vascular plexus from the pia (fh Fig. 56). In fine,

therefore, the primitive aula remains open at beginning and end, but is closed over its middle

portion by myelinated fibres. These belong to the fornix and callosum, structures which we shall

discuss in the following section.

In the brain of the primates (the apes and man), the conformation of the paraceles undergoes

yet another change, due to the large development of the occipital portion of the hemispheres.

The outer wall of each paracele pushes vigorously backwards before it takes the curve

downwards, so that the cavity itself is prolonged in the same direction. We thus have a

postcornu (cp Fig. 51, p. 123) in addition to the precornu and medicornu. The backward growth

of the prosencephalon stops, as it were, with a jerk, to continue forwards and downwards. This

fact is attested both by the outward appearance of the occipital region, and by the shape of the

postcornu, which is drawn out into an even finer point than the medicornu. In the apes, the

postcornu is smaller than it is in man; in other mammals with strongly developed hemispheres,

as e.g. the cetacea, it is no more than a trace or rudiment of what it is later to become.[p. 132]

(b) -- Fornix and Commissural System

At the anterior extremity of the primitive aula, the two hemispheres grow together in the middle

line. The resulting strip of alba is termed the terma (lamina terminalis: bd Fig. 43, p. 113). The

backward curvature of the hemispheres round the transverse axis of the diencephalon naturally

brings with it a corresponding curvature of the terma. Its most ventral and anterior extremity

becomes a band of cross-fibres, connecting the two hemispheres, and known as the

precommissure (k Fig. 43). In its further course it divides into two lateral halves running

longitudinally from before backwards, on either side of the median fissure. We find the first

beginnings of these longitudinal fibre-tracts in the birds, but they do not attain to any high

degree of development until we reach the mammals, where they constitute the fornix. Closely

approximated anteriorly, the columns of the fornix diverge as they pass backwards. The

myelinated fibres of their anterior extremity extend ventrally to the brain-base, where they stand

in connexion with the alba of the albicantia (Fig. 53, p. 127). The fibres of their posterior

extremity are distributed in man and the apes into two bundles, the smaller of which comes to

lie upon the inner wall of the postcornu, and the larger upon the inner wall of the medicornu of

the paracele. The projection thus occasioned in the wall of the postcornu is termed the calcar

(pes hippocampi minor), that in the medicornu, the hippocampus (pes hippocampi major: Fig.

55). These prominences are, however, constituted in part of other factors, which we shall

discuss later. In the other mammals, which have not developed a postcornu, and which

therefore cannot possess the calcar, the whole mass of fornix-fibres passes over into the

hippocampus.[7]

The formation of the fornix appears to stand in intimate relation to that of another transverse

fibre-system whose appearance is even more definitely characteristic of the mammalian brain.

In the monotremes and marsupials, new fibre-tracts are observed to issue from the

hippocampus (cornu ammonis). They run dorsally of the incoming fornix-fibres, and pass above

the diencephalon to the opposite half of the brain, where they terminate, as they began, in the

hippocampus. The transverse commissure that thus arises between the two hippocampi is the

original primule of the callosum. In the non-placental mammals, in which the callosum is this

restricted to a mere cross-commissure between the two hippocampi, the precommissure [p.

133] is very strongly developed, as it is in the birds, though a free space is left between it and

the callosum. In the placental mammals, the hippocampal commissure is reinforced by

additional transverse fibre-tracts, which radiate out into the hemicerebral alba at large. They

make their first appearance at the anterior end of the future callosum, so that the development

of the callosum itself proceeds from before backwards. At the same time, the precommissure

decreases in size, and enters by way of a thin and still transverse lamella of alba (Fig. 53 ca)

into connexion with the anterior extremity of the callosum, the 'beak' or rostrum. This junction of

precommissure and rostrum results in the anterior occlusion of the intercerebral fissure.

Between the broad posterior extremity of the callosum, the splenium, and the dorsal surface of

the cerebellum, there still remains, however, a narrow passage, by which the diacele can

communicate with the surrounding space (the passage is visible in Fig. 53 as the dark space

between epiphysis and splenium). This is continued laterally as a narrow cleft, leading into the

paracele. We have, in it the remnant of the original anterior roof-slit, whereby the vascular

plexuses gain access to the three anterior brain-cavities (p. 114).

In most mammalian brains, the hippocampal commissure persists as a relatively large portion

of the entire callosum (bk Fig. 57 A). Moreover, since the occipital brain is here but little

developed, and the anterior brain-ganglia, the thalami and striata, also decrease very

considerably in mass, the hippocampus is brought forwards to the point of origin of the fornix.

The fornix itself immediately separates on either side into two divisions, the one of which forms

the anterior and the other the posterior boundary of the hippocampus (f and f' Fig. 57 B).[8] It is,

however, not until we reach the higher mammals that we find any considerable development of

the fornix. Between the callosum and the deeper-lying fornicolumns are now spread two thin

vertical lamellae of alba, enclosings a narrow cleft-like cavity. These are the septa (septa

pellucida: sp Fig. 33). Fornix and septa occlude the internal openings of the paraceles; nothing

is left but the beginning of the original aula, just behind the anterior place of origin of the

fornicolumns (porta or foramen of Monro: mo Fig. 53; cf. i Fig. 43, p. 113). Between the lateral

halves of the septa is the cleft-like cavity just mentioned; it communicates posteriorly with the

diacele, and is termed the pseudocele (cavum or ventriculus septi pellucidi). The callosal

radiations form the roof and a portion of the outer wall of the paraceles. As external capsule,

they skirt the external margin of the lenticula. On their way to the cortex, where they terminate,

they interlace at all points (the posterior strands excepted) with the fibres of the corona. The

posterior fibres, coming from [p. 134] the hippocampi and their neighbourhood, do not receive

any admixture of coronal fibres. In the lower mammals, they appear simply and solely as the

hippocampal commissure (Fig. 57 A); in the primates, they divide into two parts; an internal,

passing over into hippocampus and calcar (am and vk Fig. 55); and an external, which curves

ventrally in front of the coronal fibres running to the cortex of the occipital lobe (m' Fig. 58), and

forms the outer wall of the postcornu of the paracele. This is termed the tapetum (tp Fig. 51, p.

123).

We have seen that the fornix is the fibre-tract proceeding from the terma (lamina terminalis) of

the aula; and we have followed the course which it takes as the outgrowing hemispheres arch

over the brain-caudex. The same direction is taken by the portion

of the hemicerebral wall lying immediately anterior to the terma. There is a difference, however.

The floor of the hemispheres is continuous from the first, and the fornix has, therefore, no

investment of grey matter. This anterior portion, on the other hand, which comes to lie dorsally

to the fornix as the result of the hemispheric curvature, is not included in the original area of

continuity, and is accordingly covered with a layer of cinerea over its median surface. After the

callosum has forced a passage across the brain, it is separated by the callosal fibres from the

fornix, and forms a longitudinal gyre running dorsal to the callosum. It is termed the callosal

gyre (gyrus fornicatus or cingulum: Gf Fig. 53, p. 127).[p. 135]

In certain mammalian brains, where the frontal part of the prosencephalon is but little

developed, while the callosal gyre is large,

it can be traced anteriorly from a point directly behind the base of the olfactory tract. After

curving over the callosum, it again emerges posteriorly at the brain-base. Here it passes over

into a gyre lying behind the Sylvian and bounding the median fissure; the hippocampal gyre, the

outer wall of the hippocampus (H Fig. 53). Where the callosum begins, the layer of cinerea

ceases; the lower surface of the callosal gyre, the surface adjacent to the callosum, consists of

unmixed white matter. The sole exception is a narrow stria of cinerea, isolated from the rest of

the cortex, which has persisted in its posterior portion. This is known as the fasciola (fc Fig. 59);

it lies immediately above the callosum. The fasciola is free, over its whole extent, from any

admixture of alba; so that the longitudinal myelinated fibres upon which it rests are entirely

separated from the remaining white matter of the callosal [p. 136] gyre. When the gyre is

removed from the callosum, these fibres, together with the fasciola which invests their posterior

portion, remain attached to it, they appear as a myelinated stria, and have received a special

name, tenia tecta or lateral stria (sl Figg. 58, 59). The importance of this separation of lateral

stria, and fasciola from the rest of the myelinated and cortical substance of the callosal gyre lies

in the fact that the structures remain isolated at the point of transition from callosal to

hippocampal gyre.[9] Alba and

cortex of the callosal gyre pass

directly over into alba and cortex

of the hippocampal. Really,

therefore, the two are but one: the

only difference between the parts

being that the callosal gyre is not

invested with cinerea over its

ventral surface, the surface

adjacent to the callosum, whereas

with transition to the hippocampal

gyre the cortex spreads out again

over the entire surface of the

convolution. Now at the point

where the callosal gyre leaves the

splenium and becomes the

hippocampal gyre, -- at the point

i.e. where the cortex which has

previously invested the inner

surface only extends over the

ventral as well, -- the lateral stria

divides from the rest of the white

matter of the gyre, and appears

upon the surface of the gyrus

hippocampi. This means, of

course, that the fasciola, which

lies just below the lateral stria,

must divide from the rest of the

cortex; the lateral stria forms a

partition between cinerea and

cinerea. The result is that we

have, at the point in question, a

cortical layer covered by a

lamella

of alba, and this again covered by a grey cortex.

The two most superficial layers, lateral stria and

fasciola, are, it must be remembered, strictly

limited in area; they extend only over the

hippocampal gyre. Indeed, they cover only a

portion of that; for the white and grey areas are

not coincident. The alba of the lateral stria is

distributed over the entire cortex of the

hippocampal gyre, as an extremely thin reticular

layer of white fibres. This reticular alba is the only

white layer that appears upon the cortical surface

of the hemispheres (sr Fig. 59; cf. also H Fig. 53,

p. 127). The fas-[p. 137]ciola, on the other hand,

retains its ribbon-like form; it covers, not the

whole radiation of the white fibres of the lateral

stria, but only a certain group of them, the group

lying in the fissure which forms the interior

boundary of the hippocampal

gyre. From the peculiar toothed appearance that it has at this part of its course it is known as

the fascia dentata (fd Fig. 59). The fissure which forms the

interior boundary of the hippocampal gyre corresponds to the

hippocampal projection into the medi-cornu of the paracele. The

formation of the hippocampus -- to which, as we have seen,

fibres from callosum and fornix contribute -- is thus completed by

contributions from the various regions of the callosal gyre. The

white layer which invests the paracele-surface of the

hippocampus is formed by the fibres of callosum and fornix (Fig.

60). It is followed by a first cinereal layer, the cortex of the

hippocampal gyre (r); externally to that comes a second layer of

alba, the continuation of the lateral stria or the reticular alba (H)

distributed over the cortex of the gyrus hippocampi; and, lastly,

beyond that follows a second cinereal layer, the fascia dentata,

the continuation of the fasciola (fd). This extends, as we have

said, only to the fissure which corresponds to the hippocampal

projection. The same fissure forms the inner boundary of the

reticular alba. Along the line at which the alba ceases, the grey

matter of the fascia dentata is continuous with the cortex of the

hippocampal gyre; so that here the two cinereal layers which fill

the interior of the hippocampus are brought into contact. At the

precise point where this transition takes place, the internal white investment of the

hippocampus terminates in a free reflected border, the fimbria (fi).

We have now passed in review those divisions of the brain which appear in the general course

of neural evolution.

This articulation into parts is paralleled by

a series of changes in outward form, the

final outcome of which is dependent partly

upon the degree of general development

to which the particular brain has attained,

and partly upon the relative growth of the

individual parts. In the lowest vertebrates,

the brain has gained but little upon the

simplest embryonic form which is given

with the separation [p. 138] of the

primitive brain-vesicle into its five

subdivisions. The whole range of

structural difference is here practically

exhausted by differences in the relative

size of these subdivisions; the only further

determinant of final brain-form is the

development of the olfactory lobe as an

outgrowth from the prosencephalon. A much greater variety of configuration appears as soon

as the pallial structures begin to invest the brain-caudex. The covering of quadrigemina

(bigemina) and cerebellum by cerebral hemispheres and of oblongata by cerebellum, and the

degree of encephalic flexure, bring in their train a long series of structural peculiarities; and the

list is still further swelled by differences in the outward form of the hemispheres, by the

development or lack of development of the cerebellar pilea, by the corresponding appearance

or non-appearance of certain nidal structures (such as the olives) on the oblongata, and by the

development of a pons.

The point where the cerebral hemisphere originally rested upon the brain-caudex is marked, in

all mammalian brains without exception, by the Sylvian fossa (S Fig. 42, p. 112). In the higher

mammals, the edges of the fossa draw together, so that we

find in its place a deep fissure, the Sylvian fissure

(fissura Sylvii). The fissure usually runs obliquely,

from posterior-dorsal to anterior-ventral; its

divergence from the vertical is determined by the

growth of the occipital brain and its extension

over the posterior parts of the system (Fig. 61). In

the highest mammalian order, that of the

primates, the Sylvian fissure undergoes a final

and characteristic transformation. The frontal and

occipital brains here develope simultaneously

and the fossa formed by the growth of the

hemispheres over the caudex consequently

appears, at the very beginning of the embryonic

life, as a roughly outlined triangle, lying base

upward. The edges, above, below and behind,

then grow towards each other, and the fossa

closes to form a Y-shaped fissure (S Fig. 62),

dividing into an anterior and a posterior ramus

(s1 and s2. Cf. also Fig. 65) The part of the

hemisphere that lies between the two rami, and

roofs in the original fossa from above, is termed

the [p. 139] operculum (K). If the operculum is turned back, and the floor of the Sylvian fossa

exposed, the underlying hemispheric surface proves to be bulged out and, like all the rest,

divided by fissures into a number of gyres. The brain-region which is concealed and isolated in

this peculiar way is known as the central lobe or island (insula Reilii; Fig. 56 J, p. 130). The two

rami of the Sylvian fissure form the customary points of departure for the division of the

hemicerebra of the primate brain into separate regions or lobes. The portion lying anteriorly of

the anterior ramus is termed the frontal lobe (F Fig. 62); the space included between the rami,

the parietal lobe (P); the region behind the Sylvian fissure, the occipital lobe (O); and the area

situated ventrally of it, the temporal lobe (T). These lobes pass into one another, on the convex

surface of the brain, without any sharp line of demarcation.

Not only does the Sylvian fissure divide the surface of the hemispheres at large into a number

of lobes: there are certain other and smaller areas that are marked off from their surroundings

by furrows or fissures. Thus the longitudinal fibre-tract running dorsal to the callosum from

before backwards, and then curving ventrally round the splenium, -- the area with which we

have become familiar as the callosal gyre, -- may be recognised by the presence of definite

fissures, separating it from the surrounding parts (Gf Fig. 53, p. 127). In all mammalian brains,

in particular, we can trace on the median surface of the hemispheres the margin along which

the investment of the inner portion of the callosal gyre is deflected into the medicornu of the

paracele (hippocampal fissure: fh Fig. 56, p. 130); while in most of them the callosal gyre is

also bounded during its course upwards over the callosum, by a longitudinal fissure, the

callosal fissure (sulcus callosomarginalis: C Fig. 53). In the same way, the olfactory lobe or

olfactory gyre at the base of the prosencephalon is almost always set off by an inner and an

outer fissure, the entorhinal and ectorhinal fissures; though in the human brain the two have

fused to one (sr Fig. 52, p. 125). [p. 140] All these fissures and furrows are occasioned, then,

by the growth of the hemispheres round their point of application to the diencephalon (Sylvian

fissure), by the occlusion of the external fissure of the medicornu (hippocampal fissure), or by

the course of determinate bundles of myelinated fibres appearing on the ventral and median

surface of the hemispheres (callosal, entorhinal and ectorhinal fissures). Since the structural

relations that condition them are characteristic of the mammals as a class, they form, as soon

as they can be traced at all, entirely constant features of the mammalian brain.

But there are other fissures, less uniform in their course, which give the brain-pallium of the

mammals a variously convoluted appearance. The surface of cerebrum and cerebellum is split

up by them into numerous gyres. The cerebellar gyres are, on the whole, arranged with more

regularity than the cerebral; they form narrow ridges, set vertically upon the underlying alba,

and following for the most part a transverse direction. On the cerebrum, whose folds are not

unlike the convolutions of the intestine, it is often difficult to recognise any definite law of gyre-

formation. The common cause of all these ridges and foldings of the brain-surface is evidently

to be found in the disproportionate growth of the cortex and of the myelinated tracts that radiate

into it. When a body increases in mass, its surface, of course, enlarges less rapidly than its

volume. But the cells of the brain surface have to take up the fibres of the interior alba; and so

surface-extent must be roughly proportional to volume, and this relation must be maintained

with approximate constancy throughout the whole period of development. It is obvious, then,

that the cortex has no way of keeping pace with the increase of alba except by folding. And it is

for this reason that, both in the organic series and in the course of individual development, the

convolution of the brain-surface increases with increase of the size of the brain.

The convolution of the cerebellum is found in its simplest form in the birds. The avian

cerebellum has no pilea, and so appears, in dorsal aspect, as an unpaired structure of a more

or less spherical or ovoid form. The surface of this organ us split up into transverse folds,

roughly circular or elliptical in shape, all intersecting in an axis laid transversely through the

centre of the sphere or ovoid: the transverse axis is, therefore, in this case the common axis of

convolution for all gyres visible upon the surface of the cerebellum (Fig. 49 A, p. 121). If we

bisect the organ at right angles to the direction of this axis, we find that the depth of the fissures

bounding the separate prominences varies: the ridges fall into groups of two or three, marked

off from one another by shallow depressions, but divided from the neighbouring groups by

deeper fissures (Fig. 40 B, p. 110). In the mammals, the convolution becomes more

complicated; each of the groups of ridge-like prominences marked off by the deeper fissures

contains a large number of [p. 141] separate gyres. Moreover, it frequently happens that

several of these groups are isolated by dividing fissures, and so form still larger units, lobes.

The consequence is that most of the gyres come to lie in the depth of the larger fissures, and

that only the terminal lamellae appear on the surface. Hence we have in sections the well

known representation of a tree, with its spreading leaves and branches, termed by the older

anatomists the arbor vitae (av Fig. 50, p. 121; cf. also W Fig. 53, p. 127) Further, while this

fissural differentiation is in progress, the median portion or vermis of the cerebellum is

reinforced by the large, bilaterally symmetrical pilea. If the arrangement of gyres upon the pilea

is fairly regular, as in man, the principal axis of convolution is again the transverse. The rule is

broken, however, along the anterior and posterior margins, where the gyres gradually change

their course to take an oblique or even longitudinal direction, all alike converging towards the

point of attachment of pileum to vermis (Fig. 50). For the rest, many mammalian cerebella

evince a great variety in the course of their gyres, more especially upon the pilear surfaces, so

that no definite law of gyre-formation can be made out. This is most apt to occur in brains

whose cerebellum is richly convoluted. In the human cerebellum itself there are certain

divisions, isolated by important fissures,[10] in which the course of the gyres diverges more or

less widely from the general trend.

An increase by convolution of the superficial extent of the cerebrum is found only in the higher

vertebrate classes. And the brains even of the lowest orders of mammals show, at most, only

the fissures and gyres described above (Sylvian fissure, hippocampal fissure, etc.), which are

due to special causes, distinct from those that produce the remaining ridges and furrows. When

once these latter have appeared, however, they persist in practically the same pattern

throughout the mammalian series, up to and including the primates. The rule is that all fissures

and gyres running from before backwards take a course which is approximately parallel to that

of the median fissure; most of them are also reflected in a curve round the Sylvian fissure (cf.

Fig. 61 I, II, III, p. 138). That is to say, the gyres take the same direction that the hemispheres

take in their growth round the brain-caudex, -- the direction from before backwards; and they

are reflected round the point of application to the diencephalon in a curve which repeats the

curve of hemicerebral growth. The degree of curvature is determined by the depth and extent of

the Sylvian fossa or fissure. The number of longitudinal folds which thus appear on the surface

of the cerebrum varies in general, in the various mammalian orders, between two and five. In

many cases, one or another of them forms a junction, at some [p. 142] point of its course, with

the neighbouring fold; and very frequently less well marked, secondary folds are formed,

making an angle with the primary. The convolution thus appears as an irregularly meandering

system, in which the general law of direction is more or less obscured. The case is very

different with the formation of folds in the frontal region of the cerebrum. A little anteriorly to the

Sylvian fissure, the longitudinal gyre-tract is replaced, suddenly or gradually, by an

approximately transverse ridging. At the same time, the secondary cross-fissures are often

disposed radially about the Sylvian fissure. In accounting for this mode of fissure-formation in

the frontal brain-region, we must remember that in all mammals, except the cetacea and

primates (the orders in which the olfactory gyres are to some extent degenerate), the callosa1

gyre comes to the surface in the frontal area, and is separted from the more posterior gyres, at

its point of issue, by a transverse or oblique fissure; anteriorly, it passes directly over into the

olfactory gyre, from which it is also separated, though as a rule less distinctly, by another cross-

fissure (Fig. 61, Gf). The point at which the callosal gyre comes to the surface lies in some

instances at a very short distance from the anterior boundary of the brain. This is the case e.g.

in the carnivores, where the gyre is exceedingly broad, so that it and the olfactory gyre together

occupy the entire area elsewhere taken up by the frontal brain. In other brains, the point of

issue lies more posteriorly. The exposed portion of the callosal gyre is then, as a general rule,

longer than it is broad; it merely fills a narrow space to either side of the anterior part of the

intercerebral fissure. The cross-formation of which we are speaking is not confined, however, to

the folds caused by the protrusion of the callosal and olfactory gyres; all the other fissures of

the anterior brain-region take the same transverse direction. In some cases, the folds that run

longitudinally in the occipital area are deflected anteriorly into a transverse course; in others,

the longitudinal furrows are suddenly interrupted, and replaced by cross-fissures. We have a

salient instance of the former arrangement in the brain of the carnivores (Fig. 61), which is

characterised by the regularity and symmetry of its convolution; while the second type is

represented by most of the other richly convoluted mammalian brains, though here, too, certain

of the longitudinal fissures are not infrequently continued in the transverse direction. There are,

as a rule, two principal fissures of this and crossing the frontal brain, and the number holds,

whether the fissures are completely independent or pass over posteriorly into longitudinal

fissures. To these must be added, further, the posterior limiting fissure of the callosal gyre, and

the fissure separating the callosal and olfactory gyres. There is thus, upon the average, a total

of four transverse fissures in the frontal brain-region.[11][p. 143]

All these folds, longitudinal and transverse alike, are ordinarily visible only on the upper and

outer surface of the hemispheres.

The base of the brain is, in most instances,

entirely occupied by the gyres and fissures

previously described, -- anteriorly, i.e., by the

olfactory gyre, and posteriorly by the

hippocampal gyre (ob, H Fig. 61). If anything else

can be seen, it is at most simply a narrow margin

belonging to the outermost gyres of the brain-

surface. Again, the greater number of brains,

when viewed in median section, show only the

callosal gyre and its continuations, posteriorly

into the hippocampal gyre, and anteriorly into the

olfactory gyre (Fig. 63); though in certain cases,

where these structures are not prominent -- as in

the brains of the cetacea, of the apes and of man

-- portions of the superficial gyre-tracts may also

make their appearance. But these brains deviate

markedly in other respects as well from the

general law of fissuration evinced by the mammalian brain. In the cetacea, whose organs of

smell, peripheral and central, are completely atrophied, the callosal gyre does not come to the

surface, and no olfactory gyre is present at all. The brain is extraordinarily broad; and the

principal superficial fissures run over its whole length from before backwards, as they do in the

occipital brain-region of the other mammals.

In the primates, the fissuration of the brain follows a special law of evolution, characteristic of

the order. The olfactory gyre, which has dwindled into the olfactory bulb, lies concealed upon

the brain-base. The callosal gyre appears on the surface, but only in the occipital, not in the

frontal region of the brain. As the gyre curves posteriorly round the splenium, on its passage

into the uncus, it sends off a branch to the surface of the occipital lobe. This divides into two

lobules, the cuneus and precuneus (Pr, Cn Fig. 64). The intruding area forms a sort of island,

bounded before and behind by other gyres, from which it is ordinarily divided by fissures. The

cuneus and precuneus are also themselves separated by a deep transverse fissure, the

occipital fissure (O). The same transverse arrangement of gyres and fissures obtains, further,

over the entire occipital portion of the brain, from

the anterior extremity of the main ramus of

the [p. 144] Sylvian fissure (the Sylvian

fissure in the narrower sense) to the

extreme posterior limit of the occipital lobe.

Anteriorly, the principal fissure which runs in

the transverse direction, from above

downwards, is the central fissure (fissure of

Rolando: R Fig. 65). It is bounded before

and behind, in the brain of man and of the

higher apes, by two transverse folds, the

precentral and postcentral gyres (VC, HC

Fig. 65). These are separated from the

adjacent parts -- the precentral from the

frontal gyres, and the postcentral gyre from

the precuneus -- by shorter cross-fissures.

Finally, a deep transverse fissure is found at

the posterior limit of the occipital brain. This

is the calcarine fissure, which separates the

cuneus from the gyres extending downward

to the brain-base (O'). We have, then, in all,

on the surface of the occipital brain, five well-marked transverse fissures, three of which belong to

the offshoots of the callosal gyre and the adjoining parts. On the other hand, the fissures and

gyres of the frontal and temporal areas -- i.e. of the region anterior to the presylvian and ventral to

the subsylvian fissures -- tend in general to pursue a longitudinal course, during which they are

necessarily reflected in a curve round the main ramus of the Sylvian fissure. Three of these

longitudinal folds can be distinguished in the frontal, and three in the temporal region; they

constitute the three frontal and temporal gyres (F1-F3, T1-T3), all of which run far enough to be

visible at the base of the brain (Fig. 52, p. 125). The three temporal are adjoined posteriorly to the

three occipital gyres (O1-O3). In the area of transition from the occipital to the temporal region, the

folds follow a course that lies midway between the transverse and the longitudinal; so that the

three parietal gyres (P1-P3) show a gradual passage from the one to the other direction. There is

no parallel to this in the frontal region: the three frontal gyres are interrupted suddenly and at right

angles by the downward course of the precentral. The essential difference, then, between the

brain of the primates [p. 145] and that of the other mammals lies in the fact that the primates have

the transverse fissures in the occipital, and the longitudinal fissures in the frontal region, whereas

most of the other mammals have these relations exactly reversed. There is a corresponding

difference in the course of the callosal gyre. In the primates, the callosal gyre comes to the

surface in the posterior, in the lower mammals in the anterior region of the brain: a fact which can

be brought out most clearly by the comparison of a primate brain and the brain of another

mammal in median section (Figg. 63, 64). These differences are probably referable to the

divergence of the laws of growth in the two brain-forms. In most mammals, the developing brain

has a strong lateral growth in the occipital region, while the frontal area remains narrow; so that

the whole organ is wedge shaped, tapering from behind forwards. In the brain of the primates, on

the contrary, the growth of the occipital

region is predominantly in the longitudinal, and the growth of the frontal region predominantly in

the lateral direction. The organ thus assumes the form of an ovoid, whose lateral halves are

closely apposed anteriorly, but posteriorly gape apart; while the lesser height of the posterior

portion leaves room for the growth of the cerebellum beneath it. Embryology tells us that the

cross-fissures upon the cerebrum of man (and probably of the primates at large) represent the

original foldings of the cortex.[p. 146] According to ECKER, they begin to form upon the smooth

cerebral surface as early as the fifth month, whereas the first traces of longitudinal fissuration

make their appearance in the course of the seventh month.[12] Four or five of these transverse

fissures, disposed radially

with more or less of

regularity about the

Sylvian fissure, may be

traced upon the foetal

brain. The most strongly

marked among them

becomes the central

fissure. In the apes, the

central fissure is less

prominent than in man;

but this defect is

compensated by the

greater development of

the more posteriorly

situated occipital fissure,

which has accordingly

been termed the ape-cleft

(Affenspalte).[13] The

calcarine fissure, lying still more posteriorly, can hardly been seen at all on the human brain

except in median section (Fig. 53, p. 127, and O', Fig. 65). It is this fissure whose projection into

the postcornu of the paracele forms the calcar of the primate brain (vk Fig. 55, p. 129). In man,

it joins the occipital fissure at an acute angle, so that the cuneus shows as a wedge-shaped

lobule, apparently isolated from the callosal gyre (Cn Fig 53). In the apes, the calcarine fissure

is shallower, and the connexion of the cuneus with the callosal gyre can accordingly be seen at

once (Fig. 64). But while the portion of the primate brain that lies behind the central fissure thus

developes several well marked cross-fissures, the transverse fissuration of the anterior half is

much less pronounced. In place of it, we find throughout the frontal and temporal regions the

longitudinal fissures and gyres which we have mentioned as appearing at a later stage of

embryonic development. The three longitudinal folds that are distinguishable upon every

primate brain give rise to the three frontal (super-, medi-, sub-) and the three temporal (super-,

medi-, sub-) gyre-tracts (Fig. 65). These, however, do not form (as they do in many of the other

mammals) a, continuous convolution, arching over the Sylvian fissure. On the one hand, the

three frontal gyres are interrupted by the precentral gyre. On the other, only one of the temporal

gyres (the supertemporal) runs a complete course to the postcentral gyre, curving boldly

upwards round main ramus of the Sylvian fissure; the other two are deflected by the precuneus

and cuneus, the two lobules bounded by the other radial fissures of the occipital brain, and then

continued on the parietal brain-surface into the three parietal gyres (P1-P3). These latter,

situated as they are at the meeting point of different gyre systems, show the least regular

course of all: the third parietal gyre (P3), which bounds the posterior extremity of the Sylvian

fissure, has been named from its shape and position the angular gyre. On the median surface

of the brain, the first and second parietal gyres form the boundary of the precuneus and cuneus

(Pr, Cn, Fig. 53, p. 127) which here appear as direct continuations of the callosal gyre (Gf). On

the brain-base, the subtemporal gyre is connected anteriorly with the club-shaped termination

of the hippocampal gyre; posteriorly, it is continuous with the external ramus of an U-shaped

gyre-tract which occupies the base of the occipital brain and whose inner ramus is lost in the

stem of the hippocampal gyre (O Fig. 52, p. 125). The anterior portion of the brain-base is

occupied by the ventral deflections of the three frontal gyres. The medifrontal and subfrontal

become continuous on the margin of the Sylvian fissure (F2, F3, Fig. 52)[14][p. 147]

The law of fissuration of the brain-surface is, in the author's opinion, the resultant effect of two

sets of causes: of the tensions produced by the growth of the brain itself, and of the influence

upon the brain of the enclosing skull- capsule. The former condition may be appealed to in

explanation of the fissures that appear at the earliest period of development. If a surface is to

increase its extent by fold-formation, its ridges will of necessity follow the direction of least

resistance. If the surface is more tensely stretched in the transverse than in the longitudinal

direction, therefore, it will fall into transverse folds, or ridge up about a transverse axis, just like

a damp piece of paper that one holds in the hands and pulls out to right and left. If, on the other

hand, the tension is greater in the longitudinal direction, then the folds will be longitudinal, the

ridges disposed about a longitudinal axis. Suppose, then, that the fissuration is uniform, all the

folds takings a single direction: this will mean that the difference of surface-tension remained

constant over the whole period of growth. Irregularity of folding will mean, on the contrary, that

the line of greatest tension has varied. Now if a structure grows in different directions with

varying rapidity, its surface-tensions must vary in these same directions; and the direction of

greatest tension must lie at right angles to the direction of greatest energy of growth. A growing

structure, i.e., may be regarded as a coherent elastic body, in which the deformation caused by

growth at any given point increases the tension at all other points, the tension reaching its

maximum in the parts where the intrinsic deformation is minimal. The fissuration of the

cerebellum, where the laws of growth and folding are comparatively simple, seems to confirm

this principle; and the confirmation is all the more striking, since the position of the organ may

be supposed to exempt it from the influences of the skull-form. The growth Of the cerebellum,

during the entire period of its development, is predominantly in the longitudinal direction. Its

greatest surface-tension must, therefore, lie at right angles, in the transverse direction; and this

is, as a matter of fact, the direction of its fissuration. We should expect, on the same principle,

that the fold-formation of the primate cerebrum would show two stages, corresponding to two

distinct periods of growth: a first, in which the direction of maximal growth is the same for the

whole brain, from before backwards; and a second, in which the energy of growth over the

frontal and temporal regions is greater in the transverse direction. As a matter of fact, a

comparison of embryonic brains at different stages of development shows, at the first glance,

that the ratio of the two principal diameters of the human brain changes very considerably

during the period of structural elaboration. In the first weeks of development, the whole brain is

approximately spherical in form; the longitudinal diameter is but little larger than the greatest

transverse diameter. The line of greatest breadth crosses the brain behind the Sylvian fissure,

or, more correctly, since the temporal lobes are not yet developed, behind the Sylvian fossa. As

the fossa closes to form the fissure, the greatest transverse diameter is shifted anteriorly,

crossing the brain at the point where the temporal lobes grow over the margin of the fissure.

During this whole period, however, the longitudinal diameter of the hemispheres is gaining

more and more upon the cross-diameter; so that as early as the third month the ratio of the two

is 1: 0.9, while in the course of the fifth and sixth it has fallen to 1: 0.7. This is the time of

formation of the first permanent fissures, all of which, without exception, take the transverse

direction.[p. 148]

During the fifth month, the central, occipital, and calcarine fissures make their appearance; and

in the course of the sixth, the other primary radial fissures are added to their number.[15] From

the end of the sixth month, the laws of brain-growth begin to change. The total form of the

brain, as expressed in the ratio of the longitudinal to the greatest transverse diameter, remains

practically the same; but the growth of the various parts differs widely from the rule that has

previously obtained. If we compare the dorsal aspects of foetal brains, from the sixth to the

seventh month, we remark at once that the region extending posteriorly from the central fissure

is increasing in length and breadth in approximately the same ratio, but that the frontal portion

of the brain is growing more strongly in the transverse than in the longitudinal direction. The

temporal lobe undergoes a similar modification. Even in the foetus of six months, its anterior

extremity extends almost as far as the ventrally deflected margin of the frontal lobe; but it is still

narrow, so that the Sylvian fossa lies wide open. In the following months, the growth of the

temporal lobe is most vigorous in the upward, and comparatively slight in the longitudinal

direction, and, as a result, the fossa closes to form the fissure. Now the changes here sketched

are exactly coincident with the formation of the second system of folds, the longitudinal

fissures. Since the growth of the frontal brain is predominantly a transverse growth, the frontal

gyres must take a predominantly longitudinal direction. And since the most rapid growth of the

temporal lobes is upwards, the folds formed upon them must again run from behind forwards,

taking a curved path round the Sylvian fissure. Not only do the newly formed fissures follow this

direction, over both regions of the brain surface, but some of those that were originally

disposed radially about the Sylvian fissure have their course changed to the longitudinal or

arching type. Thus the central fissure itself assumes an oblique position; and the subfrontal and

supertemporal fissures, whose primules in the sixth month are radial or transverse, have their

direction altered, and take their places in the longitudinal fissure system. The case is different

with the portion of the brain-surface that lies between the central fissure and the posterior

margin of the occipital lobe. Here the transverse fissures, while they increase in depth and

extent, for the most part retain their original positions, and only gradually, as they approach the

temporal lobe, pass over into a longitudinal path.[16]

The resistance of the skull-capsule is, necessarily, antagonistic to brain-growth. It is probable,

however, that this influence is exerted only in the latest period of the embryonic life, and for a

time after birth, before the skull has acquired its permanent form. Its principal condition is the

varying rapidity of bone-growth along the various sutures, and the successive closure of these

as growth advances. If the growing brain meets with an external resistance such as would be

offered by the skull, it must evidently fold upon itself; and the direction of the folds will be the

direction of least resistance. Where the skull is of the dolichocephalic form, the course of the

fissures will thus be predominantly longitudinal, and where it is brachycephaiic, predominantly

transverse in direction. As a matter of fact, a connexion of this kind between skull-form and

domi-[p. 149]nant gyre-trend has been made out by L. MEYER [17] and RÜDINGER.[18] But

the actual convolution of a given brain will, of course, always be the resultant of the two sets of

conditions: the intrinsic tension; of growth, and the extrinsic resistances. The former will

manifest themselves most clearly in the course of the original fissures; the latter in the

modifications superinduced upon the original convolution at a later stage of development.

Notes

[1] The 'ventricles' are counted from before backwards, with the mesocele (Sylvian aqueduct)

omitted: two lateral ventricles (paraceles), third ventricle (diacele), fourth ventricle (metacele or

metepicele).

[2] HIS, Die Formentwickelung des menschlichen Vorderhirns. In Abh. d. k. sächs. Ges. d.

Wiss., math.-phys. Cl., xv., 1890, 675.

[3] In birds, the sinus rhomboidalis is not closed by proliferation of the nerve-mass, but remains

permanently open. It thus resembles the fossa rhomboidalis, the continuation of the myelocele

in the oblongata, which persists as an open depression in all vertebrate forms.

[4] Cf. below, Ch. vii. This 3

[5] These are the oculomotor and the trochlear nerves. The third oculomotor nerve, the fifth

cranial or abducent ocular, arises in the most anterior region of the oblongata.

[6] Many anatomists restrict the name striatum to the caudatum alone, i.e., do not extend it to

embrace the lenticula, The claustrum and amygdala must be regarded, from the form of their

cells, not as true ganglionic nidi, but as parts of the cortical cinerea, from which they are

separated by an intercalated lamina of alba.

[7] The question whether the apes have, like man, a postcornu to the paracele and a calcar

(pes hippocampi minor) led to a not very profitable controversy between OWEN, who took the

negative side, and HUXLEY. Cf. HUXLEY, Evidence as to Man's Place in Nature, 1863, 100,

113. The older writers on the ape-brain figure the postcornu. Cf. e.g. TIEDEMANN, Iconces

cerebri, 1825, 54. OWEN himself, in his later work, describes the rudiment of a postcornu in the

dolphin (Anatomy of Vertebrates, iii. 10). In the anthropoid apes, as HUXLEY has shown, the

calcar exists as well as the postcornu, only at a lower stage of development than in man.

[8] In human anatomy, the portion of the callosum which connects the two hippocampi is

termed the psalterium.

[9] The nerves of Lancisi, or striae mediae, belong not to the callosal gyre, but to the callosum

itself. See sm Fig. 58.

[10] Here belong more especially the flocculus (fl Fig. 52, p. 125) a small feather-like outgrowth

from the posterior margin of the middle peduncle (crus ad pontem), and the amygdala or tonsil

(to), an ovoid prominence covering the oblongata between the inferior portion of the vermis and

the pileum of either side.

[11] In the first three editions of this work, the statements of the text are illustrated by cuts of a

series of mammalian brains. See 3rd ed. Fig. 48, p. 86.

[12] ECKER, Archiv f. Anthropologie, iii., 1868, 203. PANSCH, ibid., 227.

[13] The morphological relations of the Affenspalte (fissura perpendicularis externa; Wilder's

pomatic fissure) are, in reality, more complicated than the author here represents them. --

TRANSLATOR.

[14] BISCHOFF, Abhandl. d. bayer. Akad. d. Wiss., x., 1868. ECKER, Die Hirnwindungen des

Menschen, 1869. PANSCH, Die Furchen und Wülste am Grosshirn des Menschen, 1879.

FLATAU and JACOBSOHN, Handbuch der Anatomie und vergleichenden Anatomie de

Centralnervensystems der Säugethiere, i., 1899.

[15] ECKER. Archiv f. Anthropologie, iii., 1868, 212. Cf. the cuts of embryonic brains in the first

three editions of this work. 3rd ed., Fig. 52, p. 93.

[16] In the first edition of this work (p. 101), I have cited measurements of embryonic brains,

which serve to confirm the statements of the text.

[17] Centralblatt f. d. medicin. Wiss., 1876, No. 43.

[18] RÜDINGER. Ueber d. Unterschiede d. Grosshirnwindungen nach d. Geschlect beim Fötus

u. Neugeborenen, 1877, 5 ff.

Footnotes

[1] This figure [Fig. 53], with several others, has been printed without change in all editions of

the Physiologische Psychologie. The reader will observe that the line from the abbreviation cb

is carried to what has the general position of the terms (lamita terminalis). The real copula (the

weisse Bodencommissur of the German; Henle's commissara baseos alba) is neither

designated nor even shown in the Fig. It would be waste of space to point out in detail all slips

of this sort. They are, fortunately, irrelevant to the course of the author's argument. But the

reader should be warned that these old figures would not pass muster with modern anatomists.

-- TRANSLATOR.

[2] This Figure [Fig. 65] would, perhaps, be more nearly typical if the postcentral fissure were

separated off from the rest of the fissural complex with which the author has connected it.

Again, it is not probable that the calcarine fissure ever appears thus plainly on the lateral

surface of the brain (cf. the author's remarks in the text below); though, indeed, but little is

certainly known of the fissuration of the occipital lobe. -- TRANSLATOR.

Work done in this way is termed work of disgregation. It may be transformed back again into

molecular work, as the particles return to their original positions. In general then, molecular

work may be transformed either into mechanical action or into work of disgregation, and both of

these in their turn may be transformed into molecular work. Now the sum of these three forms

of work remains unchanged. This is [p. 61] the, principle of the conservation of work: or, if we

choose a name which will permit us, in other contexts, to abstract from that mechanical

interpretation of natural processes to which we here stand committed, the principle of the

conservation of energy.

This principle is applicable not only to heat, the most general and most widely diffused form of

motion, but to other forms as well. In every case, it is always just the one term in the chain of

the three interchangeable motions, the character of the molecular work, that is changed. Work

of disgregation and mechanical work can be done, e.g., by electricity as well as by heat. There

are, therefore, various kinds of molecular work; but there is in the last resort only one work of

disgregation, as there is only one form of mechanical work. Disgregation is the name given, in

every instance, to a permanent change of the distances separating the molecules, no matter

what cause has produced it. When we distinguish a simple increase in the volume of a body

from a change of its aggregate condition, and this again from chemical decomposition, or

dissociation, we are really distinguishing nothing more than three degrees of disgregation.

Mechanical work, in the same way, consists always and everywhere in the change of position

of ponderable masses. It should be noted that the different forms of molecular work may also,

under certain circumstances, be transformed into one another. Thus, a certain quantum of

electrical work may give rise, simultaneously, to heat, disgregation and mechanical work.

It is from mechanical work that the idea of work, in the abstract, has been derived. And it is

mechanical work that is selected, from the various forms of work mentioned above, to serve as

a common measure of work at large. The reason is, that mechanical work can be most

accurately measured, and that the only possibility of a comparison of the different forms of work

is given with the reduction of all to one. This measure, now, is applied in the special case by

help of the principle of the conservation of work, which lays it down that a given amount of

molecular work or work of disgregation is equivalent to the mechanical work into which it is

transformed or from which it is generated. In the performance of mechanical work, a

ponderable body may be lifted, against the force of gravity, or moved by its own weight, or

accelerated in spite of friction, and so on. In the latter event, the portion of mechanical work

necessary to overcome friction is transformed into heat. Where the body is lifted, we suppose

that the work employed for the lifting is stored up within it, since this work call be passed on

again to other bodies, by a subsequent fall of the weight from the same height. Disgregation

behaves, in this regard, just as the lifted weight does: a certain quantity of molecular work,

mostly in the form of heat, is used up in its production, and this same quantity must reappear

as soon as the disgregation is abolished. But a lifted weight remains lifted [p. 62] so long as its

weight is held in equilibrium by some other form of work, e.g. by the heat-motion of expanded

steam. In the same way, the disgregation of the molecules of a body persists, so long as their

reunion is prevented by some form of internal work, e.g. by heat-vibrations. Hence, between the

moment at which the weight is lifted or the disgregation of the molecules effected, and the

moment at which the work required for these operations is reproduced by the fall of the weight

or the union of the molecules, there may intervene a static condition, continuing for a longer or

shorter time, throughout which just so much internal work is being done as is necessary for the

maintenance of equilibrium, -- so that no alteration takes place in the existing status, in the

position of bodies and their molecules, in temperature, in electrical distribution. Only at the

moment when this state of equilibrium is disturbed, when the weight falls or the molecules

approach one another, do transformations of work set in again. The mechanical work or work of

disgregation is now transformed first of all into molecular work, usually into heat, and this may

in its turn pass over in part into mechanical action or disgregation of molecules; the

transformations continuing, until circumstances occur that favour the reinstatement of the

stationary condition. Since, now, there is a certain sum of work available, in a lifted weight or in

disgregated molecules, we may consider every lifted weight and every disgregation as potential

work or work of position. The amount of this potential work is always precisely the same as the

amount of work that was required to effect the lift or the disgregation, and as the amount of

work that may reappear in consequence of fall or of aggregation. The law of the conservation of

work may, accordingly, be expressed in other terms as follows: the sum of actual and of

potential work, of work of position and work of motion, remains constant. It is clear that this is

only a special way of formulating our previous law of the conservation of the sum of work; for

we always mean by work of position a lift or a disgregation accomplished by expenditure of

actual work, and maintained by a stationary condition of tension or motion. If we could observe

the smallest oscillatory movements of the atoms as well as the motions of bodies and the

permanent changes of position that they undergo, the law would hold of these atomic

movements also, that the sun of actual and potential work remains unchanged. In actual fact,

however, where the particles of the mass are in constant motion about approximately the same

positions of equilibrium, matter appears to us to be at rest. We accordingly term the work done,

invisibly to us, in a stationary condition, 'internal' molecular work, thus distinguishing it from the

molecular work which arises when there is a change in the state of equilibrium as regards

temperature, electric distribution, etc., and which we call 'external' molecular work.[p. 63]

These stationary conditions are continually alternating with changes of state. The stage of

nature is thus occupied, in never-ending succession, with the passage from internal to external

and from external back again to internal molecular work. It will suffice here to give illustrations

of the processes that have the most direct bearing upon our own problem, -- illustrations of

disgregation and its reversal. Differences in aggregate condition depend, it is supposed, upon

different states of molecular motion. The molecules of a gas repel one another, and

consequently continue to move, in rectilinear paths, until such time as they strike the wall of the

containing vessel, or other molecules, from which they rebound. In liquids, the molecules

oscillate about instable, in solids, about stable positions of equilibrium. If, now, we are e.g. to

transform a liquid into a gas, we must increase the work of the molecules. We do this by the

application of heat. So long as only the molecular work of the liquid increases, nothing results

but an increase of its temperature. But if, at the same time, we allow the liquid to expand, then

a part of its molecular work is further transformed into disgregation. Finally, if the application of

heat is continued, and the disgregation carried to the point at which the particles of the liquid

travel beyond the spheres of their mutual attraction, the liquid is suddenly transformed into gas

or steam: it now enters upon a new state of equilibrium, in the production of which a large

amount of molecular work, i.e. of heat, has been consumed. If heat is now withdrawn from the

steam, so that its internal work is diminished, a point will be reached, on the backward path, at

which the average distance between the molecules is sufficiently reduced to bring them once

more within the limits of their mutual attraction. With the supervention of this original position or

equilibrium, molecular work must be done, i.e. heat be liberated, as a result of the renewed

activity of the forces of attraction; and the amount of heat thus disengaged is precisely the

same as that consumed in the first instance.

What holds in this case holds, in practically the same way, for the decomposition and

recomposition of chemical compounds. In every substance we can distinguish between the

state of physical and the state of chemical equilibrium. For every molecule, in the physical

sense, consists of a number of chemical molecules or (to use the term applied to the

indecomposable chemical molecule) of a number of atoms. Just, then, as the molecules may

exist in different conditions of motion, varying with the aggregate state of the body in question,

so may the atoms also, according to the character o the chemical compound. Modern

chemistry regards all bodies as compounds; chemically simple bodies are looked upon as

compounds of homogeneous atoms. Hydrogen gas is thus every whit as much a chemical

compound as is hydrochloric acid: in the former, [p. 64] two atoms of hydrogen are

compounded together (H.H), in the latter, one atom of hydrogen is compounded with one of

chlorine (H.Cl). Here again, however, what appears to be matter at rest is in reality only a

stationary condition of motion. The atoms in a chemical compound oscillate, it is supposed,

about more or less stable positions of equilibrium. The character of this motion is, at the same

time, strongly influenced by the aggregate condition of the compound, regarded as a physical

body. Thus, in gases and liquids, the state of motion of the chemical atoms is, as a rule,

comparatively free; atoms are occasionally torn from their connexions, and at once compound

again with other atoms that have been similarly released. In hydrochloric acid, for instance,

gaseous or liquid, the average composition of all chemical molecules is HCl. Nevertheless,

separate atoms H and Cl are constantly occurring in the free state, though they cannot maintain

it, but are always compelled at once, by the forces of chemical attraction, to enter again into

combination. From this point of view we gain a satisfactory explanation of the ready

decomposability of gases and liquids in face of heat, electricity and other chemical compounds.

[2]

We find, once more, in the aggregation of chemical molecules, differences analogous to those

which we have noted in the aggregate states of physical bodies. There are relatively stable and

relatively instable chemical compounds. In the former, the forces of attraction, in virtue of which

the particles vibrate about certain determinate positions of equilibrium, are stronger; in the

latter, weaker. These differences of chemical aggregation are, of course, altogether

independent of the physical since the physical molecules are always, to start with, chemical

aggregates. Very stable compounds may accordingly occur in the gaseous state, and very

unstable in the aggregate state of solidity. In general the compounds of homogeneous atoms,

the chemically simple substances, belong to the less stable compounds; most of them, certain

of the metals excepted, decompose fairly easily to form compounds with heterogeneous atoms.

The same thing is true, on the other hand, of extremely complex compounds, which readily

break up into simpler. Here belong most of the 'organic' substances. It follows, then, that stable

chemical compounds are to be found predominantly among the simpler connexions of

heterogeneous atoms. Thus, carbonic acid, water, ammonia, and many of the metallic oxides

and inorganic acids are decomposed only with difficulty. Just, however, as the different

aggregate states can be transformed into one another, so may relatively instable compounds

be transformed into stable, and conversely. There is, as ST. CLAIRE DEVILLE proved, no

compound so stable that it cannot be dissociated by the application of heat in sufficient

quantity. Here, as in the change of a liquid into a gas, a certain amount of the [p. 65] internal

work of the heat disappears, transformed into work of dissociation. When the dissociation is

complete, the atoms are in a new state of equilibrium. In the dissociation of water, e.g., the

more stable connection H2O gives place to the less stable forms H.H and O.O, in which the

vibratory condition of the atoms differs from that in the stable compound H2O very much as the

vibratory condition of the molecules of steam differs from that of the molecules of water; that is

to say, the atoms in their new, instable connexions will, on the whole, describe longer paths,

and consequently do more internal molecular work. To make up this deficiency, heat is

necessary. The work thus expended upon dissociation is, however, still present as potential

work: for when the new state of equilibrium of the dissociated molecules is disturbed, they are

able to compound again, and the work of dissociation once more makes itself apparent in the

form of heat. The chemical molecules have, at the same time, passed into their former

condition of equilibrium, where the stationary work which they perform in movements about

their positions of equilibrium is diminished by the amount of the internal work released in the act

of composition. We see, then, that the phenomena connected with composition and dissociation

are identical with the phenomena observed in the alternation of aggregate states, save only that

much larger amounts of work are usually required for dissociation than for disgregation, and

that in the former case the exchange between work of position and work of motion attains

proportionately higher values.

Nervous System

The tissues of the living organism are the seat of chemical processes which, by their great

regularity of occurrence, furnish a remarkable illustration of the alternations of potential and

actual internal and external work. In the plants, we have a constant dissociation of stable

compounds. Carbonic acid, water, ammonia, the nitric acid and sulphuric acid of the nitrates

and sulphates, are taken up by the plant, and decomposed into less stable compounds -- wood

fibre, starch, sugar, albumins, etc. -- in which a large amount of potential work is stored; at the

same time, oxygen is eliminated. These compounds, produced by the plant, are transformed in

the animal body, by help of atmospheric oxygen (i.e. by a process of combustion), into the

more stable compounds from which the plant had derived them; at the same time, the potential

work stored up in the organic compounds goes over into actual work, partly in the form of heat,

partly in that of external work of the contractile substance. The central station, from which all

these processes of the animal body are directed, is the nervous system. [p. 66] It maintains the

functions that subserve the processes of combustion; it regulates the distribution and radiation

of heat; it determines the activity of the muscles. In many cases, it is true, and especially in

cases of muscular action, the issuance of impulses from the nervous system is itself directed

by external movement-processes, the sense-stimuli. The true source of its functional capacity

lies, however, not in these, but in the chemical compounds of which nerve-mass and contractile

substance are composed, and which are taken over, almost without modification, from the living

laboratory of plant-tissue. These contain the store of potential work, which under the influence

of external stimulation is transformed into actual work.

The compounds of which the nerve-mass consists remain, so long as stimulus-processes do

not intervene to modify them, approximately in that stationary condition which appears to

outward observation as a state of rest. This rest is, however, here, as in all such instances of a

stationary condition, only apparent. The atoms of the complex chemical compounds are in

continual motion; now and again, they travel beyond the sphere of operations of the atoms with

which they have hitherto been combined, and come within that of other atoms, freed like

themselves. There is, therefore, in a liquid so easily decomposable as the nerve-mass, a

constant alternation of decomposition and recomposition of chemical compounds, and the

mass appears stationary simply for the reason that, on the average, there are as many

processes of the one kind going on as of the other. In this particular instance, however, we

cannot in strictness say even so much: not even during their period of rest is the state of the

nervous elements really constant and unchanged. With compounds of such complexity, it

invariably happens that certain of the atoms which have been removed from their former sphere

of operations do not, in reuniting, enter into their old connexions, or into connexions of the

same order, but combine afresh to form similar and more stable compound. This process is

termed intrinsic decomposition. In the living organism the disturbances arising from intrinsic

decomposition are compensated by the removal of the products of decomposition, and by the

intake of new materials for the renewal of the constituents of the tissues.

We may, then, consider resting nervous substance as a semisolid mass given in a stationary

condition of motion. In such a mass, there is no release of external work; the work values

produced by the individual atoms cancel one another. This cancellation takes place, in large

measure, within the complex chemical molecules. As the atoms of the molecule oscillate about

their positions of equilibrium, each one of them does a certain work, which, however, is

counteracted by the work of other atoms, and consequently is not perceptible outside the

molecule. This internal [p. 67] molecular work is far more considerable in an instable chemical

compound, owing to the greater freedom of movement possessed by the atoms, than it is in a

stable compound. It is this, therefore, that represents the potential work of the compound. For if

the existing state of equilibrium be disturbed, the relatively instable may pass into a relatively

stable compound; in which event the surplus of internal molecular work contained in the former

is at once transformed into external. To a certain extent, however, the establishment of

equilibrium takes place without the chemical molecule. Where atoms are continually passing

from less stable to more stable connexions, work must appear: where, on the other hand,

atoms are transferred from more stable to less stable connexions, work must correspondingly

disappear: and in both cases it is external molecular work, generally heat, that is produced and

consumed again. We may term the work that appears with the origination of the more stable

compound 'positive' molecular work, and the work that disappears with the formation of the less

stable compound, 'negative' molecular work. The condition of true equilibrium in a

decomposable liquid like the nerve-mass will then be this: that the internal molecular work or

potential work be kept unchanged, by the continual compensation of the existing quantities of

positive and negative external molecular work. Or, to but the same thing in different words: the

internal molecular work must be kept constant by the renewal (through retransformation into

internal molecular work) of all that it loses in external molecular work. What changes, now, are

brought about in this stationary condition of the nerve by the development of the process of

stimulation?

§2. The Course of the Processes of Stimulation in the Nerve-Fibre

(a) -- Course of the Muscular Contraction following Stimulation of the Motor Nerve

The simplest of all the external phenomena that can inform us of the nature of the processes of

stimulation in nerve is the muscular contraction which sets in, and runs its course in time, as a

result of stimulation of a motor nerve. Fig. 24 shows the course of a contraction of this kind in

the gastrocnemius of the frog; a

lever with writing-point was

attached to the muscle, and

recorded the phases of the

contraction directly upon a quickly

moving smoked-glass plate, carried by a heavy pendulum. The conditions under which the

tracing was obtained were made as simple as possible, in order that the course of the

contraction might really be symp-[p. 68]tomatic of the stimulatory process. The muscle carried

no weight beyond the light writing lever, and gave a twitch in response to stimulation.

Numerous observations have shown that the loading of a muscle increases its irritability. Under

the present circumstances this intensifying effect may be regarded as relatively small and the

influence that it exerts upon the various experiments whose results we are to compare, as

sufficiently uniform.[3] The vertical stroke to the left indicates the moment at which the stimulus

was applied to the nerve. The resulting curve, whose axis of abscissas appears by reason of

the movement of the pendulum as an are of a circle, shows that the twitch sets in perceptibly

later than the stimulation, and that the contraction rises at first quickly, then more slowly, to be

followed in like manner by a gradual relaxation. If the stimulus is momentary, the whole twitch is

generally completed in 0.08-0.1 sec. Provided that the nerve is stimulated directly above the

muscle, about 0.01 sec. of this time is lost between the application of stimulus and the

beginning of the twitch; this interval is known as the stage of latent stimulation, or the latent

period. The experiment makes it probable that the movement process in nerve is relatively

slow. Since, however, we have not determined how much of this retardation of the processes is

referable to the inertia of the muscular substance, the result obtained is not of decisive value.

We come closer to the movement in nervous substance itself when we stimulate the nerve at

two different points of its course, the one remote from the muscle, the other as near to it as

possible, and when the experiment is so arranged that the stimulation is timed to occur in both

cases at the same point upon the

axis of abscissas above which the

curve of contraction is described.

If the two stimuli have the same

intensity, and the nerve is kept in

as constant a condition as

possible, the resulting curves evince a twofold difference. In the first place, as HELMHOLTZ

discovered, the curve of contraction given by the more remote stimulus begins later -- has [p.

69] a longer latent period -- than the other. Secondly, as PFLÜGER first showed, the twitch

released higher up the nerve is the stronger; its curve is higher and also, as the author pointed

out, of longer duration. If, therefore, the experimenter desires to obtain two muscle-curves of

the same height, he must apply a somewhat weaker stimulus to the part of the nerve that is

more remote from the muscle. Even then it usually happens, provided the experiment be made

on the living animal, that the corresponding contraction lasts for a little longer time. The two

curves will accordingly differ in the manner indicated in Fig. 25. There is a brief interval between

the starting-points of the contractions, which evidently corresponds to the time which the

excitation requires for propagation from the upper to the lower point of stimulation; and the

twitch released higher up, although in this case it was excited by a weaker stimulus, reaches

the axis of abscissas later than its initial retardation would lead us to expect. We may, then,

conclude from these experiments, first, that the movement-process of stimulation is relatively

slow, -- for the frog-nerve at ordinary summer temperature it averages 26, for the nerves of

warm-blooded animals at normal body temperature 32 m. in the 1 sec., -- and, secondly, that it

consists, in all probability, not in a simple transmission and propagation of the external stimulus

movement, but in a chain of movement-processes released from one point to another within the

nerve itself. This latter inference is borne out, more particularly, by the lengthening of the

contractions which goes with increased distance of the point of stimulation from the muscle.

The phenomenon is altogether constant, and may be observed most strikingly in the uncut

nerves of the living animal.[4]

In order, now, to gain a deeper insight into the course of the phenomena of stimulation, we

must endeavour to inform ourselves of the state of the nerve at each successive moment of the

time following upon stimulation. We may do this, always in terms of the external effects of

nervous activity, by investigating the behaviour of the nerve, at every moment of the period of

stimulation, in face of a second, test-stimulus of constant magnitude.[p. 70] Here, in the case of

the simple muscle-contraction, the properties of the muscular substance itself naturally

contribute their share to the total result. We can, however, eliminate their influence, very much

in the same way that we did in the experiments on the propagation of stimulation. Where the

conditions residing in the muscle remain contact, the observed changes must necessarily

depend upon the processes taking place in the nerve.

(b) -- Excitatory and Inhibitory Processes in Nerve-Stimulation

We must suppose, if we apply the principle of conservation to the processes in the nerve, that

every processes of stimulation produces two opposite effects in the nerve-fibre. The one set of

operations will be directed upon the production of external work (muscular contraction,

development of heat, secretion, stimulation of nerve-cells), and the other upon the recovery of

the work thus liberated. We may term the former the excitatory, and the latter the inhibitory

effects of stimulation. The whole course of the stimulation is then dependent upon the

constantly varying play of excitation and inhibition. In order to demonstrate, by means of our

test-stimulus, which of these processes, excitation or inhibition, has the upper hand, we may

employ either of two different methods. We may work with stimulation-processes of so little

intensity that they are unable of themselves, without the intervention of the test-stimulus, to

release any muscular contraction at all; or we may eliminate the influence of the contraction

itself during the time of its occurrence. We can do this, in cases where we are concerned to

demonstrate an increase of irritability, by overloading the muscle, i.e. by attaching to it so heavy

a weight that both the original twitch and the contraction normally released by the test-stimulus

are suppressed, or at most only a minimal (what is called an 'isometric') twitch remains

possible. If now, during the process of the first stimulation, the test-stimulus nevertheless

releases a more than minimal contraction, we have evidence of an increase of the excitatory

effects and, in the height of the muscle-curve, a rough measure of their magnitude. gives an

illustration of this procedure. The stimulation-process here under investigation was set up by

the closing of a

constant current in

the ascending

direction: the

positive electrode,

that is, lay nearer

to the muscle, and

the negative

farther away from it. The current was closed at the point a. In response to the stimulation, the

muscle (not overloaded) gave the twitch [p. 71] recorded as a'. The load was now attached, and

the muscle-curve reduced by it to the minimal height R. The test-stimulus, employed to test the

state of the nerve in successive phases of the stimulation-process, was the break shock of an

induction-current, applied a short distance below the length of nerve stimulated by the constant

current. So long as the latter was open, the twitch produced by the shock in the overloaded

muscle was also minimal. A series of experiments was then preformed, in which the nerve of

the overloaded muscle was first of all stimulated at a by make of the constant current, and then

again, after a definite period, by application of the test-stimulus. If the two stimuli were

coincident (a), the height of the muscle-curve remained minimal. Where the test-stimulus came

later, the successive times of stimulation b, c, d, e, f, g, gave the contractions b', c', d', e', f', g'.

The course of these curves shows clearly that the stimulated nerve undergoes a change of

state, which manifests itself as an increased irritability. The change begins shortly after the

stimulation a; reaches a maximum that corresponds approximately with the highest points of

the contractions a' and R (e e'); and then gradually decreases again, -- though it persists, as is

shown by the final test g g', for a considerably longer time than the primary twitch a'.

Where it is not, as in the instance here taken, the excitatory, but the inhibitory effects that have

the upper hand, the method of overloading naturally ceases to be applicable. We can, however,

readily infer the presence of inhibitory influences from the magnitude of the effect produced by

the test-stimulus during the progress of the contraction. If, e.g., the test-stimulus produces no

effect whatever, we can argue with perfect certainty to the preponderance of inhibitions. An

illustration of this state of things is given in Fig. 27. The stimulation process here under

investigation was again set up by the making of an ascending constant current; and the test-

stimulus was, as before, the break shock of an induction-current applied below the portion of

nerve stimulated by the constant current. A and B represent two successive experiments, in

each of which the current was closed at a and the test-stimulus thrown in at b. The primary

object of both experiments was to investigate, first, the effect of the current without the test-

stimulus, and secondly, the effect of the test-stimulus without preceding closure of the constant

current: this gave the contractions C and R, which are precisely alike in A and B. In the next

place, the test-stimulus was

applied, at b, immediately after

the closure of the constant current

at a. The results obtained in the

experiments A and B were now

entirely different. In A, a simple

contraction C was recorded,

precisely as if the test-stimulus R

had not operated at all (RC=0); in

B, the curve of contraction was at

first coincident with C, but, when

the time came for the beginning of the contraction R, rose so far above C that RC is higher than

the curves R and C [p. 72] taken together. From this difference of result we may conclude that

in A a strong inhibition persisted during the progress of the stimulation C, while in B there was

either a preponderance of excitatory effects or no change of irritability at all. To decide between

these alternatives, we have only to overload the muscle, in the manner indicated above, and so

to reduce the contractions C and R to zero or to minimal height. Adopting this method, we find

that, as a matter of fact, in experiment R the excitatory effects had the upper hand. -- Now the

difference between the experimental conditions of A and B was this: that in A the test-stimulus

was applied very near the part of the nerve stimulated by the constant current, while in B it lay

nearer the muscle. Hence the experiments show that, in one and the same process of

stimulation, the inhibitory effects may predominate in one portion of a nerve, and the excitatory

in another.[5]

We must not omit to mention the fact that, in all these cases, it depends upon the nature of the

test employed whether the one or the other of the opposed effects, the excitatory or the

inhibitory, is the more clearly demonstrable. Weak stimuli are, without exception, better for the

proof of inhibition, strong stimuli for that of excitation. If, however, we test the same stimulation-

process with weak and strong stimuli alternately, we find in most instances that, during the

greater part of its course, the excitatory and the inhibitory effects are both alike enhanced. At a

phase of the stimulation-process when the effect of weak test-stimuli is wholly suppressed, the

effect of strong stimuli may be increased.[6]

It follows from the above results that, if we desire to gain a quantitative expression of the

relation which the inhibitory effects hear at any given moment to the excitatory, we shall best

have recourse to 'isometric' contractions and to stimuli of moderate intensity, that are, on the

whole,[p. 73] equally sensitive to inhibition and to excitation. Experiments made under these

conditions show that tee stimulation-process developing as the result of a momentary stimulus,

e.g. of an electric shock or mechanical concussion, runs its course as follows. At the moment of

stimulation, and for a brief period afterward, the nerve does not react to the weak test-

stimulation at all; the process takes precisely the same form as it would if the stimulus had not

acted.[7] If, therefore, we apply to the same point upon the nerve or to two neighbouring points,

first a stimulus R (Fig. 28), then a stimulus C, and finally the two stimuli R and C together, the

curve RC recorded in the third case is identical with the more intensive of the two single-

stimulus curves R, C: in our illustration, with R (Fig. 28 A). We obtain the same result if we

allow a very brief interval to elapse between the times of stimulation a, b. So soon as this

interval becomes noticeable, however, the combined stimuli provoke a stronger contraction

than either of them gives separately. Even while the time difference is less than the ordinary

latent period, it not uncommonly happens

that RC is greater than the sum of R and

C: taken together; and the more nearly

minimal the contractions, the greater does

the excess become (Fig. 28 B). This

enhancement of irritability increases up to

a point corresponding roughly with the

maximum of contraction, and then gives

place to a decrease; at the same time, it can be demonstrated for a considerable period after

the conclusion of the twitch. Fig. 26 (p. 70 above) gives a picture of the whole process. We may

say, then, in summary, that the course of the stimulation-process is in general divisible into

three stages: the stage of inexcitability: the stage of increasing, and the final stage of

decreasing excitability.

Oftentimes, however, this third stage is interrupted by a brief interval, during which irritability

shows a sudden marked decrease, quickly followed by another increase. The decrease always

coincides with the conclusion of the twitch. It passes so rapidly, that it can be recognised only

by the increase of the latent period of the test-stimulus; and it is of regular occurrence only

where the functional capacity of nerve and muscle is very high. An instance of this transitory

inhibition at the conclusion of the twitch is given in Fig. 29 A. The contraction to the left

corresponds to the stimulation process under investigation; the unlettered twitch to the right is

the result of simple application of the test-stimulus; while RC is the twitch released by the test-

stimulus under the influence of previous stimulation.[p. 74] The curves of A were obtained from

a fresh nerve, those of B from a nerve that had already been subjected to repeated stimulation.

[8] In this phenomenon, the period following the conclusion of the twitch forms a precise parallel

to the latent period preceding contraction. In both these cases, however, it is not impossible

that the result is in some measure due to conditions residing in the muscle itself. Thus, the

stage of inexcitability,

which appears at the

beginning of stimulation,

may very possibly be

attributable to the fact that

the contractile substance

requires a certain time to

initiate a contraction. In the same way, the stage of diminished excitability that coincides with

the conclusion of the twitch may be explained by the assumption that opposing influences

within the muscle -- already at work, perhaps, during the rapid progress of the contraction --

now operate in full force. Nevertheless, the reactive effect in both stages alike must, in all

probability, be regarded as a phenomenon for which nerve and muscle are jointly responsible.

This view is borne out by the fact that the duration of the two stages of inhibition is largely

determined by the character of the stimuli which affect the nerve. If, e.g., we apply a stimulus to

a portion of nerve that lies within the sphere of operation of the anode of a constant current, the

duration of the inhibitory stages is considerably lengthened.

We are now in a position to discuss the relation of the excitatory and inhibitory effects within the

nerve fibre, in abstraction from the properties accruing to the reacting muscle. We may

conceive of them as follows. On the occurrence of stimulation, excitatory and inhibitory effects

are produced simultaneously. At first, the latter are very much the stronger. As time goes on,

however, they increase more slowly, while the excitatory effects advance more quickly.

Oftentimes, these last appear to maintain their ascendancy until the conclusion of the whole

process. If however, the functional capacity of the: nerve is very high, the inhibitory effects may

again acquire the upper hand for a brief

period immediately following the

conclusion of the twitch. This fact

indicates, at the same time, that the

process is not entirely continuous, but

that the rapid result produced in

contraction by the excitatory effects is

always followed by an inhibitory reaction.

The release of excitation thus resembles

a sudden discharge, in [p. 75] which the

available forces are quickly consumed, so

that for a short time the opposite effects

are in the preponderance. Fig. 30 is an

attempt to show this sequence of events

in graphic form. Stimulation occurs at rr'.

The curve ab represents the course of

the excitatory, the curve cd of the inhibitory effects, measured by the magnitude of the

downward directed (negative) ordinates of the curve cd. We assume that excitatory and

inhibitory impulses are already present in the nerve, before the application of stimulus, but that

they are in equilibrium. These pre-existent impulses we make proportional to the ordinates xa

and xc. The curve of inhibition is characterised by a rapid rise at its commencement, the curve

of excitation by the gradual fall at its conclusion. What we term the functional capacity of a

nerve is a function at once of excitation and of inhibition. The more functionally capable the

nerve, the greater is the efficacy of both the inhibitory and the excitatory forces contained within

it. In the exhausted nerve, both alike are diminished, but the inhibitory in higher degree. Here,

therefore, the irritability is enhanced, and the transitory inhibitions at the conclusion of the

twitch, which may perhaps be referred (as indicated by the dotted curve m) to an oscillatory

repetition of the inhibitory process, are no longer observable.

Our study of the changes of excitability which take place in a nerve during the process of

stimulation has shown two things: that the effect produced in the nerve disappears not abruptly

but quite gradually, and that it always persists for a noticeable time after the conclusion of the

twitch. We have now to consider another phenomenon, which evidently proceeds from the

same causes. If several stimuli are successively applied at such intervals that each falls within

the period of decline of the stimulation set up by its predecessor, the irritability of the nerve is

increased. Indeed, under favourable conditions this increase may be so considerable that a

weak stimulus, which at first could not provoke any contraction at all finally releases a maximal

contraction. At the same time, the contractions become longer; while the longer after-effect

shows further that the course [p. 76] of the excitation has increased not only in intensity but

also in duration. These phenomena occur both with stimulation by electric shocks and with

instantaneous mechanical stimuli. They are therefore bound up with the stimulation process as

such; although, where electrical stimuli are employed, they undergo modification as a result of

certain processes developed at the two electrodes: these processes, which we shall discuss

presently, are essentially different at anode and cathode.[9] If, on the other hand, we apply the

stimuli in very quick succession, so that the twitch provoked in any given case begins before

the twitch released by the preceding stimulus has run its full course, we obtain the permanent

contraction known by the name of tetanus, and consisting essentially of a summation of the

super-posed twitches.[10] This summation of contractions is here of no further interest to us;

we note simply that the properties of the contractile substance have an important part to play in

its origination. If we abstract from it, we may say that the phenomena of increased excitability in

consequence of preceding stimuli, which we are now considering, are in the main indicative of

the behaviour of the nervous substance; the muscle is, in all probability, concerned in their

production only in so far as it resembles nerve in the general character of its irritability. This

conclusion is borne out more especially by the fact that the increase of excitability by

stimulation is independent of the occurrence of contraction. Where the stimulus employed is so

weak that it cannot release any contraction at all or where the muscle is so overloaded that the

contraction is entirely suppressed, the increase of excitability is just as noticeable as it is when

the muscle is allowed to contract. Nay more: since, under these circumstances, the phenomena

of fatigue (which we discuss below) are ruled out, it becomes as a general rule still more

noticeable. Taking the whole group of facts into account, we may therefore designate this

increase of excitability as the elementary phenomenon of the process of practice. For when we

speak of practice, in connexion with the function of nervous organs, we mean precisely that

certain processes of excitation are facilitated: a result that can be produced most directly by an

enhancement of excitability within the nerve-paths which the excitation travels. In saying this,

we must, however, remember that the facts in question are facts of direct practice: that is, we

must abstract from all the effects which practice can produce in other tissues, -- muscles, joints,

tendons, bones, -- but which always make their appearance after a considerable interval'

through these, in their gradual [p. 77] summation, constitute, of course, a very important part of

the phenomena included in the usual definition of practice.

Suppose, however, that we allow the muscle to make the contractions which are the natural

consequence of the stimuli applied to the nerve. We then invariably meet, after a certain lapse

of time, with another phenomenon, which compensates the elementary phenomenon of

practice described just now, and which presently reverses all the features of the picture. This is

the phenomenon of fatigue. We can, therefore, observe both processes, practice and fatigue, in

their simplest typical sequence, by making a muscle do work upon a weight of moderate size,

which it has to lift, and by applying the stimuli at the appropriate intervals, with a brief interlude

between twitch and twitch. Under these conditions, we have, first of all the effects of practice;

the functional capacity of the nerve increases, quickly at the beginning, then more slowly. Then,

from a certain point onwards, the height of lift remains the same, while the duration of the

contraction is quite considerably increased. After a little while, however, the height of lift

decreases, and the contraction is more and more prolonged. Finally, a single stimulus shock

releases a weak but very slow contraction, -- similar to that provoked in the fresh muscle by the

direct application of a constant galvanic current to the muscle-substance or, most markedly, by

the passing of such a current through a muscle whose nervous excitability has been destroyed

by curare poisoning.[11] The general character of these phenomena makes it probable that

they have their principal seat, not in the nerve, but in the muscular substance itself. This

hypothesis is, as a matter of fact, borne out by a number of different observations upon the

phenomena of fatigue, which prove that they constitute, in this regard, a direct antithesis to the

elementary phenomena of practice as described above. The latter can he obtained even when

the muscle is entirely inactive; indeed, it is in such circumstances that they appear at their best.

The fatigue phenomena, on the other hand, refuse to show themselves, so long as means are

taken to prevent the contraction of the muscle during the application of stimuli to the nerve.

With a sufficiently overloaded muscle, e.g., no amount of repeated stimulation win bring ot the

signs of fatigue. If, in this case, a test-stimulus is applied to the nerve before and after the

overloading of the muscle, the resulting contraction is just the same. Similar observations have

been made upon animals temporarily deprived of the use of their muscles by poisons like

curare or atropin, which paralyse the terminal apparatus of the motor nerves in muscle, but

leave the nerve-trunks intact. If stimuli are applied to a nerve, during the action of the poison,

there is no indication of nervous fatigue.[12][p. 78]

We must conclude from these results that the elementary phenomena of practice and fatigue

are of radically different origin. The prime condition of the processes of practice is given in the

nerve-substance, which is so constituted as to be very readily changed by stimulation: the

change manifesting itself in a continuously increasing effectiveness of subsequent stimuli. All

direct practice may be referred to this elementary phenomenon. Where it is shown by muscle,

we may, in all probability, ascribe it to the nerves which the muscle contains, or to certain

fundamental properties of contractile substance which nerve and muscle possess in common.

With indirect practice, which appears as the result of actual, more especially of repeated

exercise of function, the case is different. Here, we must admit, muscle plays a leading part: the

increase of blood-supply, due to frequent repetition of contractions, means a more adequate

nutrition, and consequently a higher functional capacity. These indirect effects of practice do

not, however, differ in any essential respect from the changes produced in tendons, joints,

bones, -- increased extensibility of tendons, smoothing of articular surfaces, etc., -- by frequent

repetition of the same movement. They are secondary phenomenon, sharply marked off from

the primary by the fact that they arise only by the mediation of changes in the blood-supply. On

the other hand, the phenomena of fatigue resulting from the performance of mechanical work

are as characteristic of muscle as the phenomena of practice are of nerve: they reside almost

exclusively in the muscle-substance. And a like statement applies, by all analogy, to the other

organs appended to the nervous system, the sense-organs and glands. Nervous substance

itself seems to be, in large measure, exempt from fatigue. To explain this peculiarity, we must

assume that it contains regulatory mechanisms, of a high degree of perfection, whose office is

to prevent exhaustion. We have already spoken of the alternations of excitatory and inhibitory

forces, evinced during the progress even of a simple process of excitation. It is these, then, we

may suppose, that underlie the phenomenon of nervous inexhaustibility, guaranteeing a long

period of functional activity, and protecting the nervous substance against injury of all kinds. At

the same time, the inexhaustibility is, of course, a relative matter. There is good evidence that

the effects of nervous exhaustion, when once it has set in, are all the more permanent, and that

recuperation is all the more difficult. In view of this fact, the comparatively rapid fatigue of the

peripheral organs appears in some sort as a measure of defence; it prevents any destructive

consumption of nervous forces, by throwing the external instruments of nervous activity out of

function before the nerves themselves are affected. [p. 79]

(d) -- Stimulation of Nerve by the Galvanic Current

We must now devote a special paragraph to the stimulation of nerve by the constant galvanic

current. We are trying to gain an idea of the processes in nerve from a consideration of the

course of stimulation-phenomena at large; and the phenomena which accompany this mode of

stimulation will help us to fill in various details of the picture.

In general, the galvanic current affects the nerve by way of excitation, both at make and at

break; but in both cases the processes of stimulation at anode and cathode are markedly

different. With currents of not excessive intensity, the processes that follow directly upon the

make of the current in the neighbourhood of the cathode are of the same character as those

occurring after momentary stimulation throughout the whole length of the nerve; the only

difference is that the excitatory and inhibitory effects persist, with diminished intensity, so long

as the current is kept closed, while at the same time the excitatory processes remain constantly

in the ascendant. In the neighbourhood of the anode, on the other hand, inhibitory forces of

considerable intensity make their appearance. They increase, with increasing intensity of

current, far more quickly than the excitatory effects; so that with fairly strong currents, when the

anode lies nearer the muscle, the inhibition there set up prevents the propagation to the muscle

of the excitation beginning at the cathode. The result is that, with increased intensity of the

ascending current, the making contraction very soon decreases again, and presently

disappears altogether. The anodal inhibition begins at the anode as soon as the current is

made, and then diffuses slowly and with gradually diminishing intensity to a considerable

distance. Its rate of travel, varying with the intensity of the current, is not more than 80 to 100

mm. in the 1 sec., -- very much slower than the rate of the excitatory process, which moves

with a rapidity of 26 to 32 m. It should be noted, however, that this rate increases markedly with

increase in the intensity of the current, so that the inhibition finally extends into the region of the

cathode. If the current is now broken, the differences present during make disappear, more or

less quickly, and at the same time inhibitory effects gain a temporary ascendency at the

cathode; the break-stimulation thus consists in a process of compensation. It proceeds mainly

from the region of the anode, where the inhibition maintained during make is transformed into

excitation, -- the reversal occurring the more quickly, the stronger the current employed.

The peculiar features of the stimulation-processes released by the constant current may, then,

be stated in summary as follows. The excitatory and inhibitory effects, which with other modes

of stimulation are distributed uniformly throughout the nerve, here vary with the position of the

electrodes: at make, the excitatory forces predominate in the neighbourhood [p. 80] of the

cathode, the inhibitory in the neighbourhood of the anode; at break, a process of compensation

sets in, which for a time exactly reverses the distribution of the two classes of effects.[13]

The phenomena of nervous stimulation are attended by other phenomena besides that of

muscular work. Only the thermal and electrical changes, however, have so far been worked out

in any detail. We may appeal to these to supplement, perhaps in some measure to check, the

conclusions we have drawn from the phenomena of irritability. But we find, as was indicated

above (p. 57) that the tale of results is exceedingly meagre. No one has as yet been able to

demonstrate the occurrence of thermal changes in the nerve itself, in consequence of

stimulation: but this simply means, of course, that the changes are too slight to be taken

account of by our measuring instruments. On the other hand, heat is always set free when work

is done by muscle, while at the same time the relation between development of heat and

amount of mechanical work varies as the principle of the conservation of energy requires, --

increase of mechanical energy involving decrease of the relative quantity of heat developed.

This fact comes out clearly, if we so arrange an experiment that the muscle shall make

maximal contractions of equal height, while lifting weights of different sizes; the greater the

weight to be raised, the smaller is the amount of heat generated.[14] In contradistinction to

these differences in thermal phenomena, the electrical changes accompanying the process of

stimulation in nerve and muscle have been shown to be alike. In both tissues, the point of

excitation always becomes negatively electrical to any other unstimulated part. These changes

cannot, however, be brought at present into any intimate relation with the processes of

stimulation; our knowledge of the chemical conditions upon which they depend is glaringly

defective. That a certain relation exists is shown, however, by their temporal course; the rate of

propagation of the current of action in the nerve-fibre coincides with the rate of propagation of

the stimulation-process itself. This coincidence extends, further, to the transmission of the

inhibitory processes set up by the constant current, as described above; the changes occurring

at the anode also travel much more slowly than those occurring at the cathode. The latter

proceed with the same rapidity as the stimulus-wave at the rate of some 32 m. in the 1 sec.; the

anodal wave of inhibition travels, as BERNSTEIN found, at the rate of only 8 to 9 m. in the 1

sec.[15]

§3. Theory of Nervous Excitation

The molecular state, which our general ideas of the mechanics of complex chemical processes

lead us to predicate of nervous substance, was described above as a state in which there is

constant performance at one and the same time of positive and negative molecular work. The

positive molecular work, so soon as it gains the upper hand, will manifest itself either in

disengagement of heat or in some form of external work, such as muscular con-[p. 81]traction.

The negative molecular work will counteract these positive effects: heat will become latent; the

progress of a muscular contraction will be inhibited. Equilibrium of the two opposed kinds of

molecular work brings about the stationary condition of the nerve, during which there is no

change of temperature and no accomplishment of external work. Hence, whenever we find that

the action of an external stimulus releases a process which gives rise to a muscular

contraction, or, for that matter, simply induces an increased irritability in presence of the test-

stimulus, we may argue to an enhancement of the positive molecular work. Whenever, on the

contrary, the progress of a muscular contraction is arrested, or the reaction to the test-stimulus

reduced, we may be sure that the negative molecular work is in the ascendant. Whether, now,

the one or the other of these effects is produced, whether, that is, the positive or the negative

molecular work gains the upper hand, depends upon circumstances. We are thus led to the

general conclusion that the stimulus shock increases both the positive and the negative

molecular work of the nerve. This means, in terms of our preceding discussions, that the

stimulus shock does two things: it assists the atoms of complex chemical molecules to unite in

more stable connexions, while it also favours the disruption of these compounds and the return

of the atoms to less stable and more complex relations. The restitution of the complex

molecules corresponds to the recuperation of the nerve; the process of combustion, which ends

in the formation of more stable and less readily decomposable compounds, is the source of the

work which it performs, but is also the condition of its exhaustion. The only way in which the

stimulus can bring about external work (muscular contraction the excitation of nerve-cells) is by

furthering the positive molecular work more effectively than the negative. The positive

molecular work then becomes the source of the external work of excitation, which may he

transmitted to particular organs, and so still farther transformed into other modes of work. At

the same time, the positive and negative molecular work must be distributed over the course of

stimulation in the sequence determined by the relation of the excitatory to the inhibitory effects.

First of all, that is, there must be a storing-up of potential work, corresponding to the stage of

inexcitability; the stimulus shock releases a number of molecules from their existing connexion.

Thereupon begins a process of combustion, starting with the freed particles, and extending

from them to the readily combustible constituents of the nerve-mass at large; during this stage

a large quantity of potential is transformed into actual work. If the combustion proceeds with

great rapidity, it is followed for a short time by a restitution of the complex molecules (transitory

inhibitions; preponderance of negative molecular work). As a rule, however, there remains at

the conclusion of the contraction a surplus of positive molecular work, which disappears only

gradually; we trace it [p. 82] in the enhanced effect of a second stimulus supervening upon the

first. It follows, then, that the same curves which we employed to illustrate the relations of

excitation and inhibition (Fig. 30, p. 75) will serve here to show the relation of positive to

negative molecular work. The equilibrium of the two, during the state of rest, is indicated by the

equality of the initial and terminal ordinates, x a, x c and x b, x d. We must, however, suppose

that the internal condition of the nerve, after the process of stimulation has run its course, is not

in general precisely the same as before: there will, on the whole, have been more given out in

positive work than has been acquired in negative in potential work. Nevertheless, we must also

infer, from the fact of the relative inexhaustibility of nerve, that this difference is extremely

small; so that the equilibrium of forces is re-established in large measure: and in nerves of high

functional capacity probably in full measure, during the actual progress of the contraction. This

tendency to the maintenance of equilibrium between positive and negative molecular work,

between loss of work-equivalents and gain of potential work, appears to be a specific property

of nervous substance, founded in its chemical constitution and distinguishing it from all other

tissues. It is expressed symbolically in Fig. 30, which shows the molecular processes of

stimulation; the upper and lower curves each include an approximately equal area. This implies

that the process of stimulation consists essentially not in a permanent disturbance of the

equilibrium between positive and negative molecular work, but simply in their different

distribution in time during the progress of the stimulation. The nature of this difference is given

at once with the changes of irritability that can be traced from moment to moment in the figure.

We must, now, not lose sight of the fact that it is never more than a certain position of the total

sum of positive molecular work, set free in the nerve by stimulation, that is transformed into

excitatory effects, or, as we may phrase it, into work of excitation: another portion may become

heat, a third be changed back again into potential (negative) work. Similarly, it is only a portion

of the work of excitation that is employed in the production of external stimulus effects

(muscular contraction or stimulation of nerve-cells); we have seen that there is always an

enhancement of irritability, both during and after the contraction. Hence a supervening stimulus

will invariably and the nerve possessed of a surplus of work of excitation. If no new stimulus

shock supervenes, this surplus in all probability passes over into heat. After the work of

excitation has once been set up, at the point of stimulation, it exerts an influence upon

neighbouring parts, where the store of molecular work is in its turn transformed in part into work

of excitation, and so on. But, as we know, the process released by the momentary stimulus

persists for a considerable time. Hence, while work [p. 83] of excitation is released, new

stimulus-impulses are conveyed to the part affected from the neighbouring parts. We are in this

way able to explain the heightening of excitation observable when different points of the nerve

are subjected to stimulation (p. 68).

The main difference between these general stimulation-processes and stimulation by the

constant current is, obviously, to be found in the uneven distribution of the sums of positive and

negative molecular work which obtains in the latter case. While the current is closed, there is

preponderance of negative molecular work in the neighbourhood of the anode, of positive in the

region of the cathode. This difference becomes intelligible, when we remember that the

resulting electrolysis must produce internal changes in the nerve-substance. At the positive

electrode electronegative, at the negative electrode electropositive constituents are given off. At

both places, that is, the word of the electric current produces dissociation. The immediate

consequence is, that work must disappear; but as soon as the wandering partial molecules

tend to enter into more stable compounds than those from which they have been separated,

the positive molecular work may begin to increase, that is, a part of the work which has

disappeared may be set free again. The phenomena of stimulation lead us to infer that the first

of these processes takes place regularly in the neighbourhood of the cathode, the second in

the region of the anode. The precise chemical changes involved are as yet unknown to us; but

the phenomena of electrolysis supply an abundance of analogous instances of the interchange

of forces. Thus, in the electrolysis of stannous chloride, we have at the cathode a deposition of

tin, in which the work employed for its separation remains stored as potential work, while at the

anode we obtain chlorine, which at once unites with the stannous chloride to form stannic

chloride, liberating heat in the process. Similar results may appear in all cases where the

products of electrolysis are liable to chemical interaction. At break of a current passing through

a length of nerve, on the other hand, a less well-marked process of electrolytic decomposition

sets in, as a consequence of its polarisation, in a direction opposed to that of the original

current. This, together with the gradual compensation of the chemical differences, occasions

the phenomena of the break-stimulation.

We may say a word, in conclusion, of the relation of the processes whose general mechanism

we have here described to the electrical changes in the stimulated nerve. It is a noteworthy fact

that the current of action which follows upon a momentary stimulation of the nerve reaches its

conclusion, on the average, as early as 0.0006 to 0.0007 sec. after the application of the

stimulus,[16] and therefore falls completely within the period of nervous in-[p. 84]excitability.[17]

It would appear, then, that the variation is connected with the inhibitory forces, or with the

passage of positive into negative molecular work. We must, however, have further information,

as regards the character of this connexion, before we can think of turning the electrical

processes to theoretical account.

The phrases 'positive' and 'negative molecular work' are meant to suggest the general line of

thought followed by the science of mechanical energy or, as we may put it more briefly, by

mechanical energetics. Modern physiologists not infrequently substitute for them the words

'assimilation' and 'dissimilation,' antithetical terms borrowed from the vocabulary of metabolic

phenomena, and thence transferred to that of the general mechanics of the nervous system. It

is, the author hopes, hardly necessary to point out in this place that the words and phrases

employed in the text are not translations back again from the language of physiological

chemistry into that of mechanics; though in such an idea, might possibly arise, in view of the

popularity of the antithesis 'assimilation-dissimilation' at the present time and the variety of

contexts in which it appears. The contrary is true: the phraseology adopted in the text, and

modelled upon that of a general mechanical energetics, was followed in the first edition of this

work (1874) and in the still earlier "Untersuchungen zur Mechanik der Nerven" (i., 1871), before

the terms 'assimilation' and 'dissimilation' had begun to play their part in physiology. Its

retention in the present edition is not due to any prejudice on the author's part in favour of the

original form in which his thought was cast, but rather to the objections which he feels may be

urged against the alternative wording. The processes of metabolism which, for want of more

precise terms, we name tentatively 'assimilation' and 'dissimilation' are, apart from the effects

from which the names are derived, altogether unknown to us; and the effects themselves are

simply that, in the one case, an existing tissue is reinforced by complex tissue-materials of the

same order, while, in the other, existing tissue-materials are brought to disappearance. We

have good ground for the assumption that, in dissimilation, the decomposition of the complex

molecules, and the combustion-processes resulting from it, have the upper hand; and we may

suppose that in assimilation, conversely, the chemical processes introduced are predominantly

synthetic in character. But when we ask how in detail the interchange of nerve is effected in the

two cases, the reply is that as regards dissimilation very little, and as regards assimilation

practically nothing is known. The various processes involved certainly do not conform to any

simple pattern, but depend upon a series of chemical interactions so complicated that at

present we have no means of tracing them. This complication is vouched for by the fact, now

fairly well established for all such processes, that in dissimilation there is a constant interplay of

decomposition and recomposition; existing chemical connexions are dissolved, and new

compounds formed, in continual interdependence. As a rule, energy is liberated during these

dissimilations, in the form of heat or of mechanical work. But again, we cannot say which phase

of the dissimulation process is responsible for this result; nor do we know if a similar

interchange [p. 85] of energy is necessarily involved in every instance of dissimilation, or if

there may not be processes, whose chemical effects would lead us to class them with the

others as dissimilations, bat whose general and final effect is accompanied by a transformation

of energy in the opposite sense. Our knowledge of the chemism of the metabolic processes is,

in the author's opinion, far too defective to permit of our answering these questions. The theory

of assimilation and dissimilation thus attempts to illuminate the processes in the nervous

system by analogies that are more obscure than the processes themselves; and such a

proceeding can hardly inspire confidence. It is sometimes said that dissimilation is the correlate

of fatigue, and assimilation of recuperation. But to say this is, after all, merely to set in place of

certain complex symptoms conditions that are at least as complex, and far less open to

demonstration.[18] Fatigue and recuperation are symptomatic terms whose meaning is, roughly

at any rate, clear to everyone. It is, as we have seen, highly probable that the metabolic

processes which give rise to both groups of symptoms are extremely complicated. When we

consider, therefore, that the words 'assimilation' and 'dissimilation' used to denote these

processes are words that have no chemical significance, but are in the last resort purely

teleological concepts, we cannot but suspect that the symptomatic terms 'fatigue' and

'recuperation' are simply coming back to us in changed form. If we wish to analyse these latter

in detail, there are, as it seems to the author, but two ways open, in the present state of our

knowledge. We may, on the one hand, limit ourselves to the symptoms, but, while we do this,

attempt so far as may be to reduce the phenomena, given in an extremely complicated

syndromes, to their simplest components. We thus discover that every process of fatigue and

recuperation contains two elementary terms, variously interrelate in the particular case, --

excitation and inhibition. Or, on the other hand, we may attempt to refer the effects in question

to the more general concepts furnished by mechanical energetics. We then arrive at the notion

of positive and negative molecular work, in the sense in which these phrases have been

employed in the text. This restriction of the hypothetical foundations of nerve physiology to

straight forward physical analogies appears to the author to be especially desirable, in view of

the great extension of the rival terms. We find the antithesis of 'assimilation' and 'dissimilation'

in the most diverse contexts, -- in the theory of visual sensations, in the theory of auditory

sensations, applied to all other conceivable physiological and psychophysical phenomena. It

almost seems, indeed, as if this pairs of terms is gradually coming to play the part in modern

physiology that was played in SCHELLING'S nature-philosophy, at the beginning of the

nineteenth century, by the phrase "polar opposites," which found application not only to

electricity, magnetism and chemical process, but also to sensibility and irritability, light and

darkness, and many other things besides.

§4. Influence of the Central Parts upon the Processes of Excitation

(a) -- Course of the Reflex Excitation

We begin our investigation of the processes in the central nervous substance by stimulating a

peripheral nerve, and endeavouring to find out how the course of stimulation is altered, if it is

compelled to pass through central [p. 86] elements. The easiest way to perform this experiment

is to avail ourselves of the phenomenon of reflex excitation. We first of all apply an electric

shock of the proper intensity to a motor nerve root, whose connexion with the myel on the one

hand and with its dependent group of muscles on the other is kept intact; and we then

stimulate, in the same way, the central end of some sensory root. The two twitches are

recorded by the muscle, and the experiment is so arranged that the times of stimulation

correspond to the same point upon the axis of abscissas of the two muscle-curves. The

differences in appearance and progress of the two contractions then give us a measure of the

influence exerted by the intercalated central substance.

The first thing that we observe, under these circumstances, is that much stronger stimuli are

required to produce contraction by way of the sensory root. If we make our shocks as nearly

instantaneous as may be, e.g. by using induction-shocks, we shall frequently find it impossible

to release any reflex contraction whatsoever; the currents required are of such intensity that

their employment would bring with it the danger of leakage to the myel.[19] Provided, however,

that the reflex irritability is high enough to permit of our making the experiment, we obtain two

curves which repeat, on a greatly enlarged scale, the same characteristics that distinguish the

curves obtained in a previous experiment by stimulation of a motor nerve at two points

unequally distant from the muscle (cf. Fig. 24, p. 67) The reflex twitch is extraordinarily late in

appearing, and is of much longer duration. Suppose, e.g., that we stimulate a motor and a

sensory root which enter the myel at the same height and on the same side, and that we so

regulate the stimuli as to equalise the heights of the muscle curves; we get the result shown in

Fig. 31. The only marked difference between these contractions and those released from

different points upon the motor nerve is that a stronger and not a weaker stimulus must be

employed, to bring

the reflex

contraction to the

same height as

the other. The

differences in the

course of the excitation are, however, in this instance so considerable, that no increase in the

intensity of stimulus is able to change their character. It is true that intensification of stimulus

increases not only the height but also the duration of the contractions, while decreasing the

latent period. But [p. 87] the weakest reflex contractions are always noticeable by their long

duration, and the strongest by the length of their latent period, even when we compare the

former with the strongest and the latter with the weakest direct contractions.[20] It is clear, now,

that the time which the stimulation requires to pass from a sensory into a motor root is given by

the interval separating the beginnings of the two contractions, the direct and the reflex. The

nerve-roots are so short, that the portion of this interval taken up by the peripheral conduction

may be considered negligible; and we may accordingly designate the interval as a whole, the

reflex time. To determine it, we must -- since the latent period is dependent upon the intensity

of the stimuli -- have recourse once more, as we did in our measurement of the rate of

propagation in the nerves, to experiments in which the muscle-nerves are of the same height.

This presupposed, we may proceed to investigate the reflex time under various conditions. The

simplest case is shown in Fig. 31, where the stimulation is transferred from a sensory root to a

motor root belonging to the same nerve-trunk: we may term this the case of same-sided reflex

excitation. Next in order comes the propagation of stimulus from a sensory root to a motor root

which leaves the myel at the same height but upon the opposite side: we term this the case of

crossed reflex excitation. In the third place, we may have propagation along the length of the

myel, which we may call the longitudinal conduction of reflexes; as, e.g., in the transference

from the sensory root of a nerve of the arm to the motor root of a nerve of the lower extremities.

In no one of these three cases is the reflex time sensibly dependent upon the intensity of the

excitations. It is, as might have been predicted, relatively shortest for same-sided reflex

excitation, where under normal circumstances it amounts to 0.008 to 0.015 sec. It is, however,

as one would be less likely to expect, relatively greater with crossed than with longitudinal

conduction. Thus, if we compare the crossed with the same-sided reflex, there is an average

difference to the disadvantage of the former of some 0.004 sec. If we then compare the reflex

released in the thigh by stimulation of the root of a sensory nerve of the arm with the same-

sided reflex, the difference between the two times is as a rule somewhat smaller.[21] Since the

path travelled by the stimulation in the latter case is at least six to eight times as long as that

traversed in the former, it is evident that the retardation in crossed conduction is much more

serious than it is in longitudinal conduction. An explanation is, without any doubt, to be found in

the fact that longitudinal conduction (as we shall see presently, in Chapter V., when we come to

discuss the morphology of the myel) is sub-[p. 88]served for the most part by the fibres of the

white substance, while crossed conduction must be mediated almost exclusively by the cell-

reticulum of the grey substance. We have, then, in the results of this set of experiments, a

confirmation of the inference, already suggested as probable by the long duration of the reflex

time, that the central elements offer incomparably more resistance than the nerve-fibres to the

progress of an excitation. The same conclusion may be drawn from the further fact that a

retardation of conduction, amounting on the average to 0.003 sec., occurs in the spinal ganglia

of the frog, and again from the related observation that the sensory nerve roots are more

irritable than the nerve-fibres below the spinal ganglia. It is noteworthy, in connexion with this

latter result, that the ramifications of the sensory nerves in the skin are, in their turn, more

easily excitable than the nerve-branches that run to the skin. Just, then, as there are

mechanisms in the spinal ganglia which diminish the irritability of the incoming nerves, so must

there he mechanisms in the skin which discharge a precisely opposite function. It follows from

all this that the irritability of the nerve-trunks and their branches is reduced to a minimum: a

characteristic, we need hardly say, that is eminently fitted to protect the central organs from the

advent of useless sensory excitations.[22]

(b) -- Enhancement of Reflex Excitability

The temporal relations of reflex conduction have made it appear probable that the central

elements, while on the one hand they offer greater resistance to incoming excitations, are able,

on the other, to develop a greater amount of stored energy. This hypothesis is confirmed by

many other facts. We notice, first of all that in almost every case, where the excitability of the

myel is not enhanced by artificial means,[23] a single, momentary stimulus-shock is unable to

release a reflex contraction. To obtain a response, we must repeat the stimulation; and the

contraction thus set up usually takes on a tetanic character.[24] Within certain limits, the reflex

makes its appearance after the same number of single stimulations, whether these be given in

quick or slow succession. On the other hand, the duration of a [p. 89] reflex tetanus is not

directly dependent upon the duration of the stimulation, as is that of the contraction aroused by

tetanic excitation of the motor-nerve. If the stimulation be of short duration, the tetanus outlasts

it; if it be of longer duration, the tetanus disappears earlier than the stimulation itself.[25]

Another phenomenon, that shows very clearly the differences in excitability between the

peripheral and the central nervous substance, is the following. If we stimulate the motor nerve

by induction-shocks, repeated with not too great rapidity, the corresponding muscle falls, as

was first pointed out by HELMHOLTZ [26] into vibrations of the same frequency. These may be

perceived as a tone, or may be recorded, by means of a fitting instrument, upon a cylinder

which rotates with uniform speed. If, now, we stimulate the myel in the same way, the muscle

again falls into vibrations, But the

frequency of vibration is considerably

diminished. Fig. 32 shows two curves of

vibration obtained from the muscle of a

rabbit by KRONECKER and HALL. With

42 stimuli in the 1 sec., the muscle traced

the upper curve when the motor nerve was stimulated, the lower, when the stimulation was

applied to the myel, which had been severed below the oblongata. [27] Closely connected with

this is BAXT'S observation that voluntary movements, however simple they may be made,

always last a considerably longer time than simple contractions, released by the stimulation of

a motor nerve. BAXT found, e.g., in experiments upon himself, that the index finger of the right

hand, moving in response to stimulation by the induction current, required on the average 0.166

sec., while movement initiated by voluntary innervation required 0.296 sec.[28]

We can easily see the reason for the greater effect produced upon the myel by frequent

repetition of the stimulus. Every stimulation leaves behind it an enhanced reflex excitability.

Here, again, however, the central substance merely exhibits, on a larger scale, phenomena

with which we are already familiar in the case of the peripheral nerve. On the other hand, there

are certain chemical effects, that are able in some unknown way to produce a similar change of

irritability, which appear to be peculiar to the central nerve-substance. The agents in these

effects are termed 'reflex poisons'; the chief place among them is taken by strychnine, which

brings abort the changes in question with unfailing certainty. Strychnine prob-[p. 90]ably owes

its power in this regard to the circumstance that its effect is limited almost exclusively to the

central substance of the myel; whereas other nerve-poisons set up changes in the peripheral

nerves, or in higher nerve-centres, which may serve, in greater or less degree, to counteract

the effect under discussion.[29]

The action of these poisons is, in general, as follows. (1) Much weaker stimuli are sufficient to

release a reflex contraction; indeed, a point is very soon reached, at which the reflex irritability

becomes greater than the irritability of the motor nerve. (2) Even when the stimuli are reduced

to the lowest limit of effectiveness, the contraction is higher and, in particular, of longer duration

than under normal conditions; if time effect of the poison be increased, it passes over into a

tetanic contraction. (3) The beginning of the contraction is more and more delayed; so that the

latent period may have more than twice its ordinary duration. At the same time, the difference in

the length of the latent period with strong and weak stimuli is enormously increased: when the

action of the

poison is at its

height, the reflex

tetanus hardly

shows any

difference of

degree, whether

the weakest or the

strongest stimuli

be applied, but in the former case sets in with extraordinary slowness. An illustration of these

changes is given in Fig. 33. The curve A was taken as the action of the poison was beginning;

the curves B, when it was at its full height: a was released by a strong, b, by a weak momentary

stimulus: in both cases a direct contraction has been recorded, for purposes of comparison.

There can be no doubt that this lengthening of the latent period is directly connected with the

enhancement of irritability. When the central substance is modified by the poison, the after-

effect of the stimulus is prolonged, so that the excitation call be released after the initial

inhibition has been overcome. The phenomenon is somewhat like that of the summation of

stimulations, only that here the external stimulus is not repeated. We must, accordingly,

suppose that the stimulus brings about a number of successive stimulations, whose summation

presently leads to excitation. This suggests the idea that the processes of molecular inhibition

are not sensibly changed by the alteration of the [p. 91] nervous substance, but that the positive

molecular work is seriously affected. In the normal state, it returns to the potential form, in

whole or in great part, immediately after its liberation; in the present instance, it appears to be

recovered but gradually. We may note that similar, though weaker, effects are produced on the

myel by the action of cold.[30]

We have spoken so far only of the influences which enhance the excitability of the central

elements. Here, however, just as in the case of the nerve-fibre, the excitatory are paralleled by

inhibitory effects. The fact that first drew attention to these inhibitions is a discovery of old

standing in physiology: the fact that the reflex excitability of the myel is increased after removal

of the brain. SETSCHENOW, starting out from this fact, found that the stimulation of certain

parts of the brain in the frog, -- thalamus, bigemina, medulla, --prevents or delays the

appearance of the reflexes.[31] He was therefore inclined to think that the inhibitory function is

confined to certain definite central parts. Further experiments showed, however, that the same

effect is produced by the stimulation of other sensory nerves, or of the sensory columns of the

myel; [32] so that it became necessary, in terms of SETSCHENOW'S hypothesis, to suppose

that these specific inhibitory centres are distributed over almost the entire cerebrospinal organ.

But if any given sensory excitation may be inhibited by the stimulation of any other sensory

element, the sphere of inhibition -- as GOLTZ justly observed [33] becomes coextensive with

the sphere of sensory excitation; and the assumption of specific inhibitory centres falls to the

ground. At the same time, while any possible sort of sensory stimulation, whether it affect other

sensory nerves or sensory central parts, may arrest the progress of a reflex [p. 92] excitation,

the inhibitory effect is not by any means invariably produced; the supervening stimulation may,

on the contrary, enhance the reflex, -- as always happens, of course, when two excitations

meet in some motor fibre, or in a motor central area. Let us term the meeting of two excitations

in the same central territory, quite generally, an interference of stimulations. Then the result of

such interference is dependent upon four things. It depends (1) upon the phase to which the

one excitation has attained when the other begins. If the muscle contraction released by the

first stimulation is still in course, or only just over, when the second arrives, we have as a rule

an enhancement of the stimulus-effect. If, on the other hand, the original stimulation occurred

some time before the application of the second, this latter is more easily inhibited. It depends

(2) upon the intensity of the stimuli. Strong interference-stimuli inhibit a given reflex excitation

more easily than weak; sometimes, indeed, strong stimuli will inhibit the same excitation that

weak stimuli enhance. It depends (3) upon the spatial relation of the nerve-fibres stimulated.

Sensory fibres that enter the myel at the same height and upon the same side, i.e. that belong

originally to one and the same nerve-trunk, effect a much weaker inhibition (or, in other terms,

are much more ready to enhance the excitation) than fibres that come in from different sides or

at different levels. Lastly, it depends (4) upon the state of the central organ. The more

completely the normal functional capacity is preserved, the more certainly, other conditions

being favourable, may one look for inhibition of the reflexes ; the more the functions of the

organ have been impaired by cold, by strychnine or other reflex poison, by loss of nervous

force due to fatigue, malnutrition, etc., the more likely is it that an enhancement of stimulation

will take the place of inhibition. This decrease of inhibition is evidenced, first of all by the fact

that stimuli of longer duration and greater intensity are required to evoke it. It always

disappears first with stimulation of the nerve-fibres belonging to the same root; but in a state of

extreme functional incapacity, or of serious derangement by cold or strychnine, it disappears

altogether, so that no inhibitory symptoms can he observed at all.[34]

It is, perhaps, tempting to think of these inhibitory effects due to an interference of oscillatory

stimulus-motions, analogous to the interference of light and sound vibrations; to conceive, i.e.,

that the stimulus-waves meet together and, in whole or part, cancel one another.[35] Such an

hypothe-[p. 93]sis is, however, wholly unable to explain the simple effacement of excitation that

occurs, e. g., in the ventral nerve-cells of the myel, when the motor fibres issuing from them are

stimulated. Moreover, it gains no support from the known facts of the course of excitation. On

the contrary, the varying results of stimulus-interference indicate, quite clearly, that in the

stimulation of the central elements, as in that of the nerve-fibre, excitatory and inhibitory effects

are released at one and the same time. It is also clear, however, that the phenomena of

inhibition are in this case much more pronounced than they are in the peripheral nerve-fibre.

The special conditions under which the two opposed results of central stimulation are obtained

make it probable, further, that the external effect of inhibition is produced more particularly

when the stimuli are so conducted as to interfere in the same sensory central area; whereas

summation of stimuli seems to occur whenever the excitation travels from different sensory

central areas, simultaneously stimulated, to the same motor elements. In general both of these

effects may he produced, side by side, by the simultaneous stimulation of any different sensory

elements, and it will depend upon the special circumstances whether the one or the other of

them gains the upper hand.

(d) -- Chronic Effects of Excitation and Inhibition: Positive and Negative Tonus

If we inquire into the nature of these specific circumstances, we find as the most important, the

connexions in which the various nervous elements stand with one another and with their

appended organs. This conclusion is suggested at once by certain phenomena observed in

nerves and muscles whose functional connexion with their central points of origin has remained

intact. Thus, in the first place, a muscle which is united by its nerve to the central organ is kept

permanently in a certain tension, which ceases at once, as may be observed in the slight

lengthening of the loaded muscle, when the nerve is cut through.[36] This permanent tension in

the state of rest is known as the tonus of a muscle. Its disappearance when the nerve is cut

indicates that it has its ground in a chronic excitation of the nerve, transmitted to the fibre from

the central elements. Its maintenance seems to depend, further, upon the connexions in which

the central elements stand with one another. For the tonic excitation which travels to the

skeletal muscles along the motor nerves of the myel may be abrogated, not only by section of

the motor nerves themselves, but also by section of the sensory roots of the spinal nerves.[37]

We must therefore [p. 94] suppose that a portion of the forces which release the excitation

reach the motor nerve-cells only by way of their connexions with sensory elements; while the

related observation, often made and confirmed, that tonus persists after the severance of such

sensory connexions, points us to the cells of origin of the motor nerve-fibres as a co-ordinate

and independent source of excitatory forces. On the other hand, however, the central elements

appear also under certain conditions, according to the circumstances in which they are placed

by their nearer or more remote connexion with other like elements, to generate, and to transmit

to their peripheral continuations, a surplus of inhibitory forces. Here, e.g., belongs the

observation that increase of tonus in a determinate muscle-group is regularly followed by

decrease of tension in the antagonists: so that increased excitation of the flexor muscles of a

limb brings with it a decrease of excitation in the extensors, and conversely. [38] We may term

this phenomenon that of negative tonicity. It then becomes evident that the two opposing forms

of tonus may be brought into relation with the fundamental phenomena of excitation and

inhibition, which we have seen to be observable, first in the peripheral nerve, and then, on an

enlarged scale, throughout the central organs of the nervous system. We have only to add,

what is shown by all these observations, that the continual shift of government from excitation

to inhibition, and back again, is very largely dependent upon the influences to which the central

elements are subjected, in virtue of their connexion with other like elements and with the

stimulation processes which these latter convey to them.

§5. Theory of Central Innervation

(a) -- General Theory of the Molecular Processes in the Nerve-Cell

The phenomena of central innervation have referred us to the same two classes of opposed

molecular effects that we traced in the process of excitation in the nerve-fibre, The general view

that we were led to take of this latter will, therefore, serve as our point of departure in the

present instance. We begin, accordingly, by postulating for the central substance a stationary

condition, similar to that which we assumed in the nerve-fibre; a condition, i.e., in which there is

an equilibrium of positive and negative molecular work. The application of stimulus, here as

before, means an increase in the amount of both forms of work. But everything points to the

conclusion that, in the nerve-cell, there is at first a marked prepon-[p. 95]derance in the

increase of the negative molecular work; so that a momentary stimulus-shock is, as a rule,

unable to release any excitation whatever. If, however, the stimuli are repeated, then, as one

follows another, the amount of negative molecular work is gradually diminished, in proportion to

the positive, until at last this latter attains such dimensions that excitation arises. We may

therefore suppose that the typical process of stimulation in a nerve-cell is analogous to that set

up at the anode in a nerve-fibre by the making of a constant current. Under the action of the

stimulus, the processes which transform more stable into less stable compounds, i.e. which

subserve the storage of potential work, are thrown into increased activity. There is, however, a

difference. When the current is applied to the nerve, its electrolytic action introduces

decomposition-processes which do not normally take place in the nerve-fibre. When, on the

other hand, the nerve-cell is stimulated, we have no right to assume anything more than an

enhancement of activity which, under ordinary circumstances, is directed mainly upon the

formation of complex chemical molecules, i.e. upon the accumulation of potential work. This

difference between nerve-fibre and central substance, whose importance is sufficiently evident,

is attested by other physiological considerations. The nerve-cells are really the laboratories, in

which the materials that compose the nerve-mass are prepared. In the nerve-fibres, these

materials are very largely consumed, in consequence of physiological function, but -- if we

abstract from the inadequate and partial restitution which accompanies decomposition in every

case of stimulation -- cannot, obviously, be reformed. For the fibres, if we separate them from

their cells of origin, lose their nervous constituents, and the renewal of these proceeds always

from the central points.[39] Even in the state of functional inactivity, therefore, the interchange

of materials and forces within the nerve-cell is not in perfect equilibrium. But the balance dips

on the one side in the cell, on the other in the fibre. Characteristic of the nerve is the formation

of definite products of combustion, with the performance of positive work; characteristic of the

cell is the production of compounds of high complexity, in which potential work is stored. It is

true that the work done in the animal body, as a whole, is pre-eminently positive work, the

combustion of the complex organic compounds; but it is altogether wrong to look upon this as

the only means for the interchange of forces and materials within the organism. There are

always going on, alongside of the positive work, reductions, dissolutions of more stable into

less stable compounds, with the resulting accumulation of potential work. The nervous system,

in particular, is the scene of great activity in this regard. The compounds which enter into the

formation of nervous substance are, in some cases; more complex, [p. 96] and possessed of a

higher combustion-value, than the nutritive materials from which they are derived; that is, are

compounds in which a large amount of potential work is stored up.[40] The nerve-cells, the

architects of these compounds, are in a certain sense akin to the plant-cells. These, too,

accumulate potential work, which may remain latent until need arises, and then be transformed

back again into actual work. The nerve-cells, in the same way, are the storehouses in which

materials are laid up for future use. And the chief consumers of these stores are the peripheral

nerves and their terminal organs.

Putting all this together, we may gain some idea of the relation obtaining between the central

substance and the nerve-fibres that issue from it. We have, first of all the transmission from cell

to fibre of those molecular motions that we term processes of excitation. But this is by no

means all. There is, further, a constant movement of material, in the direction from centre to

periphery; so that the fibre is in continual receipt of substances in which potential work is stored

up. Here, it is plain, we have the explanation of the nutritive influence which the central

substance everywhere exerts upon the nerve-fibres connected with it, and, through their

mediation, upon the organs which they supply. This nutritive function belongs to all nerve-

centres and nerve-fibres, and is intimately connected with the general mechanics of central

innervation. The hypothesis that there is a special class of nerves, specially devoted to trophic

functions, seems therefore to have nothing in its favour. The conditions under which this

movement of material takes place must, however, necessarily react upon the phenomena of

irritability and the course of excitation. Suppose, e.g., that a certain central area has enjoyed a

long period of rest, and has consequently accumulated a large store of potential work. The

actual work, sensory or motor, done in this area itself and in the nerve-fibres connected with it

will, in general, be more intensive and of longer duration than would have been the case under

different conditions. It is, also, not improbable that the movement of material may serve to

develope neurodynamic interaction between adjoining central parts, as a result of which the

actual work done at any given point may be increased by the conveyance of potential work from

neighbouring points.[41] The differences in the response of the nerve-cells to the stimuli

conducted to them proves, further, that every cell is divided into two distinct regions, the one of

which resembles in excitability the peripheral nerve- substance, while the other shows a

marked degree of divergence. We will term the former the peripheral, the latter the central

region of the nerve-cell.[p. 97] The central region, we may suppose, is devoted pre-eminently to

the formation of the complex compounds of which the nervous substance is composed; it is,

therefore, the place of storage of potential work. A stimulus-movement conducted to it simply

accelerates the molecular processes in the direction in which they are already moving, and

accordingly disappears without external effect. It is different with the peripheral region. Here,

too, something is done towards the transformation of actual into potential work. But, besides

this, there is already a fairly rapid consumption of materials, derived in part from the central

region, and a consequent production of work. If a stimulus strikes this peripheral region, its first

result is, again, a relatively greater increase of the negative than of the positive molecular work.

But the negative soon sinks back to its ordinary level, while the positive persists for a

considerable time; so that, perhaps after an unusually long latent period, certainly if the original

stimulus is reinforced by new stimulus-impulses, it is able to produce an excitation. For the rest,

here as in the nerve, it is only a portion of the positive molecular work that passes over into

work of excitation, and again only a portion of this that shows itself in external excitatory effects;

another portion may be transformed back again into negative molecular work, and the work of

excitation may be changed, in whole or part, into other forms of molecular motion. Further,

when once excitation has arisen, the accumulated work of excitation is consumed very rapidly;

the process suggests that of explosive decomposition. At the same time, the greater strength of

the inhibition has meant the storage of a correspondingly greater amount of work of excitation;

so that the stimulus-effect, when it appears, is greater than in the case of nerve-stimulation. In

this respect, the irritable region of the nerve-cell stands to the peripheral nervous system in

somewhat the same relation that a steam-boiler with stiffly working valve bears to a similar

boiler whose valve moves easily. The expansive force of the steam must be made more

considerable, in the former case, if the valve is to be opened; but, when this is done, the steam

rushes out with a correspondingly greater force. It should be added that the peripheral region of

the nerve-cell probably evinces a different conduct in different cases, approaching sometimes

more, sometimes less nearly to the character of the peripheral nerve-substance. Thus, the

sensory excitations conducted upward by the cells of the dorsal cornua of the myel are certainly

less changed than the reflex excitations which are mediated as well by the cells of the ventral

cornua. These differences may be conditioned upon the number of central cells which the

stimulation has to traverse. But it is also conceivable that there is a continuous transition from

the one to the other of the two cell-regions which we have named the central and the peripheral

and that certain fibrils terminate in middle regions, in which inhibition is not yet [p. 98] complete,

while at the same time difficulties are placed in the path of the stimulation.

We are now in a position to interpret the peculiar enhancement of reflex excitability produced

by the repetition of stimulus or by the action of poisons. Under these conditions, the positive

molecular work, once liberated, can be retransformed into negative work either not at all or, at

least, less completely than usual. It therefore accumulates, until excitation arises. The effect of

these two modes of interference is, therefore, to prevent the restitution of the nerve-substance,

and so to make it possible for comparatively weak external impulses to set up a rapidly

extending decomposition, in consequence of which the stored forces are soon exhausted.

There are still two things to be explained: the phenomenon of the mutual inhibition of

excitations conveyed to the same nerve-cells from different quarters, and the fact that the

stimulation can traverse certain cells only in one direction, and is inhibited so soon as it

attempts the other. To account for them, we must suppose that stimulations which act upon the

central region of the nerve-cell serve to propagate the inhibitory processes (the negative

molecular work) there in progress to the peripheral region; while, conversely, stimulations which

act upon the peripheral region effect a diffusion of the excitatory processes (the positive

molecular work) there released over the central region. The intrinsic probability of this

hypothesis is vouched for by the well known fact that in all chemical processes, in which the

state of equilibrium of complex molecules has once been disturbed, the disturbance is normally

transmitted to other molecules. The explosion of the very smallest quantity of nitrogen chloride

is enough to decompose many pounds of this substance, and a single blazing chip may set a

whole forest on fire. There is, it is true, an apparent difficulty in the case before us: molecular

processes of opposite character are distributed over one and the same mass, according to the

direction from which the stimulation proceeds. We must, however, remember that these

processes are constantly in progress, side by side, in both regions of the cell; and that, as the

constant exchange of materials demands, there is a continuous and gradual transition from the

one region to the other. We may again return to the illustration of the nerve-fibre under the

action of the constant current. In the neighbourhood of the anode there is a preponderance of

inhibitory, in that of the cathode a preponderance of excitatory molecular processes. But it may

be demonstrated, by aid of test-stimuli of varying intensity, that there is increase at the anode

not only of the inhibitory but also of the excitatory processes; while, on the other hand, as the

strength of current is increased, the inhibitory process is propagated to the cathode and beyond

(cf. pp. 97 f.). Similarly [p. 99] with the nerve-cell. Fig. 34 may illustrate, e.g., the behaviour of

the cells of the dorsal and ventral cornua of the myel to incoming and outgoing fibres. M

represents a cell of the ventral, S a cell of the dorsal cornu; c and c' are their central p and p'

their peripheral regions. In the ventral half of the myel, stimulation can travel only from m' to m;

in the dorsal half, only from s to s': a

stimulus proceeding from m, s' is inhibited

in c, c'. A stimulation passing between S

and M can travel only in the direction

from S to M, and not contrariwise; for a

stimulus operating at m will be arrested in

c, and a stimulus applied at m' may be

conducted as far as c', but cannot go

farther. Finally, the reflex excitation

proceeding from s must be inhibited by a

stimulation acting at s', because the

molecular motion of inhibition arising in c'

tends to spread over the peripheral

region, and thus destroys, in whole or

part, the excitation there set up. The

morphological facts put it beyond

question that the part of the ganglion cell here designated the central region is the place of

origin of the axis-cylinder or neurite, and that the peripheral region gives rise to the dendrites.

The latter region belongs accordingly to the actual periphery of the ganglion-cell, though it may

perhaps also extend some little distance into the central ground-reticulum.[42]

The results which follow from stimulation of peripheral ganglia, such as those of heart, blood-

vessels, intestine, lend themselves readily to the same interpretation. Whether stimulation of

the nerves which run to these ganglia produces excitation or inhibition depends likewise upon

their mode of connexion with the nerve-cells. Thus, the inhibitory fibres of the heart will

terminate in the central the accelerating fibres in the peripheral region of the ganglion-cells of

this organ; it is not necessary to assume the existence of separate apparatus for the two

processes. The result of stimulation can be modified in only one way. The ganglia are, at the

same time that they are stimulated from without, in a state of continuous automatic stimulation,

so that the incoming nerves can do no more than regulate the movements made. For the rest,

the nerve-cells, here as before, show the phenomena of accumulation and summation of

stimuli. Intensive excitation of the inhibitory nerves of the heart will, it is true, [p. 100] arrest the

heart-beat after a very short interval; but stimulations of more moderate intensity can produce

this effect only after several beats have been executed. The phenomenon appears still more

plainly in the case of the accelerating nerves, where several seconds regularly elapse after the

beginning of stimulation before acceleration sets in. On the other hand, the after-effect of the

stimulus always persists for a considerable time after the stimulus itself has ceased to act; the

heart returns only gradually to its original frequency of beat. There is also a further point in

which the conditions here evidently differ somewhat from those obtaining in the skeletal

muscles. In all regions of innervation that do not stand under direct voluntary control, the

mechanisms which subserve the production and accumulation of excitatory and inhibitory

effects are to be found, in part, in the muscles themselves; so that, under these circumstances,

the muscle-substance is endowed, within limits, with the attributes elsewhere reserved for the

peripheral and central nerve-substance.[43] In view of the close relation subsisting in other

respects between nervous and contractile substance, we may perhaps look upon this result as

a simple enhancement of the powers possessed by muscular tissue in its own right, an

enhancement due to the independence attained by the peripheral organs.

We may now turn to the elementary phenomenon of practice which, as will be remembered, is

exhibited by the peripheral nerve-substance. The phenomenon recurs, with modifications

conditioned upon the law of propagation of molecular processes within the ganglion-cell, in the

central substance. Its effects are by no means simple, as may be observed in the following

instances. We find, on the one hand, that co-ordinated movements; whose first performance

was difficult and required continuous voluntary control, gradually become easier and, at last,

altogether in voluntary. We find, again, that functional disturbances, set up by the destruction of

central elements, are gradually compensated, without restitution of these elements themselves.

In the first of these phenomena, we have an increasing facilitation of the excitatory processes in

consequence of their frequent repetition. The second suggests that, under suitable conditions,

the stimulation may strike out new paths within the central substance: we may accordingly

designate this latter effect of practice, in contradistinction to direct practice by repetition of

function, as path-making or canalisation.[44] We are thus led to the following conclusions. First,

when an excitatory process is frequently conducted through a ganglion [p. 101] cell in a given

direction, the cell thereby acquires a prepotent disposition to conduct any future stimulations

that may reach it in the same direction. Secondly, the processes of conduction in the central

substance at large cannot be confined within fixed limits; elements in which, under normal

circumstances, the excitations are annulled by concurrent inhibitions must be able, under the

new conditions of practice introduced by the destruction of former conduction paths, to enter

into new functional connexions. Translated into terms of the hypothesis developed above, this

would mean that the frequent repetition of conduction, in a certain direction, so modifies the

portion of the central substance which lies along this particular path that it takes on, more and

more completely, the character normally attaching to the peripheral region. But, as a matter of

fact, this sort of transformation is just what might have been predicted from the general laws of

stimulation. We have seen that, in the peripheral nerve, the inhibitory forces are further and

further reduced, under the action of repeated stimuli; so that at first, before functional capacity

is exhausted, frequent repetition of stimulation means enhancement of irritability. Repetition of

stimulus, that is, always and everywhere brings with it an alteration of the nerve-substance,

which thereby loses the power of exerting the inhibitory influence connected with restoration of

its internal forces. It is to this fact that we must turn for explanation of the principle of practice,

in its special significance for the central functions, noting at the same time that the principle

divides into two less general principles, each of importance for the understanding of these

functions, and each in various ways supplementing the other: the principles of localisation and

of vicarious function. We shall find, when we come to consider the functions of the central

organs of the nervous system, that both alike are indispensable aids to an interpretation of the

phenomena.

(b) -- Relation of Nervous to Psychical Processes

These considerations yield, however, another and a more general result important not only for

the physiological but also for the psychological aspect of vital phenomena. We have taken, as a

measure of the effects which the nervous substance can produce within itself and can transmit

to other elements of the body, resembling it in certain general properties, the effects exhibited

by the muscle; and we have done this, partly because the muscular effects are most easily

accessible to observation, partly because they can be subsumed, with the least possible

ambiguity, to metric principles of universal validity. Now we have no right to suppose that the

laws which govern the transference of nervous molecular processes to the contractile

substance are at all different from the laws which regulate [p. 102] their transmission to other

substances, whose properties show them to be related to the nervous elements, -- more

especially, therefore, to the substances that are of peculiar import for the psychical aspect of

vital phenomenon, the elements of the sense-organs. On the contrary, the identity of these laws

is a matter of course. It follows, therefore, that the changes set up by the action of stimulus in

the sensory cells, and in the peripheral and central portions of the nervous system connected

with them -- whether the stimulus be applied from without or arise within the system itself --

consist always in those forms of positive and negative molecular work whose general laws we

have sought to trace in the symptoms presented by the muscular system. We have seen that

all these forms can readily be brought under the general point of view of the principle of energy;

we have had, as illustrations of them, the decomposition and recomposition of chemical

compounds, the liberation and absorption of heat, the increase and decrease of actual

mechanical work. Now the processes thus analysed remain, always, physical and chemical

processes. It is never possible to arrive, by way of a molecular mechanics, at any sort of

psychical quality or process. If, then, experience teaches us that the molecular processes

within our nervous system may have psychical concomitants, we can only say that we are here

in presence of a fact which lies altogether beyond the cognisance of a molecular mechanics of

nerve-substance, and consequently beyond the cognisance of any strictly physiological inquiry.

It would fall within the scope of physiology only if we were able in some way to interpret the

psychical processes themselves as molecular processes, i.e. in the last resort, as modes of

motion or as physical energies. This, however, we cannot do: the attempt fails at once, under

whatever guise it may be made. Psychical processes refuse to submit to any one of our

physical measures of energy; and the physical molecular processes, so far as we are able to

follow them, are seen to be transformed, variously enough, into one another, but never directly

into psychical qualities. In saying this, we do not, of course, reject the idea that psychical

processes may be regularly attended by an interchange of physical forces, which as such forms

a proper object of co-ordinate investigation by the molecular mechanics of the nervous system;

nor do we deny, what would naturally follow, that psychical symptoms may he taken as

indicative of definite physiological molecular processes, and that these in their turn, if it ever

happens that we know more about them, may be taken, under certain circumstances, as

indicative of psychical conditions. But such a relation between the two departments is entirely

compatible with their separate independence, with the impossibility, at any time or by any

means, of the reduction of the one to the other. As a matter of fact, we can no more derive the

mechanics of nerve-substance from the connexions and relations [p. 103] of our sensations and

feelings, than we can derive the latter from molecular processes. We have, then, no choice as

to the road which we shall take in the following Chapters. We must first of all occupy ourselves

with the investigation of the bodily substrate of the mental life as a physiological problem; our

task being, in the main, simply to apply the principles which we have discovered in the general

mechanics of nervous substance to the complex connexions of nervous elements presented in

the nervous system of the animals, and more especially of man. Psychological facts will here

be accorded merely a symptomatic importance, in the sense defined above: we shall depart

from this rule only when the critical discussion of certain hypotheses of psychological character,

which have taken shape within nerve-physiology, requires us to raise the question whether and

how far these hypotheses receive adequate support from the physiological facts themselves.

But the general question, as to the nature of the relations which unite the mechanics of nerve-

substance and of its complex effects in the nervous system, on the one hand, and the

phenomena of the mental life, on the other, -- this question presupposes the analysis of both

sets of facts, the physiological and the psychological: so that its investigation must, naturally,

be postponed to the conclusion of the present work.

Notes

[1] A good summary of the most important facts regarding the mechanical properties of muscle

will be found in TIGERSTEDT'S Lehrbuch der Physiologie, ii., 98, 128 ff. The reader should

compare with this the recent papers of ROLLETT (PFLÜGER'S Arch f. d. ges. Physiol., lxiv.

and lxxi.), SCHENCK (ibid. lxii., lxiii., lxiv., lxv., lxvii., lxxii.) and KAISER (Zeitschr. f. Biologie,

xxxiii., xxxv., xxxvi., xxxviii.).

[2] CLAUSIUS, Abhandlungen zur mechanischen Wärmetheorie, ii., 1867, 214.

[3] Muscle-curves of this kind are termed, as proposed by A. FICK, 'isotonic' (curves of equal

tension), and distinguished from the 'isometric' curves, described when the muscle is

prevented, by over-loading, from making contractions of any considerable extent (cf. Fig. 26, p.

70). There is, of course, no such thing as a purely isotonic or purely isometric curve. In the

twitch of the muscle, there must necessarily occur changes of tension, increasing in general

with the amount of load, while an absolutely isometric muscle would not describe any curve

whatsoever. Curves may also be obtained under the further condition that the tension increases

during contraction, as when the muscle is made to pull against a spring ('auxotonic' curve), or

that it is suddenly augmented during contraction by the application of a load, and so on. The

different properties of the resulting curves are, however, of interest only for a mechanics of

muscle.

[4] Cf. my Untersuchungen zur Mechanik der Nerven und Nervencentren, Abth. i., 1871, 177.

The increase of the height of the muscle-curve with the distance of the point of stimulation from

the muscle, first observed by PFLÜGER (Untersuchungen über die Physiologie des

Elektrotonus, 140), has peen referred by many physiologists, following HEIDENHAIN (Studien

des physiol. Instituts zu Breslau, i., 1), to the effect of the section or, where connexion with the

myel is retained to the unequal decay of the nerve-tissue. If this hypothesis be correct, we must

suppose that the excitability of the living nerve is the same at all points along its course. I have,

however, shown, and the observation has been subsequently confirmed by TIEGEL

(PFLÜGER'S Arch. f. d. ges. Physiol., xiii., 598), that the greater excitability of the parts more

remote from the muscle obtains also in a living animal in which the circulation is maintained. I

found, in particular, that the lengthening of contraction, which I had myself observed to be

connected with increased length of nerve, is especially noticeable in the living nerve. This is, no

doubt, the reason that it was not seen by experimenters who worked only with muscle-nerve

preparations.

[5] Experiments on the superposition of two contractions were first made by HELMHOLTZ

(Monatsber. d. Berliner Akad., 1854, 328). He found in opposition to the results noted above,

that there was never anything more than a simple addition of the contractions. The greater

heightening of the curve of summation has, however, been confirmed by KRONECKER and

STANLEY HALL (Archiv. f. Physiologie, 1879, Supplementband, 19f.). The later experiments of

M. VON FREY (ibid. 1888, 213) and J. VON KRIES (same vol. 537) also agree in all essential

points with my own results.

[6] Mechanik der Nerven, i., 109 ff.

[7] Mechanik der Nerven, i., 63, 100.

[8] Mechanik der Nerven, i., 86, 190, 200.

[9] WUNDT, Arch. f. Anatomie u. Physiologie, 1859, 537; 1861, 781. Untersuchungen zur

Mechanik der Nerven, i., 177 ff.

[10] HELMHOLTZ, Monatsber. d. Berliner Akademie, 1854, 328. These phenomena of

summation are discussed with more accuracy of detail as regards the time-relations of the

component stimuli by J. VON KRIES, in DU BOIS-REYMOND'S Arch. f. Physiologie, 1888,

538.

[11] WUNDT, Arch. f. Anatomie u. Physiologie, 1859, 549.

[12] BOWDITCH, Journal of Physiology, vi., 1887, 133. Arch. f. Physiologie, 1890, 505.

[13] PLÜGER, Untersuchungen über die Physiologie des Elektrotonus, 1859. WUNDT,

Untersuchungen zur Mechanik der Nerven, i., 223 ff.

[14] A. FICK, Mechanische Arbeit und Wärmeentwickelung bei der Muskelthätigkeit, 1882.

[15] BERNSTEIN, Monatsber. d. Berliner Akademie, 1880, 186.

[16] According to BERNSTEIN'S investigations, PFLÜGER'S Arch. f. d. ges. Physiol., i., 190;

Untersuchungen über den Erregungsvorgang im Nerven- und Muskelsysteme, 1871, 30.

[17] The negative variation of the muscle-current is of somewhat longer duration; it lasts

approximately 0.004 sec. (BERNSTEIN, Untersuchungen, 64.) This time however, also falls

within the limits of the period of inexcitability.

[18] BIEDERMANN, Elektrophysiologie, 1895, 71 ff.

[19] It is therefore advisable, in order to induce a reflex excitability that shall suffice for

experimental series of some length, to help things out by minimal doses (0.002 to at most 0.004

mg.) of strychnine. I have convinced myself, by experiments specially directed upon the point,

that minimal quantities of this poison do not affect the temporal course of the reflex

contractions. See Untersuchungen zur Mechanik der Nerven und Nervene ntnen, ii., 1876, 9.

[20] Exceptions to this rule may occur, though very rarely, in cases of minimal reflex excitation

and minimal motor stimulation: op. cit., 21.

[21] Op. cit., 14, 30, 37.

[22] Op. cit., 45 f.

[23] Cf. Note 1, p. 86, above.

[24] KRONECKER and STIRLING, Berichte d. k. sächs. Ges. d. Wissensch. zu Leipzig, math.-

phys. Cl., 1874, 372. These observers declare, further, that the reflex twitch is invariably

distinguished from the simple muscle-contraction by its more tetanic character (Arch. f.

Physiologie, 1878, 23). I cannot follow them in this statement. It may evidently be explained by

the fact that KRONECKER and STIRLING did not avail themselves of the minimal doses of

poison referred to above, and were therefore obliged to use stronger stimuli for the excitation of

reflexes. I would not, however, be understood to maintain that any hard and fast line can be

drawn between simple contraction and tetanus at large. On the contrary, the acceleration of the

course of the simple muscle-contraction, in its ascending branch, proves that even in it several

successive excitatory impulses are at work.

[25] BEAUNIS, Rech. expér. sur les conditions de l'activité cérébrale et sur la physiologie des

nerfs, 1884, 106.

[26] HELMHOLTZ, Monatsberichte d. Berliner Akademie, 1864, 307.

[27] KRONECKER and STANLEY HALL, Arch. f. Physiologie, 1879. Supplementband, 12.

Similar observations are recorded by HORSLEY and SCHAEFER (Journ. Of Physiology, vii.

96), and, on the human subject, by GRIFFITH (ibid., ix. 39).

[28] HELMHOLTZ and BAXT, Monatsber. d. Berliner Akad., 1867, 228; 1870, 184 Experiments

by VON KRIES (Arch. f. Physiol., 1886, Supplementband. 1 ff.) gave like results.

[29] Untersuchungen zur Mechanik der Nerven, ii., 64.

[30] Op. cit. 56 f. ROSENTHAL (Monatsber. d. Berliner Akad., 1873, 104; 1875, 419) speaks of

a decrease of the latent period in strychnine tetanus, and BIEDERMANN (Elektrophysiologie,

1895, 501) accepts his statement. I do not understand this result; though with a high degree of

strychnine poisoning, and with stimuli of moderate intensity, the increase of the latent period is

not so pronounced as to be obvious at once, without the aid of some chronometric instrument.

[31] SETSCHENOW, Physiol. Studien über die Hemmungsmechanismen für die

Reflexthätigkeit des Rückenmarks, 1863. SETSCHENOW and PASCHUTIN, Neue Versuche

am Hirn und Rückenmark des Frosches, 1865.

[32] HERZEN, Sur les centres modérateurs de l'action reflexe, 1864, 32. SETSCHENOW,

Ueber die elektrische und chemische Reizungder sensibeln Rückenmarksnerven, 1868, 40.

[33] GOLTZ, Beiträge zur Lehre von den Functionen der Nervencentren des Frosches, 1869,

44, 50. That other parts of the brain, besides those designated by SETSCHENOW, are able to

inhibit reflexes, was demonstrated by GOLTZ in his croak experiment. Frogs whose cerebral

hemispheres have been removed may be made to croak, with almost mechanical certainty, by

a gentle stroking of the skin of the back; while with uninjured animals the same procedure very

frequently fails of its effect. It appears, then, that the cerebral hemispheres also have the power

of inhibiting reflexes (GOLTZ, Op. cit., 41). Experiments made by LANGENDORFF (Arch. f.

Physiol., 1877, 133) and BÖTTICHER (Ueber Reflexhemmung, in Sammlung physiol. Abhandl.,

ii. Reihe, Heft 3) show that the same result may be obtained by blinding the animals.

[34] Untersuchungen, etc., ii., 84 ff., 106 ff. Morphine, on the other hand, seems, at a certain

stage of its action, to increase the central inhibitions. For it was found by HEIDENHAIN and

BUBNOFF that the contractions produced in animals by stimulation of the motor areas of the

cerebral cortex were, in the normal state, enhanced, but in morphine narcosis inhibited, by

mechanical stimulation of the skin. See PLÜGER'S Arch. f. d. ges. Physiol., xxvi., 137 ff.

[35] E. CYON has turned this idea to account for a theory of the central inhibitions: Bulletin de

l'Acad. de St. Pétersbourg, vii., Decr., 1870. The facts which he adduces in support of it, so far

as they are taken from the phenomena of vascular innervation, have been called in question by

HEIDENHAIN. PFLÜGER'S Arch. f. d. ges. Physiol., iv. 551.

[36] BRONDGEEST, Over den Tonus der willekeurigen Spieren, Utrecht, 1860.

[37] CYON, Berichte d. k. sächs, Ges. d. Wiss., math.-phys. Cl., 1865, 86. Cf. on the other side,

G. HEIDENHAIN, in PFLÜGER'S Arch. f. d. ges. Physiol., iv., 1871, 435.

[38] H. E. HERING and SHERRINGTON, in PFLÜGER'S Arch. f. d. ges. Physiol., lxviii., 1897,

222 ff.

[39] Cf. p. 53, above.

[40] Cf. pp. 55 f., above.

[41] Cf. the discussions in Part V. of the abnormal enhancement of excitability in the cerebral

cortex, which presumably underlies certain forms of derangement of consciousness (dream,

hypnosis).

[42] See above, Ch. ii, pp. 42 ff. I may be permitted to remark that this theory of the directions

of central conduction as formulated, on the ground of purely physiological considerations

(Untersuchungen, etc., ii., 1876, 116), long before RAMON Y CAJAL used the morphological

facts to develope his views of the functional significance of the twofold origin of the nerve-fibres.

Cf. also p. 50.

[43] T. W. ENGELMANN, PFLÜGER'S Arch. f. d. ges. Physiol., lvi., 1894, 149 ff.

[44] I take this very useful term Bahnung from S. EXNER, who first proposed it (Entwurf einer

physiologischen Erklärung der psychischen Erscheinungen, i., 1894, 76), without meaning

thereby to commit myself in any way to the views and hypotheses put forward by this author.

(As the word 'facilitation' does not fit the present passage, we seem to have no better English

term than 'canalisation.' -- TRANSLATOR.)

[p. 150] CHAPTER V

Course of the Paths of Nervous Conduction

§ I. General Conditions of Conduction

Our examination of the structural elements of the nervous system led us to conceive of the

brain and myel, together with the nerves issuing from them, as a system of nerve-cells, inter-

connected by their fibrillar runners either directly or through the contact of process with

process. Our recent survey of the morphological development of the central organs lends

support to this conception. We have found a series of cinereal formations which collect the

fibres running centralward from the external organs and mediate their connexion with other,

especially with more centrally situated grey masses. The paths of conduction that begin in the

myelic columns pass upwards first in the crura and then in the corona until they penetrate the

cerebral cortex. There we have the commissures, pointing to the inter-connexion of the central

regions of the two halves of the brain, and the intergyral (arcuate) fibres, indicating the

connexion of the various cortical zones of the same hemisphere. Hence from whichever point of

view we consider the outward conformation of the central organs, we are presently met by the

question as to the course taken by the various paths of nervous conduction. We know, of

course, that the cell-territories stand, by virtue of the cell-processes, in the most manifold

relations. We shall accordingly expect to find that the conduction-paths are nowhere strictly

isolated from one another. We must suppose, in particular, that under altered functional

conditions they may change their relative positions within very wide limits. But we may fully

admit such a relative variability of functional co-ordination as is suggested by the neurone

theory, and yet with justice raise the question of the preferred lines of conduction; -- of the lines

which, under normal circumstances, are chiefly concerned to mediate determinate connexions,

on the one hand, between the central regions themselves, and on the other, between the

centre and the peripheral organs appended to the nervous system. This answered, we may in

certain cases proceed to ask a second question, regarding the auxiliary paths or bypaths which

can replace the regular lines of transmission in particular instances of interrupted conduction or

of inhibition of function.[p. 151]

We distinguish two main kinds of conduction-path, according to the direction in which the

processes of stimulation are transmitted: the centripetal and the centrifugal. In the former, the

stimulation is set up at some point on the periphery of the body, and travels inwards, toward the

central organ. In the latter, it issues from the central organ, and travels toward some region of

the periphery. The physiological effects of a centripetally conducted stimulation, when they

come to consciousness, are termed sensations. Frequently, however, this final effect is not

produced; the excitation is reflected into a movement, without having exerted any influence

upon consciousness. Nevertheless, the paths of conduction traversed in such a case are, at

least in part, the same. We therefore give the name of 'sensory' to the centripetal conduction-

paths at large. The physiological effects of centrifugally conducted stimulation are very various:

it may find expression in movements of striated and non-striated muscles, in secretions, in

heightened temperature, and in the excitation of peripheral sense-organs by internal stimuli. In

what follows, we shall, however, confine our attention for the most part to the motor and the

centrifugal-sensory paths, since these are the only parts of the centrifugal conduction-system

that call for consideration in psychology. The muscular movements that result from the direct

translation of sensory stimulation into motor excitation are termed reflex movements; those that

have their proximate source in an internal stimulation within the motor spheres of the central

organ we shall call automatic movements. In the reflex, i.e., centripetal is followed by

centrifugal conduction; in the automatic movement, centrifugal conduction alone is directly

involved.[1]

So long as the stimulation-process is confined within the continuity of determinate nerve-fibres;

as occurs e.g. in the peripheral nerves, which often traverse considerable distances, it remains

as a general rule isolated within each particular fibre, and does not spring across to

neighbouring paths. This fact has been expressed in the law of isolated conduction. The law

has usually been regarded as valid not only for the periphery, but for the conduction-paths

within the central organs as well; on the ground that an external impression made upon some

precisely localised part of a sensitive surface evokes a sharply defined sensation, and that a

voluntary impulse directed upon a definite movement produces contraction of a circumscribed

group of muscles. Really, however, these facts prove nothing more than that the processes in

the principal paths are, as a rule and under normal conditions, separate and distinct. It has not

been demonstrated with certainty that the stimulation is strictly confined to a single primitive

fibril, even during the peripheral portion of its course. And in the central parts, any such

restriction is entirely out of the question, as appears both from the general [p. 152]

morphological features discussed in Chapter II., and from the phenomena of vicarious function

which we shall speak of presently. The only principle that can be recognised here is a principle

of preferential conduction. There is in every case a principal path, but this is supplemented by

auxiliary or secondary paths.

§ 2. Methods of Investigating the Conduction-Paths

We may avail ourselves of three distinct methods, in our examination of nervous conduction.

Each one of them has certain imperfections, and must therefore be supplemented, where

possible, by the other two. The first method is that of physiological experimentation; the second

is that of anatomical investigation; and the third is that of pathological observation.

(I) Physiological experimentation attempts: to reach conclusions as to the course of the nervous

conduction-paths in two ways: by stimulation-experiments, and by interruptions of conduction

due to a division of the parts. In the former case, we look as a general rule for enhancement, in

the latter for abrogation of function, in the organs connected with the stimulated or divided

tissue. When we come to the investigation of the central paths, however, we find that both

methods alike are attended by unusual difficulties and disadvantages. Even in the most

favourable instances, when the stimulation or transsection has been entirely successful, we

have established but one definite point upon a path of conduction; to ascertain its full extent, we

should have to make a large number of similar experiments, from the terminal station in the

brain to the point of issue of the appropriate nerves. Such a task holds out absolutely no hope

of accomplishment, since the isolated stimulation or section of a conduction-path in the interior

of the brain presents insuperable obstacles. There are, therefore, only two problems to which

these methods can be applied with any prospect of success. We may use them to determine

the course of conduction in the simplest of the central organs, the myel, and in the direct

continuations of the myelic columns, the crura; and we may use them to discover the

correlation of definite areas of the brain cortex with definite organs upon the periphery of the

body. The answer to the former question has been attempted, for the most part, by isolated

transsection of the various myelic columns; the answer to the second, by experiments upon the

stimulation and extirpation of definitely limited cortical areas. Even with this limitation, however,

it is difficult to secure valid results. A stimulation will almost inevitably spread from the point of

attack to the surrounding part. This objection applies with especial force to the electric current,

almost the only form of stimulus which fulfils the other requirements of physiological

experiment, and a stimulus which the physiologist is therefore practically compelled to employ.

The same thing is true of the disturbances consequent upon a division of [p. 153] substance.

And if one is at last successful in securing the utmost degree of isolation of experimental

interference, there will still be many cases in which the interpretation of the resulting

phenomena is uncertain. The muscular contraction that follows upon a stimulation may, under

certain circumstances, be due to a direct excitation of motor fibres, just as well as to a reaction

upon the sense-impressions. And the derangements of function that appear as a result of

transsections and extirpations always require a long period of observation before they can be

accurately determined. This means that the certainty of the conclusions is, again, very largely

impaired: the disturbances set up as the direct effect of operation for the most part disappear

as time goes on, the explanation being that the principal path is functionally replaced by the

secondary paths of which we spoke just now.

(2) The gaps left in our knowledge by the physiological experiment are largely filled out by

anatomical investigation. The anatomist has followed two methods in the prosecution of his

task: first, the macroscopic dissection of the hardened organ, and, later, its microscopic

reduction to a series of thin sections. Of late years, the former of these two methods has fallen

into disrepute, on the score that it runs the risk of substituting artificial products of the

dissecting scalpel for real fibre-tracts. Carefully applied, however, it is a valuable means of

orientation with regard to certain of the wider roads of brain-travel; while its critics are inclined,

on their side, to underestimate the danger of error in the interpretation of microscopic

appearances. And this danger is the more serious, the farther we are from the actual

attainment of the ideal goal of a microscopic examination of the central organ, its complete

reduction to an infinite series of sections of accurately known direction. For the rest,

microscopical anatomy has been brought in recent times to a high degree of perfection by the

application of the various methods of staining. The advantage of these is that they permit of the

more certain differentiation of nerve-elements from the other elementary parts, and thus enable

us to trace the interconnexion of the nerve-elements much farther than had before been

possible.[2] Anatomical investigation is, further, very materially supplemented by embryological

research. Embryology shows that the formation of the myelinic sheath in the various fibre-

systems of the central organs occurs at different periods of foetal development, and thus puts it

in our power to trace out separately certain paths of travel that in all likelihood are

physiologically interconnected. This method, however, like the others, has its limits: the

systems that develope simultaneously may [p. 154] still include numerous groups of fibres,

possessing each a different functional significance.[3]

(3) Pathological observation is equally concerned with functional derangement find with

anatomical change, and so in a certain measure combines the advantages of physiological and

of anatomical investigation. The observations of pathological anatomy have been especially

fruitful for the study of the nervous conduction-paths. Abrogation of function over a determinate

functional area means that the fibres belonging to that area undergo secondary degeneration.

The pathological anatomist can, therefore, appeal to a law very similar to that upon which

embryological investigation is based. Unless there are extrinsic conditions present, which

render an accidental concurrence of the degeneration probable, he can assume that all fibres

which suffer pathological change at one and the same time are functionally related.[4] The

observation of secondary degenerations is of especial value when conjoined with physiological

experimentation. The joint method may follow either of two different paths. On the one hand, a

severance of continuity may be effected at some point in the central or peripheral nervous

system of an animal, and the consequent functional derangement observed. Then, after a

considerable time has elapsed, the paths to which the secondary degeneration extended can

be made out by anatomical means. On the other, a peripheral organ (eye, ear, etc.) may be

extirpated in early life, and the influence observed which the abrogation of determinate

functions exerts upon the development of the central nervous organs.[5] In the former case, the

nerve-fibres evince the successive stages of degeneration represented in Fig. 23, p. 53. In the

latter, the parts of the brain which serve as the centres for the abrogated functions sink in, and

microscopic examination shows their nerve-cells in the various stages of atrophy that lead up,

in the last resort, to complete disappearance (Fig. 22 B, p. 53).

The first extensive collection of material for the investigation of the microscopical structure of

the central organs was furnished by the researches of STILLING. The earliest attempts to

construct a structural schema of the whole cerebrospinal system and its conduction-paths by

STILLING'S method, i.e. by the microscopical examination of sections, date from MEYNERT

and LUYS.[6] MEYNERT, especially, rendered great services to the science; he brought to his

re-[p. 155]construction of brain-structure the results of a comprehensive series of original

investigations and a rare power of synthetic imagination. It is true, of course, that the schema of

conduction-paths which he published was largely hypothetical, and that it has already been

proved erroneous in many details. Nevertheless, it formed the point of departure for further

microscopical research; so that most of the later work takes up a definite attitude to

MEYNERT'S structural schema, supplementing or amending. The application of the various

methods of staining, and the consequent differentiation of the nervous elements, have played

an important part in this chapter of scientific enquiry. Embryological investigation depends upon

the fact that the myelinic sheath is formed in the different fibre-systems at different periods of

embryonic development. It is this sheath which is responsible for the coloration of the alba, so

that its appearance is easily recognisable. The signs of secondary degeneration consist on the

other hand, in a gradual transformation of the myelinic sheath. The tissue becomes receptive of

certain colour-stains, like carmine, by which it is unaffected in the normal state. Finally, the

myelinic sheath disappears altogether. At the same time, the nerve-fibres proper (neurites)

change to fibres of connective tissue, interrupted by fat granules. The value of these

degenerative changes for the investigation of conduction-paths lies in the fact that the

progressive transformation is always confined within an interconnected fibre-system, and that

the direction which it takes corresponds in all fibres with the direction of conduction (WALLER'S

law); so that the degeneration of motor fibres follows a centrifugal, that of sensory fibres a

centripetal course. Nevertheless, this law of WALLER, like the law of isolated conduction,

appears to be valid only as regards the principal direction of the progress of degeneration. In

cases where the interruption of conduction has persisted for a considerable length of time, and

more particularly in young animals, the deterioration of the fibres is always traceable, to some

extent, in the opposite direction as well. We have, further, besides atrophy of the nerves

separated from their centres, a similar though much slower degeneration of the nerve-cells

which have been thrown out of function by transsection of the neurites issuing from them. This

secondary atrophy of the central elements, the initial symptoms of which are the changes

represented in Fig. 22, p. 53 above, is, again, especially likely to appear in young animals. It

may, however, occur in the human adult, after long persistence of a defect. Thus it has been

observed that loss of the eye is followed by atrophy of the quadrigemina; nay, more, in certain

cases of the kind, a secondary atrophy of certain cerebral gyres has been demonstrated.

§ 3. Conduction in the Nerves and in the Myel

(a) -- Origin and Distribution of the Nerves

The nerve-roots leave the myel in two longitudinal series, a dorsal and a ventral. The dorsal

nerve-roots, as a simple test of function by stimulation or transsection shows, are sensitive:

their mechanical or electrical stimulation produces pain, and their transsection renders the

corresponding cutaneous areas anaesthetic. The ventral nerve-roots are motor: their

stimulation produces muscular contraction, and their transsection muscular [p. 156] disability.

The fibres of the dorsal roots conduct centripetally; if they are transsected, stimulation of the

central cut end will give rise to sensation, but not that of the peripheral. The fibres of the ventral

roots conduct centrifugally; in their case, stimulation of the peripheral cut end will give rise to

muscular contraction, but not that of the central. These facts were first discovered by

CHARLES BELL, and their general statement is accordingly known as ' BELL'S law.' They

prove that, at the place of origin of the nerves, the sensory and motor conduction-paths are

entirely separate from each other. The same thing holds of the cranial nerves, with the addition

that here, in most cases, the separation is not confined to a short distance in the

neighbourhood of the place of origin, but persists either throughout the whole course of the

nerves or at least over a considerable portion of their extent.[7] There can be no doubt that the

union of the sensory and motor roots, to form mixed nerve-trunks, finds its explanation in the

spatial distribution of the terminations of the nerve- fibres. The muscles and the overlying skin

are supplied by common nerve-branches. While, therefore, the two sets of conduction-paths

are functionally distinct, a spatial separation throughout their entire course occurs only in

certain cranial nerves, where the terminations are comparatively near to the points of origin, but

the points of origin themselves lie farther apart. Under these circumstances, a separate course

involves simpler space-relations than an initial union of the sensory and motor fibres, such as

we find in the trunks supplying adjacent parts of the body.

Not only the origin, but also the further peripheral course of the nerves is very largely

determined by the conditions of distribution. Fibres that run to a functionally single muscle-

group, or to adjacent parts of the skin, are collected into a single trunk. Hence it does not follow

that the mixed nerve, formed by the junction of ventral and dorsal roots, always proceeds

simply and by the shortest path to its zone of distribution. On the contrary, it frequently happens

that there is an interchange of fibres between nerve and nerve, giving rise to what are called

the nerve-plexuses. In explaining the occurrence of these plexuses, we must remember that

the disposition of the nerve-fibres, as they issue from the central organ, meets the conditions of

their peripheral distribution only in a rough and provisional manner; the arrangement is by no

means perfect, and requires to he supplemented later on. The plexuses are most commonly

formed, therefore. at places where there are parts of the body that need large nerve-trunks, e.g.

the two pairs of limbs. Here it is evidently impossible, from the spatial [p. 157] conditions of their

origin, that the nerves should leave the myel in precisely the order that is demanded by their

subsequent peripheral distribution. But the plexus-formation is not only supplementary; it is,

beyond question, compensative as well. The nerve-fibres that are nearest together as the

nerves leave the central organs are those that are functionally related. Now functional relation

does not always run parallel with spatial distribution. Thus the flexors of the upper and lower

leg, e.g., are functionally related, and act in common; but those of the upper leg lie upon the

ventral and those of the lower upon the dorsal side of the body, and consequently receive their

nerves from different nerve-trunks, the crural and the sciatic respectively. If, then, the nerves

for the flexors of the whole limb are in close proximity at their place of origin, there must he a

rearrangement of fibres in the sacrolumbar plexus, in order that the two trunks may pass off in

different directions. It is probable that the simpler connexions of the root-pairs are principally

useful as supplementary mechanisms, while the more complicated plexus-formations are for

the most part compensatory in function.

When BELL first established the law known by his name, he felt constrained by it to postulate a

specific difference between sensory and motor nerves, -- a difference which found expression

in this fact of the difference in direction of conduction, Physiologists for a long time afterwards

gave in their adhesion to this hypothesis. There was, indeed, a prevailing tendency to refer all

differences of function, e.g. those obtaining between the various sensory nerves, to conic

unknown specific property of the nerve-fibres.[8] Later on, the belief gained ground that the

nerves are simply in different conductors of the processes released in them by stimulation;

though the only argument at first brought forward in support of it was the not very convincing

external analogy of electrical conduction.[9] At the present time, we may say, with better

reason, that BELL'S hypothesis of a specific conductive capacity of sensory and motor nerves

is not tenable. The decisive evidence is drawn from two sources. On the one hand, the general

mechanics of nerve-substance has thrown new light upon the processes of conduction in the

peripheral nerve-fibre (pp. 80 ff. above). On the other, the morphological facts indicate that the

difference in direction of conduction depends upon the mode of connexion of the nerve-fibres at

centre and periphery. Figgs 20 and 27 (p. 50 above) gave a schematic representation of the

structural relations involved in the two cases. Every [p. 158] motor fibre, as we said in

describing them, is the neurite of a nerve-cell; and there is a certain principle of transmission of

force which supposedly holds for all neurites alike. In accordance with this principle, the neurite

is able to take up the stimulation-processes originated within the cell, or carried to it by its

dendrites; but the excitatory processes of which the neurite is itself the sent, though they are

conducted to the cell as a result of the general diffusion that every stimulation-process

undergoes in the nerve fibre, are inhibited in the central substance of the cell (p. 99). The cells

of origin of the sensory fibres always lie, on the contrary, outside of the central organ: in the

invertebrates, for the most part at the periphery of the body; in the vertebrates, at any rate

outside of the myel proper. Here, as we know, they form as it were little centres of their own;

the spinal ganglia, situated in the intervertebral foramens. These ganglia are composed

throughout of bipolar nerve-cells; that is to say, each cell sends out two, morphologically

identical processes, which probably have the character of dendrites. In the lower vertebrates,

the processes issue at different points -- in the fishes, at opposite sides -- of the cell. In man,

the same conditions obtain in the early stages of development. As growth proceeds, however,

the two processes fuse together at their point of origin, so that what were at first two distinct

and separately originated processes now appear as the branches of one single process (Fig.

21, p. 50), which nevertheless retains the character of a protoplasmic process or dendrite. The

two processes thus form a single neurone territory (N1), which divides into two halves. The one

lies within the myel, and after giving off numerous collaterals penetrates with its terminal fibres

into a second central neurone territory (N2). The other is continued in the sensory nerves, and

is finally lost either in terminal arborisation among the epidermal cells, or in special end-organs,

adapted for the support of the nerve-ramifications (H Fig. 21). We may therefore suppose, in

agreement with what was said above regarding the diffusion of excitations carried by the

dendrites (p. 42), that, where the peripheral and central processes issue separately from the

cell-body, the process of stimulation is transmitted directly by the cell. When, as in man, the two

processes unite to form one, the passage across, and then the transmission to higher neurones

(N2), may actually take place within the fibre itself. The cell Z1 seems in this case to be cut out

of the line of nervous conduction by a sort of short circuit; though this, of course, does not

diminish its importance as a nutritive centre and storehouse of force. If, then, we regard the

processes of conduction as conditioned in this way by the properties of the nerve-cells and the

mode of termination of their processes within them, the principle of conduction in a single

direction will hold only for the connexions of neurones, not for the nerves themselves; the

stimulation of any nerve, at any point of its course, must, so long as its continuity [p. 159] is

preserved, be followed by the production of a stimulus wave which spreads out centripetally

and centrifugally at one and the same time. But, as a matter of fact, there is not the slightest

reason why we should hesitate to adopt this hypothesis. It is obvious that, when a sensory

nerve is cut across, the excitations carried to the peripheral cut end must disappear at the

periphery without effect, i.e. without arousing any sensation in our mind, just precisely as the

stimuli which act upon the central cut end of a transsected motor nerve are inhibited in the cell

connected with the neurite. In both cases, it is not any property of the nerve-fibres, but the

character of the nerve-cells, that is responsible for the result. We can see, more especially

when we consider the different modes of origin of the cell-processes, that the nerve-cell is

naturally qualified to determine the direction of conduction and to regulate the mode of

transmission from one neurone territory to another. For the rest, we shall presently become

acquainted with facts that speak definitely for a centrifugal conduction of certain sensory

excitations (pp. 182 ff.).

(b) -- Physiology of the Conduction-Paths of the Myel

We have now to investigate the farther course of the nerve-paths that lead into the myel, as

they are continued in the interior of this central organ. We obtain information concerning them,

in the first place, from physiological experiments upon the result of stimulation, and more

especially upon the effect of transsection, of certain portions of the myel. We know that the

motor roots enter the ventral, and the sensory roots the dorsal half of the myel. These

experiments show that the principal lines of conduction retain the same arrangement, as they

take their course upwards. The effects of outside interference with the ventral portion of the

myel are predominantly motor; with the dorsal portion, predominantly sensory. At the same

time, they show also that even in the myel the individual fibre-systems are interwoven in the

most complicated fashion. The results of hemisection of the myel, e.g., prove that not all

conduction- paths remain upon the same side of the body upon which the nerve-roots enter the

myelic substance, but that some of them cross over within the myel from right to left and vice

versâ. It is true that the statements of various observers as to the kind and extent of conductive

disturbance after hemisection are not in complete agreement; and it is evident, also, that the

relations of conduction are not identical throughout the animal kingdom. But experiments on

animals and pathological observations on man have put it beyond question that the sensory

fibres, at any rate, always undergo a partial decussation. Hemisection of the myel does not lead

to a complete abrogation of sensation upon either half of the body. The motor paths [p. 160]

appear be more variable in this respect. Experiments on animals again point to a partial

decussation, though to one in which the greater part of the fibres remain upon the same side.

Pathological observations, on the other hand, lead to the conclusion that in the myel of man the

motor paths are uncrossed. We may, in particular, recall the well known fact that in unilateral

apoplectic effusions in the brain, it is always only the one side of the body, viz. the side

opposite to the apoplectic area, that is paralysed. Now there is, as we shall see presently, a

complete decussation of the motor paths in the oblongata. If, then, decussation occurred to any

considerable extent in the myel, the one arrangement would of necessity, so far as it went,

compensate the other. We accordingly conclude that the principal motor path is situated in the

ventral portion of the myel. And we are safe in affirming, similarly, that the principal sensory

path lies in the dorsal portion,[sic] In the animals, it is true, we have a greater number of

secondary paths, branching off to other parts of the myel, than we have in man, but even in the

animals, there can be no doubt that the great majority of the fibres run their course without

decussation. Impressions made upon the skin after hemisection of the myel upon the same

side are not sensed, though stronger, painful stimuli will still evoke a reaction. Finally, in the

lateral columns of the myel (m Fig. 66, p. 164) we have a combination of motor and sensory

paths, drawn again for the most part from the fibre-systems of the same side. If the integrity of

these columns be impaired, whether in man or in the animals, the resulting symptoms are in

general of a mixed character.[10]

These experiments upon severance of continuity at various parts of the myel have brought to

light a somewhat complicated interlacement of the fibre-systems. One of the chief factors in the

resulting formation is, undoubtedly, the cinerea which surrounds the myelocele. The presence

of this grey matter also explains the change of irritability brought about in the myelic fibres by

stimulation-experiments. While the peripheral nerves may be readily excited by mechanical or

electrical stimuli, this is so far from being the case with the myelic fibres that many of the earlier

observers declared them to be wholly irresponsive to stimulation.[11] The statement, in its

extreme form, undoubtedly overshoots the mark. Excitation can always be effected by

summation of stimuli, or by help of poisons, like strychnine, which enhance the central

irritability. At the same time, the marked change of behaviour points clearly to the intercalation

of grey matter (cf. p. 86 above). If it be asked in what way this intrusion can materially affect the

processes of conduction, we reply that a path coming in from [p. 161] the periphery will be

brought into connexion, by the cinerea, not with one but with many paths of central conduction.

These paths will, it is true, not be all equally permeable. Some will offer more resistance than

others to the passage of excitation: in certain cases inhibitory effects, of the kind with which we

have become familiar as the results of certain modes of connexion of the central elements (p.

99), may destroy or modify excitations already in progress. But there will always be various

secondary paths available, over and above the principal path. The experiments upon severance

of continuity call our attention chiefly to the principal path; but there are several ways --

increased intensity of stimulus, enhancement of irritability, destruction of the principal path -- in

which the secondary paths may be thrown into function. If the white columns are entirely cut

through, at any point upon the myel, so that only a narrow bridge of cinerea remains intact,

sense-impressions and motor impulses may still be transmitted, provided that they are

unusually intensive. And we find, similarly, that the phenomena of disability, which appear on

transsection of a portion of the white columns, disappear again, after a short interval, although

the cut has not healed.[12] The existence of these secondary paths or by-paths is attested,

further and more particularly, by the phenomena of transference from one conduction-path to

another, phenomena which prove the presence of a connecting path between different

conduction-paths. They are of three kinds: the phenomena of concomitant movement, of

concomitant sensation, and of reflex movement. As all alike are of importance fur a right

understanding of the functions of the central organs, especially of the myel, we shall be

occupied with them in the following Chapter. We are interested in them here only in so far as

they bear witness to the existence of determinate conduction-paths, preformed in the myel, but

functioning only under certain special conditions. The place of transference from motor to

motor, sensory to sensory, or sensory to motor paths must be sought, again, in the cinereal

structures. Complete severance of the cinerea, with retention of a portion of the dorsal and

ventral columns of white matter, abrogates the phenomena in question. Transferences within

the motor paths, manifesting themselves in concomitant movements, may be made, without

any doubt, either on the same side of the myel or from the one side to the other. Thus, the

innervation of a finger-phalanx is transmitted both to other fingers of the same hand and, under

certain circumstances, to the skin of other parts of the body. Bilateral concomitant movements

of this kind are especially observable in movements of locomotion and in pantomimic

movements. It is clear, on the other hand, that excitatory innervation within a definite path may

be connected with inhibitory co-excitation of the cells of origin of another motor path. We have

an instance of this state of things in the relaxation [p. 162] of tonus in the extensors that goes

with excitation of the flexors of a limb.[13] This illustration, like that of co-ordinated movements,

shows further that co-excitations within the motor paths may establish themselves as regular

functional connexions. Transferences within the sensory paths seem, contrariwise, to be

confined almost exclusively to the same half of the myel. The concomitant sensations observed

after stimulation of some part of the skin are nearly always referred to cutaneous regions on the

same side of the body. They are brought out most clearly by painful stimuli and by the arousal

of tickling: in the latter case, more particularly when the skin is rendered unusually sensitive by

enhancement of irritability. Under these conditions, stimulation of certain regions of the skin

usually evokes sensations in other regions. Certain sensory parts, e.g. the external auditory

meatus and the larynx, are also pre-eminently disposed for concomitant sensation.[14] We can

hardly explain these facts otherwise than by the hypothesis that, on the one hand, certain

preferred paths of connexion exist within the sensory conduction-paths, and that, on the other,

certain sensory areas (larynx, external auditory meatus) are peculiarly susceptible to co-

excitation. As regards the conditions under which conduction takes place, it is clear that

concomitant movements and concomitant sensations both alike depend upon cross-

conductions, that may be effected at different heights in the myel, and that differ only in

direction: the motor cross-conductions extending in all directions, while the sensory, so far as

we can tell, are almost exclusively unilateral, and for the most part follow the direction from

below upwards. For the rest, the concomitant sensations and concomitant movements that

have their ground in connexions of the myelic conduction-paths can never be certainly

discriminated from those mediated by transference within the higher centres.

This statement does not apply to transferences of the third kind, reflex connexions of sensory

and motor paths. The myelic reflexes may be observed for themselves alone, after the myel

has been separated from the higher central parts. The conclusion to be drawn from such

observation is that branch-conduction of the reflexes is effected by a large number of

conduction-paths, all of which are closely interconnected. Moderate stimulation of a

circumscribed area of the skin is followed, at a certain mean degree of excitability, by a reflex

contraction in the muscle-group, and in that only, which is supplied by motor roots arising at the

same height and on the same [p. 163] side as the stimulated sensory fibres. If stimulus or

irritability be increased, the excitation passes over, first of all, to the motor root fibres that leave

the myel at the same height upon the opposite side of the body. Finally, if the increase be

carried still farther, it spreads with growing intensity first upward and then downward; so that in

the last resort it involves the muscles of all parts of the body which draw their nerve-supply

from myel and oblongata. It follows, then, that every sensory fibre is connected by a branch-

conduction of the first order with the motor fibres arising on the same side and at the same

height; by one of the second order, with the fibres issuing at the same height upon the opposite

side; by branch-conductions of the third order, with the fibres that leave the myel higher up;

and, lastly, by branch-conductions of the fourth order, with those that emerge lower down.[15]

This law of the diffusion of reflexes may, however, as we shall see in the following Chapter, be

modified in two ways: by variation of the place of application of the reflex stimulus, and by the

simultaneous application of other sensory stimuli (cf. Chap. VI. § 2).

The conclusions which we have reached by way of physiological experimentation regarding the

course of the conduction-paths in the myel are in complete agreement with the morphological

facts revealed by histological examination of this organ. In particular, the arrangement of the

nerve-cells and of the fibre-systems which take their origin from the cell-processes as shown in

transverse and longitudinal sections, enables us to understand at once that every principal path

is here accompanied by a large number of secondary paths, and that the most manifold

connexions obtain between one line of conduction and another. We see, first of all, that the

fibres of the ventral roots enter directly into the large nerve-cells of the ventral cornua, whose

neurites they form; whereas the fibres of the dorsal roots, after their interruption by the nerve-

cells of the spinal ganglia, divide upon entering the myel into ascending and descending

systems, which there give off delicate branches at all points into the cinerea of the dorsal

cornua. Here, therefore, as in the experiments with transsection, the white columns (l, m, n Fig.

66) appear in the role of principal paths: their ventral portions as motor, their dorsal as sensory.

Secondary paths, for the conduction of unusually intensive excitations, or for the transferences

required by concomitant movements, concomitant sensations and reflexes, can be mediated in

a great variety of ways by the cellular and fibrillar system of the central cinerea (d, e). The

interrelations of these different paths of conduction, and in particular of the two groups that in

functional regard [p. 164] stand farthest apart, the motor and sensory, are then determined by

their mode of connexion with their cells of origin, and with the processes which these cells give

off. We thus find, in the properties of the neurone and its area of distribution as manifested

within the myel, a continuation of the differences that we meet with in the primitive forms

represented in Fig. 20, 21 (p. 50). Fig. 67 shows the various morphological elements in their

natural connexion. Each of the large multipolar cells m of the ventral cornu has direct control of

some peripheral region by means of its neurite n, which does not break up into its terminal

arborisation until it reaches the terminal plate of a muscle-fibre (Fig. 20, p. 50). On the other

side. the dendrites issuing from the same cell run a very short course, to enter at once into the

cinerea of the ventral cornu. The dendritic reticulum stands in direct contact with the terminal

fibrils of the neurite g of another nerve-cell, situated as a rule high up in the brain; so that the

neurones of this motor conduction cover very

extensive territories. Indeed, it is probable that in

most instances the entire motor conduction involves

only two neurones (N1, and N11, Fig. 20), the one of

which extends from the cell n of the ventral cornu to

the periphery of the body, while the other begins with

some one of the fibres that run their course in the

ventral or lateral column (l, m Fig. 66), and ends in a

cell of the cerebral cortex. At the same time, still

others of the dendrites belonging to the cells of the

ventral cornua are in contact with the processes of

the small cells s of the dorsal cornua, and with the

small intercalatory or commissural cells c that lie

scattered between ventral and dorsal cornu. In these

latter connections we have, presumably, the

substrate of reflex conduction. The sensory nerve-

paths, on the other hand, follow a very

different course. In their [p. 165] case, the spinal

ganglion-cell

sp forms the

central point of

a neurone

territory, the

one half of

which extends

by means of

the

peripherally

directed

processes h to

the sensory

termini of the organ of touch (Fig. 21, p. 50), while the

other runs centralward in the central process f, which

divides in the dorsal portion of the myel into ascending

and descending branches (a, d). Both of these branches

give off numerous collaterals, whose terminal

ramifications stand in contact with the small cells of

commissure and dorsal cornu. They themselves are

finally resolved into fibrillar reticula, connected by contact

with the dendrites of cells lying farther up and lower

down. These structural relations seem to warrant the

inference that the collaterals correspond to the various

secondary paths by which transference, and especially

reflex transference, is effected, and that the ascending

and descending fibres constitute the principal path. The

principal path of sensory conduction is, however,

markedly different front the meter. As a general rule,

there are several breaks in the line; the path consists of

a number of neurone chains, arranged one above

another. And this means, again, that the conditions of

conduction in the principal path are less sharply

distinguished from those in the secondary paths that begin in the collaterals. The whole

morphological plan of the system of sensory conduction thus suggests a co-ordination of parts

that is at once less strict and more widely variable than is the case on the motor side.

We have spoken so far only of the general properties of the myelic conductors, properties

accruing to all nerve-fibres whose mode of origin [p. 166] and connexions conform to a certain

type. In the higher regions of the myel, other conditions are at work, paving the way for that

differentiation of the conduction-paths which characterises the higher central regions. Even as

low down as the thoracic portion of the myel, certain funicles divide off from the three principal

columns already named, the ventral, lateral, and dorsal columns (l, m, n Fig. 66). The principal

paths, sensory and motor, that run their course within the length of the myel, are thus split up

into several separate tracts. The significance of these new funicles can best be understood

from their embryological connexions and from the course of the degenerations observed in

pathological cases (pp. 154 f.). It can be shown, by both lines of evidence, that the motor

division of the lateral columns ascends uncrossed in their

dorsal half, in a funicle which, as seen in cross-sections,

encroaches from the outside upon the cinerea of the dorsal

cornu. Higher up, it passes over into the pyramids of the

oblongata, and is accordingly known as the path of the

pyramidal lateral column (Fig. 68). In the same way, the

innermost division of the motor ventral columns, the part

bordering directly upon the ventral sulcus, ascends

uncrossed to the oblongata, where it too passes over into the

pryamids [sic]. It is termed the path of the pyramidal ventral

column, and is the only division of the pyramidal tracts to

remain uncrossed in the oblongata. Of the more peripherally

situated funicles of the ventral column, some take a straight

course upwards, while others enter the ventral commissure

and cross to the opposite side of the body. The division of

the lateral column which overlies the pyramidal lateral

column, at the periphery of the myel, is an uncrossed and, to

judge from the conditions of its origin, a sensory path: it

branches off to the cerebellum by way of the postpeduncles,

and is termed the path of the cerebellar lateral column. The

dorsal columns, which are exclusively sensory in function,

and therefore receive from below the great majority of the fibres that enter the dorsal roots,

divide in the cervical region into two funicles: the slender funicles or columns of Goll (fun.

graciles), and the more outlying cuneate funicles (fun. cuneati, Fig. 68).[16][p. 167]

§ 4. Paths of Conduction in Oblongata and Cerebellum

(a) -- General Characteristics of these Paths

Oblongata and cerebellum, the parts of the brain stem that correspond developmentally to after

brain and hind brain (p. 108), together with the pons that unites them, form in the brain of the

higher mammals and of man a connected system of conduction paths. The system, as may be

gathered from the general trend of the fibre-tracts that pass across it or decussate within it, is of

importance in three principal directions. In the first place, this region furnishes the passage-way

for the continuation of the sensory and motor conduction paths that come up from the myel.

Secondly, it originates new nerves: the great majority of the crania nerves spring from separate

grey nidi in the oblongata: and, in doing this, repeats, though in much more complicated

fashion, the structural patterns which we have traced, in their comparatively simple form, in the

lower central organ. Thirdly, it contains a great variety of connecting paths, them selves for the

most part interrupted by deposits of nerve cells, between the various paths that lead across or

arise within it; while, further, in the fibre tracts that run from the main conducting trunk to the

cerebellum and back again, it possesses a secondary conduction path of very considerable

extent that is interpolated in the course of the principal conduction path. It will be understood

that, under these conditions, the lines of travel in the region we are now to consider, as well as

in the adjoining regions of mid brain and 'tween brain, are extraordinarily complicated. A

complete explication of them, in the present state of our knowledge, is altogether out of the

question. But more than this: it is impossible, as things are, to put a physiological or

psychological interpretation upon many of the structural features that have already been made

out. The functional significance of some of the most prominent conduction paths, as e.g. the

entire intercalatory system that runs to the cerebellum, is still wrapped in obscurity. Hence, in

most cases, the tracing out of the fibre systems is a matter solely of anatomical interest. In

physiological regard it is useful, at the best, merely as illustrating the extreme complexity of the

conditions which here determine conduction. We shall therefore refer, in what follows, only to

certain selected instances, adapted to give a general picture of the course of the paths of

conduction in the gross; and we shall enter into some detail only in those cases which appear

to be of importance for the physiological and psychophysical relations of the central processes.

On the score of method, we must say also that the physiological expedient of isolating the

paths by transsection of individual fibre tracts, which did good service in giving us the general

bearings of the paths of conduction in the myel, can hardly come into consideration here,[p.

168] more especially in our study of the conditions of conduction in the hind and mid brain

regions. Experiments of the kind are recorded not infrequently in the older physiology. The

course of the paths is, however, too complicated and their origin too uncertain, to admit of any

but an ambiguous result. The most that the method can give us is a point of view from which to

appreciate the gross function of the organs or of certain of their parts; and we shall accordingly

say nothing of the observations made by it until we reach the next Chapter. We may add that

the method which has proved most fruitful for the problem of direction of conduction, apart from

direct morphological analysis of the continuity of the individual fibre tracts, is the tracing of the

course of degeneration in fibres separated from their centres of origin.

(b) -- Continuations of the Motor and Sensory Paths

The simplest problem presented by our preset enquiry is that of the further course of the paths

of motor and sensory conduction that come up from the myel. The two methods just mentioned

furnish us with a fairly satisfactory solution, at any rate as regards the motor paths. The

principal continuation of the main path of motor conduction that runs upward in the lateral and

ventral columns of the myel is, as we already know: the pyramidal path (Fig. 68, p. 166; cf. Fig.

46, p. 118). The course of this path in detail has been made out, with some degree of

completeness, by help of the descending degeneration which appears in it after destruction of

its terminations in the brain. It is the continuation of that division of the motor principal path

which lies in the myel in the dorsal portion of the lateral columns and along the inner margin of

the ventral columns (Fig. 68 B). The branch of this path that belongs to the ventral columns

decussates in the cervical region of the myel. Now the larger branch, from the lateral columns,

also undergoes a complete decussation, clearly visible on the external surface of the oblongata

(p, Fig. 47, p. 119). The central continuation of the path then runs to the cerebral cortex, without

interruption by cinerea, Fig. 69 gives a schematic representation of the course of these paths,

the longest and so far the best known of all lines of central

conduction. After they have traversed the pons, the fibres of

the pyramidal path enter the crusta (f, Fig. 56, p. 130)

between lenticula and thalamus, and then trend upwards in

the space between lenticula and caudatum to pass into the

corona, where their principal branches constitute the fibre-

masses that terminate in the region of the central gyrus and

the surrounding area (VC, HC, Fig, 65, p. 145).[17] The path

is thus fairly well defined. Part of it, as is proved by the

paralyses following lesion of the pyramids [p. 169] and their

continuation in the crus, undoubtedly subserves the

conduction of voluntary impulses. In the animal kingdom, the

pyramidal path affords a better measure than any other of

the fibre systems collected in the brain stem of the general

development of the higher central organs. In the lower

vertebrates, the pyramids are altogether wanting. In the

birds, they are but little developed. They steadily increase in

importance in the mammalian series, up to man; while at the

same time the tract from the lateral columns, which passes

to the opposite side of the body in the pyramidal

decussation, grows constantly larger as compared with the

tract from the ventral columns, which decussates in the

myel. A branch of the motor path which is forced inward by

the pyramids, and which remains intact after removal of the

pyramidal fibres, may be traced in part to the

mesencephalon. It consists mainly of divisions of the ventral

columns (mf, Fig. 70). Finally, certain of these remains of the

ventral columns are collected in the interior of the rounded prominences to form the

dorsolongitudinal bundle (hl, Fig. 72), which in its further course through the pons makes

connexions with the pontal nidi and more especially, as it appears, with centres of origin of the

oculomotor nerves and with the cerebellum.[18] We may accordingly suppose that these

branches of motor conduction which run to the mesencephalon serve to mediate co-excitations

in that region. The connexions of the dorsolongitudinal bundle, in particular, seem to point to

connexions of the motor innervation of the eye and of the skeletal muscles, such as are

involved in locomotion and in the orientation of the body in space.[p. 170]

The course of the sensory path through the oblongata has not been made out as fully as that of

the motor. The main reason for this defect in our knowledge lies in the difference of structure to

which we referred above. It is characteristic of sensory conduction in the myel that the path

does not pass upward in unbroken continuity, but consists of a chain of neurones. This

structural complexity is not only continued but increased in the oblongata, where large numbers

of cells, grouped together to form separate nidi, are interposed in the line of conduction. We

may suppose that these nidi serve for the most part as transmitting stations -- points at which a

path, whose course has so far been single, splits up into several branches that diverge in

different directions. The main divisions of the sensory path pass in this way, within the

oblongata, first of all into the grey masses deposited in the slender and cuneate columns (Fig.

68, A, and Fig. 46, p. 118). Further on, the sensory path continues in a bundle lying close under

the pyramids (l, Fig. 70), which appears on the ventral surface of the oblongata directly above

the pyramidal decussation (p, Fig. 47, p. 119) here in its turn suffers decussation, and then

passes on in the lemniscus of the crus, a structure lying in the enter and upper portion of the

tegmentum. The lemniscal decussation (formerly known as the superior pyramidal decussation)

thus forms yet another continuation of the decussations of myelic fibres which begin within the

myel itself. Other sensory fibres (ci, Fig. 70), down from the dorsal columns, pass into the

tegmentum proper, which thus brings together portions of the motor (mf) and of the sensory

path. All these sensory fibres terminate in the grey masses of the region of the quadrigemina

and thalami, from which, finally, further continuations of the sensory path proceed to the

cerebral cortex.

The general sketch of the course of the sensory and motor paths, given in the preceding

paragraphs, makes matters much simpler than they really are. There are two facts, not yet

mentioned, that are chiefly responsible for the complications actually found. The one of these

consists in the origination of a large number of new sensory and motor paths, which are derived

front the cranial nerves, and in their further course either join the paths formed by the myelic

nerves or strike out special lines of their own, the other, in the appearance of large groups of

central nerve cells, which serve either as transmitting stations for the conductions comings up

to the cerebrum from below, or as junctions for the important branch-conduction to the

cerebellum, here opened for travel. The difficult questions concerning the origin of the cranial

nerves, questions that have not yet in every case received their final answer, are of interest for

psychology [p. 171] only in so far as they involve that of the paths followed by the sensory

nerves. Since these belong in large part to the mesencephalic region, we may postpone their

consideration until later. It will suffice for our present purpose to refer to Fig. 72 (p. 176), as an

illustration of the conditions of origin of the

cerebral nerves at large. The Figure shows how the funicles of origin of these nerves spring

from isolated grey masses, the nerve nidi; how they then again and again strike across the

longitudinal fibre tracts; and how they finally follow the general trend of the ascending paths.

Most of the fibres of origin, however, enter into still further connexions most with other nidal

structures scattered throughout the oblongata. This statement applies in particular to the fibres

of the oculomotor nervous system, to which we return below, when we come to discuss the

conduction paths of the sense of sight, and to the mixed nerves, among which the

pneumogastric, trigeminus and facialis are of especial importance by reason of their manifold

functional relations. The nidi just mentioned are, we may conjecture, centres of excitation and

transmission for the great functions regulated from the oblongata, -- heart beat, movements of

respiration and articulation, mimetic movements. Our knowledge of the mechanics of

innervation in these cases is, however, still very incomplete.[19][p. 172]

We turn now to the grey nidi of this region of the brain. We have already mentioned the nidi of

the dorsal columns, interposed directly in the sensory path. Very much more complicated are

the functions of the largest nidi of the oblongata, the olives (Fig. 46 B, Fig. 47, pp. 118 f.) whose

principal office seems to be the giving off of branch-conductions. On the one hand, the neurites

of the cells give rise to a fibre system, the further course of which is uncertain: it is supposed to

connect partly with the cerebellum, partly with the lateral columns of the myel. On the other

hand, the dentata give rise to two fibre systems. The first of these covers the outer surface of

the olivary nidus, in the form of zonal fibres (g Fig. 48, p. 120), and then bends round into the

restes and their continuations, the cerebellar peduncles (cr Fig. 70). The second issues from

the interior of the nidus and crosses the median line, to decussate with the corresponding fibre-

masses of the opposite side. Other fibres from the olives enter the longitudinal fibre tract that

lies between them, and then run within the pons to the lemniscus of the crus (l Fig. 70); they

thus appear to join the sensory principal path to the cerebrum. Putting the facts together, we

may say that the olives are structures which stand in intimate relation with the branching off of

conduction paths towards the cerebellum Another ganglionic nidus, lying higher up -- in man

concealed by the pons, in the lower mammals projecting on its posterior border -- the trapezium

or superior olive, forms, as we shall see presently, a nodal point of great importance in the

conduction of the acoustic nerve.

(d) -- Paths of Conduction in Pons and Cerebellum

The conduction paths that branch off from the oblongata to the cerebellum, and there turn back

again to join the caudex in its course through the pons, bear a striking external resemblance to

a shunt interposed in the main current of an electrical conduction. And it seems, as a matter of

fact, that this obvious comparison fairly represents the actual relations of the nerve paths, as

they are shown schematically in Fig. 71. The sensory and motor principal paths, just described,

have also been included in this diagram, in order that the reader may obtain a rough idea of

their relation to the branch path leading to the cerebellum. The mammalian cerebellum

contains, as we have already said, two formations of cinerea: the one appearing in the

ganglionic nidi, the other in the cortical layer investing the entire surface of the organ (pp. 121

f.). Our present knowledge of the relations between the fibres that enter into and issue from the

cerebellum and these grey masses may be summarised as follows (cf. Fig. 48, p. 120). The

fibres of the restes are deflected round the dentatum, more especially over its anterior margin.

They do not appear to connect with the cinerea of the nidus, but radiate from its upper surface

towards the cortex, where [p. 173] they terminate and are lost. From the cortex itself comes a

system of transverse fibres; which cut across the more longitudinal radiations of the restes, and

draw together in stout fascicles to form the medipeduncles (brachia of the pons). The interior of

the dentata gives rise, further, to the funicles which pass into the prepeduncles (crura ad

cerebrum). And, finally, there is a connexion between the dentata and the cerebellar cortex.

This path, together with the radiation of the restis and the medipeduncle, occupies the outer

division of the alba, while the innermost portion is constituted by the prepeduncle. It is therefore

probable that all the fibres running through the postpeduncles of the cerebellum from the

oblongata have their termination in the cortex. The cortex itself gives rise to two fibre systems:

the one passes directly over into the medipeduncles, the other appears first of all to connect the

cortex with the dentatum, which then gives off the vertically ascending fibres of the

prepeduncles. These run upwards, with the continuations of the myelic columns, converging as

they proceed; just anteriorly to the upper end of the pons they reach the middle line, and

undergo decussation. Besides the two divisions of this system of ascending fibres, we find,

lastly, further radiations, whose fibres subserve the interconnexion of more or less remote

cortical areas. Some of the longer lines cross from the one, side to the other in the vermis.

The further course of the paths leading from the cerebellum to the cerebrum is as follows. The

path which is continued in the medipeduncles appears, first of all, to terminate in grey masses

in the anterior region of the pons. From these masses arise new, vertically ascending fibre,

some of which can be traced to the anterior brain ganglia, the lenticula and striatum, while

others proceed directly to the anterior regions of the cerebral cortex. The fibres collected in the

prepeduncles find their proximate termination in the rubrum of the lemniscus (hb Fig. 56, p.

130). A small number of the fibres issuing from this point probably enter the thalami; but the

greater portion pass to the internal capsule of the lenticula, and thence in the corona to the

cerebral cortex, ending in the regions posterior to the central gyre, and more especially in the

precuneus. The valvula (vm Fig. 48, p. 120), which joins the prepeduncles at the beginning of

their course, serves in all probability to supplement the connexions of the cerebellum with the

brain ganglia, by mediating a conduction to the quadrigemina.

We must believe, in view of these results of anatomical investigation, that the concurrence of

conduction paths in the cerebellum is extremely complicated. Let us consider these paths as a

branch conduction, interposed in the course of the direct conduction from myel to cerebrum as

mediated by oblongata and pons. We have two divisions, a lower and an upper. The lower

division of the branch conduction carries sensory fibres from [p. 174] the dorsal and ventral

columns (olivary path of the dorsal columns, and cerebellar path of the lateral columns), which

connect the myel with the cerebellum; and motor funicles, which branch within the pons to enter

the restes. The upper division makes two principal connexions, by way of the medipeduncles:

the one with the cerebral cortex direct, the other with the anterior brain ganglia (lenticula and

striatum). At the same time, there is a connexion, mediated by prepeduncles and valvula, with

the posterior brain ganglia (thalami and quadrigemina). The most extensive of these

conductions, that to the cerebral cortex effected by the medipeduncles, radiates out to all parts

of this organ, but is principally directed forwards to the frontal brain and the adjacent regions.

The schema given in Fig. 71 shows the main features of this conduction system. The reader will

recognise, first of all, the pyramidal path, with its crossed branch from the lateral and its

uncrossed branch from the ventral columns, running directly between myel and cerebral cortex

(p1p2, p). He will next notice the other motor paths, derived from the ventral columns, and

interrupted in the mesencephalic region by masses of cinerea. Some of these paths are

continued in a new neurone chain, and extend to

the cerebral cortex; other fibres of the same

system probably terminate in the mesencephalic

region itself (vv'). A considerable division of the

sensory path (gg') drawn from the dorsal

columns, passes in the lemniscal decussation

(k2) to the opposite side: part of it is lost in the [p.

175] grey masses of the pons, part continues in

fibre tracts which, interrupted by grey nidi, run to

the anterior brain regions and so finally to the

cortex. There is also an uncrossed sensory path

(cc), derived from the dorsal and lateral columns,

which passes into the tegmentum of the crus and

finds its proximate terminus in the tegmental grey

nidi. Another path, also sensory in origin, is the

uncrossed branch conduction (cs) carried to the

cerebellum from the restes in the postpeduncles;

it terminates in the cerebellar cortex, for the most

part in the vermis. Finally, there is a crossed

conduction (f), issuing from the grey nidi of the

olives, which, unlike the former, enters into the

nidal structures (N) of the cerebellum. These are

all incoming paths. The outgoing lines, leading to

the cerebrum, are two in number: the

prepeduncles, which start from the cerebellar

nidus, and may be traced partly into the

prosencephalic ganglia, partly to the cerebral

cortex (e'); and the fibres of the medipeduncles

(bb'), which run direct from the cerebellar cortex

to the cerebrum. These latter enter, first, into the grey nidi of the pons, and are by them brought

into connexion, in some measure, with the brain ganglia, but most extensively with the cerebral

cortex, and in that principally with the frontal region. The system is completed by the paths of

connexion between nidal structures and cortex (rr) which belong exclusively to the cerebellum.

The general relations of these incoming and outgoing paths suggest that the cerebellum brings

into connexion with one another conductions of different functional significance, This inference

finds further support in the peculiar structure of the cerebellar cortex. The characteristic

constituents of this region are, as we saw above (Fig. 15, p. 44) the cells of PURKINJE, easily

distinguished by their large size and the manifold arborisation and reticulation of their

protoplasmic processes. If, now, the cerebellar cortex serves to connect fibres of different

function, sensory and motor, as is suggested by the relations of the incoming and outgoing

paths, it is clear that we may look upon these cells of PURKINJE as elementary centres of

connexion between functionally different fibre elements. We should then have to assume, on

the analogy of the large cells in the ventral cornua of the myel, that the dendrites mediate

centripetal, the neurites centrifugal conductions: in other words, that the chief office of the

former is to take up the excitations carried in the postpeduncles, while the latter collect to form

the paths of conduction that continue in the medipeduncles to the cerebrum and there, as it

appears, are chiefly connected with the centres of innervation of the prosencephalon.

The pons is chiefly important as receiving the paths to be carried up from cerebellum to

cerebrum, and associating them to the vertical ascending fibres of the crus. Its development in

the animal kingdom thus keeps even pace with the development of all these paths of

conduction, and [p. 176] especially of the pyramids and medipeduncles. The fibres that cross

over from the one side to the other in the median line of the pons (at R Fig. 72) are decussating

fibres belonging in part to the direct continuations of the myelic columns through the pons, in

part to the medipeduncles of the cerebellum. The decussation of these latter has been

established by pathological observations: atrophy of a cerebral lobe is ordinarily attended or

followed by a wasting away of the opposite half of the cerebellum. The fibres of the

medipeduncles, probably without exception, pass through internodes of grey matter before they

are deflected into the vertical paths; and small grey

nidi are also strewn in the path of the directly

ascending prepeduncles (ba Fig. 72). These

presently decussate, and come to art end in the

rubrum of the tegmentum. In this way, by collection

of the myelic columns that come up from below, and

of the continuations from the cerebellum that join

them front above and from the side, there forms

within the pons that entire fibre tract which connects

the lower-lying nerve centres with the structures of

the cerebrum, -- the crus. At the same time, the pons

is broken root bundles of certain cranial nerves,

which take their origin higher up. The nidi of origin of

these nerves are situated partly upon the cinereal

floor of the highest portion of the fossa rhomboidalis

(metacele), partly in the neighbourhood of the

Sylvian aqueduct (mesocele), which forms a

continuation of the central canal.[p. 177]

As a result of its cleavage by cinerea and by the

cross fibres of the medipeduncles, the crus divides

into two parts, distinguishable in the gross anatomy

of the brain, and known as crusta and tegmentum. A

third division, the lemniscus, belongs to the

tegmentum so far as regards the direction of its

course, but in all other respects is clearly

differentiated from it. Neither of the two principal

parts constitutes a complete functional unit; on the

contrary, each of them includes conduction paths of

very diverse character. Nevertheless, the twofold

division of the crus seems to represent a first, even if

a rough classification of the numerous paths of

conduction to the cerebrum. Thus the inferior portion

or crusta (p--p' Fig. 72) is principally made up of the

continuations of pyramidi, remains of the dorsal

columns, and medipeduncles. Its outermost portion

carries that continuation of the dorsal columns which

passes in the lemniscal decussation to the opposite

side of the body (k2 Fig. 71). The intercalatum

(substantia nigra of SÖMMERING: Sn Fig. 73) is a

ganglionic nidus, belonging to the conduction paths

of the crusta, which separates crusta from

tegmentum. The portion of the crus which lies above

the intercalatum, the tegmentum (v'--hl Fig. 72), is at

first composed of the remains of the lateral and

dorsal columns, and of a part of the remains of the ventral columns. In its further course,

beyond the point at which the rubrum appears in cross sections of the tegmentum (R Fig. 73),

these are reinforced by the prepeduncles (mf, hi, cr Fig. 70). Finally, the lemniscus, which we

have recognised as a separate subdivision of the tegmentum (sl--sl' Fig. 72), also carries fibres

from the dorsal columns, as well as fibres from the ventral columns and the cerebellum. Taking

the origin of all these tracts into consideration, we may designate the crusta as that part of the

crus which, so far as it derives directly from the myel, is especially devoted to the conveyance

of motor paths; the tegmentum and lemniscus are of mixed, and mainly, as it seems, of sensory

origin. At every point, however, these direct continuations of the myelic systems are augmented

by intercentral paths, the conductions from the cerebellum. In this way, as may be seen from

Fig. 72, which shows a cross section taken approximately through the middle of the organ, the

structure of the pons becomes extraordinarily complex. We may add that it contains, crowded

together in a comparatively small space, the whole number of conduction paths, many of which

in their later course are widely divergent. It is, therefore, a remarkable coincidence that, besides

the epiphysis, which is not a nervous centre at all (see p, 124, above), the pons should have

been regarded with especial favour by the metaphysical psychology of past times as the

probable 'seat of the mind.' HERBART himself accepts this view. If, on the contrary, one were

asked to lay one's finger upon a part of the brain that by its complexity of structure and the

number of elements it compresses into a small space [p. 178] should illustrate the composite

character of the physical substrate of the mental life, and therewith show the absurdity of any

attempt to discover a simple seat of mind, one could hardly hope to make a happier choice.

5. Cerebral Ganglia and Conduction Paths of the Higher Sensory Nerves

(a) -- The Cerebral Ganglia

If we look at the series of cerebral ganglia, we see at once that those of mesencephalon and

diencephalon, the quadrigemina and the thalami, serve as intermediate stations on the line of

conduction: peripherally, they receive sensory and motor fibres; centralwards, they stand in

connexion with the cerebral cortex. They lie, as their function requires, directly upon the

crura, whose fibre masses partly run beneath them straight to the prosencephalon, partly curve

upwards to enter into the grey

nidi of the ganglia. There is a

difference, however: the thalamus

takes up comparatively few fibres

from below, and sends out very

considerable bundles to the

cerebral cortex; the quadrigemina

do just the reverse. Both ganglia,

as we shall see in detail later, are

of especial importance as nodal

points in the optic conduction.

Fig. 73 shows a section taken

through the middle region of this

whole area, and will assist in

some degree towards an

understanding of the structural

relations.

The position of the prosencephalic ganglia, the striata with their two subdivisions, caudatum

and lenticula, is more obscure. The incoming and outgoing fibres tell us but little of their

function. Both divisions receive fibres front the periphery, derived for the most part from the

diencephalic and mesencephalic ganglia. The crural fibres, on the other hand, pass below and

between the prosencephalic ganglia, without entering them (Fig. 74). The grey masses of the

ganglia send no further reinforcements [p. 179] to the coronal radiation. It would appear, then,

that these structures are terminal stations of conduction, analogous to the cerebral cortex, and

not intermediate stations like the thalami and quadrigemina.[20]

(b) -- Conduction Paths of the Nerves of Taste and Smell

An important place is filled in the system of conductions that falls within the region we are now

considering (prosencephalon, diencephalon, mesencephalon) by the paths of the sensory

nerves. Fortunately, these are among the conductions that have so far been most fully

investigated, and whose functional significance is at the same time relatively easiest of

interpretation. In view

of their great importance for psychology, we shall, therefore, depict their principal features in

some little detail. We begin with the conduction paths of the nerves of taste and smell; putting

these together not so much because the peripheral sense organs are closely related, both in

spatial position and in function, as rather because the two lines of conduction may in a certain

sense be regarded as prototypes of the much more complicated conditions that obtain in the

cases of sight and hearing. The [p. 180] gustatory path approaches very closely to the type

familiar to us in the sensory paths of the general sense. When the conducting fibres have left

the central organ, they pass only once more, and then in the near neighbourhood of the centre,

through bipolar

cells, analogous to the cells of the spinal ganglia; they then break up at the periphery of the

organ in a reticulum, which is distributed between non-nervous epithelial elements. The

olfactory path, on the other hand, is a pronounced instance of the second type of sensory

conduction, characterised by the outward displacement of central nerve cells to the peripheral

organ, which accordingly represents, in all essential particulars, a portion of the central organ:

cf. above, p. 47.

There is, however, a further point, in which the path of the gustatory nerves differs from those

of the other nerves of special sense. The gustatory fibres, in consequence, we may suppose, of

their distribution over a functional area of some considerable extent, run their course in two

distinct nerve trunks: those destined for the anterior portion of this area in the lingualis (L Fig.

75), and those intended for the posterior portion in the glosso-[p. 181]pharyngeus (G). This

division appears, however, to be simply external. Both of the gustatory nerve paths take their

origin from the same masses of nidal cinerea on the floor of the metacele. At first, however, the

gustatory fibres that run to the anterior portion of the tongue join the facialis, at the genu of

which (F) they pass through the cells of a small special ganglion. Thenceforward they are

continued in the chorda tympani (Ch) side by side with the lingual branch of the trigeminus. The

glossopharyngeus, on the other hand, which supplies the posterior portion of the tongue,

passes through its own ganglion.

At the periphery, as we shall see

when we come to consider the

peripheral sense apparatus the

two nerves break up into terminal

fibrils, which end in and among

the taste breakers, without, as it

appears, coming into contact with

other than epithelial terminal

structures. The course of the

fibres, as shown synoptically in

Fig. 75, accordingly corresponds

in all details with a general

sensory conduction, such as is

represented in Figg. 21 and 67

(pp. 50, 165) for the myelic

nerves: the ganglia VII. and IX.

may be regarded as analogues of

the spinal ganglia.

The paths of the olfactory nerves

follow a radically different course.

Their point of origin lies furthest

forward of all the sensory nerves,

so that they border directly upon

certain cortical regions of the cerebrum. This is the reason that the olfactorius, from the outset,

is not a single nerve, but appears in the form of numerous delicate threads, which issue direct

from a part of the brain that belongs to the cortex, the olfactory bulb (Fig. 52, p. 125).

Conduction begins at the periphery in the cells of [p. 182] the olfactory mucous membrane (A

Fig. 76), which are set between epithelial cells, and have themselves the character of nerve

cells that send their neurites centralward. These neurites, in their course to the olfactory bulb,

break up into delicate fibrils, which for the most part come into contact with the dendrites of

small nerve cells: the two sets of processes together forming a compact ball of tissue (a, b).

Each of these cells, in its turn, sends out a principal process, which passes into one of the large

nerve cells of the bulb (C). Here we must place the proximate cortical station of the olfactory

path. The dendrites issuing laterally from the cell bodies represent, in all probability, ramose

secondary conductions; while the main path of centripetal conduction is continued, in the

direction of the arrows, in the neurites, which leave the cell upon the opposite side, and pass

into the olfactory tract. This, the principal path, accordingly extends over a peripheral and a

central neurone territory. There is, now, a second group of central olfactory cells which, if we

may judge from their connexions and the direction of their processes, are probably to be

regarded as nodal points of a system of centrifugal conduction. These cells (D) send out a

single peripherally directed neurite, which breaks up within the glomeruli in a delicate reticulum

of terminal fibrils (c). The olfactory path thus shows a marked divergence from the type of

sensory conduction represented by the cutaneous nerves. The peripheral organ itself appears

as a peripherally situated portion of the cerebral cortex, and the olfactory fibres, by a natural

consequence, resemble central rather than peripheral nerve fibres. Another novel feature is

introduced in the probable existence of a secondary path of centrifugal conduction,. And, lastly,

we must note the central connexion of the olfactory region; of the two sides by the

precommissure (ca Fig. 53, p. 127). The connexion is presumably to be interpreted as an

olfactory decussation, by which the centripetal paths are carried to the opposite hemisphere,

and the neurones D c are also enabled to mediate co-excitation, in the centrifugal direction, of

the peripheral cells A of the opposite side of the body.

In man and the higher vertebrates, the cochlea of the auditory organ is, in all probability, the

only part of the labyrinth of the ear that subserves auditory sensation. If we may judge from the

character of the cochlear nerve terminations, the peripheral starting-point of the acoustic

conduction conforms in all respects to the conduction type represented by the cutaneous

nerves. The terminal fibrils of the acusticus extend among the epithelial and connective tissue

structures of the basilar membrane (cf. Ch. VIII., § 4, below), and then, in the auditory canal of

the cochlea (S Fig. 77) traverse groups of bipolar ganglion cells (g) which resemble the cells of

the spinal ganglia [p. 183] and are termed in common the spiral ganglion. The cells of this

ganglion, which accordingly corresponds to an outlying spinal ganglion, give off neurites which

run centralward, and finally break up into

terminal arborisations within various accumulations of cinerea, more especially in two large nidi

in the region of the metacele, with the processes of whose cells they are thus brought in

contact. The nidi in question are a somewhat smaller anterior nidus, the anterior acoustic nidus

(V A) and a larger posterior nidus, the tuberculum acusticum (Ta). Both of these ganglia send

off a small number of fibres, which pass upwards on the same side (uf), and a larger number,

which ascend on the opposite side of the body (kf). The [p. 184] former run partly to the

postgemina, and partly, having joined the lemniscus, direct to the cerebral cortex direct to the

cerebral cortex. There is also, in all probability a branch path from the tuberculum acusticum,

which proceeds with the postpeduncles to the cerebellum (Cb). The great majority of the fibre

tracts issuing from the two nidal masses cross, however, to the opposite side, either directly or

by way of the superior olives (Ol); in each of which the conduction is transferred to new

neurone territories. After decussation, they continue in the same direction as the uncrossed

fibres: some to the postgemina (UV), some in the lemniscus to the acoustic area of the cerebral

cortex (H). Still other neurites (r'r'), which take their origin from cells in the superior olives,

follow a shorter road, running crossed or uncrossed, to nidi of motor nerves. This latter path

must accordingly be regarded as a reflex path. The branch conduction to the postgemina is

continued, with interruption by their cell masses, to the pregemina (OV), from which again a

centrifugal fibre system (rr) runs to motor nidi, and more especially to the nidi of the oculomotor

nerves. It would seem, therefore, that this quadrigeminal path is also, in part, a reflex path;

though the quadrigemina serve at the same time as a transmitting station, from which a further

centripetal conduction is continued to the anterior brain ganglia. To the sensory conductions

already described must be added, finally, a path between the same cell stations in the

quadrigemina (OV, UV) and the acoustic nidi (VA, Ta), which, if we may judge from the

peripheral direction of its neurites, conducts in the opposite direction to that marked in the

Figure, and world therefore represent a centrifugal path. Like the centripetal path which it

accompanies, it consists of a large number of crossed and a small number of uncrossed fibres.

It is indicated by the downward pointing arrows. We thus have, in summary, the following paths

of conduction: (1) the primary path, conforming in type to the path of the spinal nerves, which

runs from the peripheral end-fibres of the acoustic nerve to the ganglion spirale (the equivalent

of a spinal ganglion) and thence to the acoustic nidi (VA, Ta) in the oblongata; (2) the principal

centripetal path, beginning in these nidi, which divides into a smaller uncrossed and a larger

crossed bundle and runs, with interruption by the superior olives or by other masses of nidal

cinerea, to the cerebral cortex; (3) a branch path, beginning in the same nidi of the oblongata,

which also divides into crossed and uncrossed portions, and runs to the quadrigemina and

thence in all probability to the anterior brain ganglia; (4) reflex paths, which lead across to

motor nidi as low as the superior olives and as high as the pregemina, and which include, more

particularly the paths of the oculomotor nerves and of the facial muscles concerned in the

movements of speech; (5) a branch path to the cerebellum, which again begins in the primary

acoustic nidi; and, finally, (6) a centrifugal sensory [p. 185] path, which issues from the

quadrigeminal nidi, and is associated in its peripheral course with the corresponding centripetal

path.[21]

This list shows us how extraordinarily complex is the network of relations into which the

auditory organ is brought by its central paths. Apart from its twofold crossed and uncrossed

connexion with the cerebral cortex, the following facts should be noted as of especial

significance. First, there is a reflex path connecting the acoustic centres with the points of origin

of muscular nerves, and among them with the centres for the movements of articulation and for

the movements of the eyes, which latter are extremely important in the spatial orientation of the

body. Secondly, we find that the conduction system, like that of the olfactory nerve, includes

centrifugal paths, whose office is, perhaps, to transmit the excitations of the auditory organ of

the opposite side, or other sensory excitations that find their nodal points in the mesencephalic

region, in the form of concomitant sensation.

We remark, in conclusion, that the acoustic nerve proper, which comes from the cochlea, is

connected over a part of its peripheral course with the nerve that comes from the vestibule and

canals. This, the vestibular nerve, is a branch of the eighth cranial, and is commonly

accounted, like the cochlear, to the acoustic nerve. In its central course, however, it appears to

follow a different road. It passes through special nidal structures, and finally, as its secondary

degenerations prove, terminates in separate areas of the cerebral cortex. [22]

(d) -- Conduction Paths of the Optic Nerve

The principal difference between the optic and acoustic conductions is that the optic surface

itself, like the olfactory surface, is an outlying portion of the central organ, displaced to the

periphery of the body. It is natural, therefore, that the optic fibres too, when they emerge from

the retina, should at once appear, as by far the great majority of them do, in the character of

central nerve fibres. The cells that give visual sensation its specific quality, the rods and cones

(S and Z Fig. 78) -- usually termed, on this account, visual cells -- are sensory epithelia which,

like the gustatory cells, are connected only by contact with the terminal fibrils of the optic

conduction. In the retinal layers that cover them are several strata of nerve cells, easily divisible

by their marked differences of form into two main groups: the large multipolar ganglion cells

(G2), which my be regarded, from the relations of their neurites and dendrites, as proximate

points of departure for the optic conduction running centripetally from the retina to the brain;

and bipolar ganglion cells (G1) to which may be added stellate intercalary cells, found far

forward in the neighbourhood of [p. 186] the elements S and Z, and not represented in the

Figure. These last two classes constitute together a neurone territory, intervening between the

last termina: fibrils of the peripheral optic conduction and the large ganglion cells (G2), which

may be considered as the extreme peripheral

member of the centripetal optic conduction.

Between its limits we find, further, terminal

arborisations of neurites (e), derived not from

cells of the retina itself but from more central

regions, -- probably from the pregemina, since

these, as we shall see in a moment, form

important nodal points in the optic conduction at

large. There is thus a further point of

resemblance between the outlying central area

represented in the retina and the olfactory

surface: here as there, the structural relations

indicate the existence of a centrifugal secondary

path, running alongside of the centripetal running

alongside of the centripetal principle path.[23][p.

187]

The fibres collected in the optic nerve conduct,

then, for the most part centripetally; though there

is, in all probability, a small admixture of

centrifugal conductors. Following its course, we

come upon the decussation of the optic nerve,

the chiasma, where a distribution is made of the

optic fibres, to the paths running further towards

the central organ, that is obviously of extreme

importance for the co-operation of the two eyes

in binocular vision. It is instructive, in this regard,

to trace the phylogenetic stages through which

the mode of distribution in the human chiasma

has gradually been attained. In the lower

vertebrates, up to the birds, there is a complete

decussation of the paths, the right half of the

brain receiving only the left optic path, and

conversely. In the mammalian series, from the

lower orders onwards, direct paths play a larger

and larger part alongside of the crossed fibres;

until finally, in man, the distribution has become

practically equal; so that the one (the temporal) half of the retina passes into the optic tract of

the opposite side, and the other (the nasal) into the tract of the same side.[24] We owe our

knowledge of this fact less to direct anatomical investigation, which finds great difficulty in the

tracing of the detailed course of the optic nerve, than to pathological observations of the partial

loss of sight resulting from destruction of the visual centre of one hemisphere or from the

pressure of tumours upon the optic tract of one side. The main results of these observations

are brought together, in schematic form, in Fig. 79. It will be seen that the corresponding halves

of retina and optic nerve are cross-hatched in the same direction. The temporal halves of both

retinas have a crossed (tt), the nasal halves a direct path (nn). Before decussation, the crossed

path lies on the outside, the uncrossed on the inside of the optic nerve; after decussation, the

crossed changes to the inside, the uncrossed to the outside of the optic tract. In

contradistinction to the retinal halves which thus receive only a one-sided representation in the

brain, the central area of the visual surface, or macula lutea, where the retinal elements are set

most thickly, is favoured with a bilateral representation. Destruction of the central optic fibres of

one side is accordingly followed by half-blindness (hemianopsia) or limitation of the field of

vision to one-half of each retina (hemiopia), with the exception of the area of direct vision

around the fixation point, which becomes blind only when the central disturbance affects both

sides of the brain.[25] We return later (pp. 229 ff.) to the relations which this peculiar mode of

distribution sustains to the function of vision.

The conduction of the optic tract of either half of the brain is thus composed of temporal paths

from the opposite retina, nasal paths from the [p. 188] retina of the same side, and macular

paths from both retinas. It divides again, on both sides -- as is shown schematically in Fig. 78,

where abstraction is made from the decussations which we have just been discussing -- into

two paths: the one of which runs first of all to the pregenicula (AK), while the other passes to

the pregemina (OV). The two paths, appear to have no connexion with each other, despite the

proximity of quadrigemina and genicula (see Fig. 48, p. 120). In the same way, the optic path to

the pregeminum runs direct to this through the prebrachia, without coming into contact with the

postgeminum. The first of these two paths, that which travels to the pregeniculum, forms the

direct optic radiation (ss) to the cortex or the occipital lobe. It passes over, in the grey nidi of the

pregeminum, into a new neurone territory, whose neurites break up into terminal fibrils in the

brain cortex. On the other hand, the large pyramidal cells of the cortex send out neurites, which

apparently join the coronal [p. 189] fibres that enter the pregemina, and thus constitute a

centrifugal conduction extending to that point (c'f'), The pregemina themselves, which contain

terminal arborisations of fibres, first beginnings of neurites from ganglion cells, and various

forms of intercalary cells, and which send out fibres both to the peripheral sense organ and to

the nidi of the oculomotor nerves, appear accordingly as intermediary stations of great

complexity, while, on the one side, they receive the central conduction coining from the brain

cortex, they serve, on the other, towards the periphery, as points of departure for sensory and

motor fibres, -- the sensory paths conducting in part centripetally to them, and in part

centrifugally away from them. If, then, we abstract from the facts of decussation, described

above, we may say that the fibres of the optic tract make up the following paths: (1) the

centripetal sensory principal path (ss), which in the large multipolar ganglion cells of the retina

(G2) receives the excitations pouring in from the periphery; has an intermediary station, again

interrupted by ganglion cells, in the pregeniculum (AK); and finally reaches the cerebral cortex

in the optic radiation of the corona: (2) a centripetal sensory mesencephalic path (cp), which

runs to the pregemina; here, in its turn, enters a new neurone territory; an finally, as it would

appear, passes into the centrifugal paths (rr) that go to the nidi of the oculomotor nerves -- the

whole path cprr thus representing a reflex path, which connects retina and oculomotor nerves

in the mesencephalon: and lastly: (3) a path which, if we may judge from the mode of

connexion of its elements, conducts centrifugally, and which divides into two parts: a central

branch c'f', running front optic cortex mesencephalon, and terminating in the pregemina; and a

peripheral branch cf, beginning in the pregemina and [p. 190] ending, as we saw above, in the

retina. In view of all these connexions we can readily understand that reactions to light

impressions can be released in the mesencephalon, without any participation of the principal

path: released as reflexes to the oculomotor system, by way of the transferences effected in the

quadrigemina, and as reflexes to other muscles of the body, by way of the other connexions.

The paths which, from the direction of their neurone connexions, must in all probability be

regarded as centrifugal conductors may be supposed to serve, on the one hand, as the vehicle

of direct reactions of central excitations to the nervous structures at the periphery, and, on the

other, as lines of transmission, by help of which excitations in the one retina mediate centrally

coexcitations of the other.[26]

6. Paths of Motor and Sensory Conduction to the Cerebral Cortex

(a) -- General Methods for the Demonstration of the Cortical Centres

We have now traced, as accurately as may be, the course of the fibre systems that run to the

cerebral cortex, whether directly from the crura, or indirectly from the cerebellum and the brain

ganglia. We have made use, in this enquiry, both of the results of anatomical investigation and

of the degenerations set up by severance of the fibres from their centres of function. But we

have not been able to say anything at all definite of the final distribution of the central fibre

systems in the cortex itself. As a matter of fact, there are still certain mazes of interlacing fibres

to which anatomists have not yet found the clue; and our two methods fail us, when we seek to

determine by their aid the precise relations in which the various regions of the cerebral cortex

stand to the deeper lying nerve centres and to the peripheral parts of the body. We therefore

ask assistance at this point from two other sources, physiological experiment and pathological

observation. The former supplies us with a certain correlation, in the animal brain, between

definite cortical areas and the various motor and sensory functions of the peripheral organs.

The latter attempts the same problem for the human brain, by a comparison of the functional

derangements recorded during life with the results of post-mortem examination. The

conclusions drawn from experiments on animals may be transferred to man only, of course, in

so far as they answer the general question [p. 191] of the representation of the bodily organs in

the cerebral cortex. When we attempt to map out, on the human brain, the terminal areas of the

various paths of conduction, we have to rely solely upon pathological observations. These

possess the further advantage that they allow us to make more certain tests of the behaviour of

sensation than do the experiments, upon animals. On the other hand, they have the

disadvantage that circumscribed lesions of the cortex and pallium are of comparatively rare

occurrence, so that the collection of data proceeds but slowly.

Experiments on animals fall into two main classes: stimulation experiments and abrogation

experiments. Under the latter heading, we include all experiments which are intended to

abrogate, temporarily or permanently, the function of some cortical area. In stimulation

experiments, the symptoms to be observed are phenomena of movement, twitches or

contractures in the muscles; abrogation experiments bring about abrogation or disturbance of

movements or sensations. Both forms of experiment are of value for the definition of the

terminal areas of the motor paths; for the sensory areas, we must have recourse in most cases

to abrogation experiments. There are, however, many regions of the cerebral cortex which form

the terminal areas of intercentral paths from the cerebellum and brain ganglia, paths which are

connected only in a very complicated and roundabout way with the lines of sensory or motor

conduction, or with both. We shall, therefore, expect a priori that not every experimental or

pathological change, induced over a limited area, will be followed by noticeable symptoms; and

that, even where such symptoms appear, they will not, as a rule, consist in simple phenomena

of irritability and disability such as arise from the excitation or transsection of a peripheral nerve.

This expectation is amply confirmed by experience. At many points, stimulation may be applied

without producing any symptoms whatsoever. Where it does produce a result, the muscular

excitations often have the character of co-ordinated movements. The symptoms of abrogation,

on the other hand, are for the most part simple disturbances of movement or impairments of

sense perception; it occurs but seldom, and in general only where the lesion is of considerable

extent, that there is complete abrogation of function, sensory or motor. It is well, therefore, in

speaking of experiments upon the cerebral cortex, to use expressions, that in some way

indicate this ambiguity of result. We shall accordingly distinguish between centromotor cortical

areas, whose stimulation produces movements of certain muscles or muscle groups, and

whose extirpation is followed by a derangement of these movements, and centrosensory areas,

whose removal brings in its train symptoms of loss or defect upon the sensory side.[27] These

terms must [p. 192] not, however, be interpreted at the present stage of the enquiry as implying

any hypothesis whether of the significance of the phenomena of stimulation and abrogation or

of the function of the cortical areas to which they are applied. The only question to be

discussed here is that of the termination of the paths of conduction in the cerebral cortex; and

all that we require to know, in order to answer it, is the functional relation obtaining between the

various regions of the cortex and the peripheral organs. How these functional relations are to

be conceived, and in what manner the different cortical areas co-operate with one another and

with the lower central parts, -- these are questions that we do not yet need to consider. There

is, however, one point, so important for the right understanding of the conditions of conduction

that it should, perhaps, be expressly mentioned in this place; a point that follows directly from

the extreme complexity of interrelation which we have found to prevail in the central parts. It is

this: that, for anything we know, there may exist several centromotor areas for one and the

same movement, and several centrosensory areas for one and the same sense organ; and that

there may quite well be parts of the cortex which unite in themselves centromotor and

centrosensory functions. Suppose, then, that we are able to demonstrate certain results of

stimulation and abrogation. They will simply indicate that the particular area of the cortex

stands in some sort of relation with the conduction paths of the corresponding muscular or

sensory region. The nature of the relation can be conjectured only after a comprehensive

survey has been made of the whole body of central functions. All questions of this kind must,

therefore, be postponed until the following Chapter.

The extreme complication of the course of the conduction paths, and the unusually complex

conditions that govern the central functions, in face of which the formation of a critical judgment

becomes a matter of serious difficulty, make us realise all the more keenly the comparative

crudeness and inadequacy of all, even the most careful, experimental methods. In stimulation

experiments, it is never possible to confine the stimulus effect within such narrow limits as is

desirable, if we are to establish the relations of conduction obtaining between distinct cortical

areas. Moreover, the central substance, as we have seen, has its own peculiar laws of

excitability, which make negative results practically worthless as data from which to draw

conclusions. Physiologists are therefore inclining more and more to attribute the higher value to

abrogation experiments. But here, again [p. 193] there are difficulties, as regards both the

performance of the experiments and the interpretation of their results. The shock given by the

operation to the whole central organ is usually so violent, that the immediate symptoms cannot

be referred to any definite cause; they may be due to functional disturbance in parts of the brain

widely remote from the point of injury. Hence almost all observers have gradually been led to

agree that the animals must be kept alive for a considerable period of time, and that only the

later, and more especially the chronic symptoms may be made the basis of inference. Even so,

however, various sources of error are still possible. Thus, as GOLTZ pointed out, inhibitory

influences may continue to be exerted, either upon the entire central organ or upon distant

regions, particularly if but a short interval has elapsed after the operation. Or, if a longer time

has passed, the injured part may have been functionally replaced by other cortical areas:

numerous pathological observations on man have put the efficacy of such vicarious function

beyond the reach of doubt. Or, finally, as LUCIANI remarked, the cortical lesion may, on the

contrary, set up a secondary degeneration of deeper lying brain centres, so that the abrogation

of functions may be extended far beyond its original scope. In view of these difficulties, which

mean that the experimental result may be obscured by sources of error of the most various

kinds and of opposed directions, it is obvious that conclusions in which we are to place any

measure of confidence, must be drawn without exception from a large number of accordant

observations, made with due regard to all the factors that might affect the issue. And when

these precautions have been taken, it is still inevitable that the conclusions, in many cases,

attain to nothing higher than a certain degree of probability. In particular, they will as a general

rule fail to carry conviction, until they are confirmed by pathological observations upon the

human subject.

(b) -- Motor and Sensory Cortical Centres in the Brain of the Dog

Centromotor areas in the cerebral cortex may easily be demonstrated, as HITZIG and

FRITSCH were the first to show, by experiments with electrical or mechanical stimuli. The

simplicity of the structural plan of the carnivore brain (Fig. 61, p. 138) makes it comparatively

easy to rediscover the irritable points, when they have once been found. In Fig. 80 there are

marked upon the brain of the dog the principal points about which the statements of the

different observers are in general agreement.[28] Besides these superficial areas, there appear

to be other cortical regions in the same neighbourhood, lying concealed in the depth of the

crucial fissure, which [p. 194] are mechanically excitable: their exact localisation is, however,

impossible, owing to their inacessible [sic] position.[29] The motor areas are all situated over

the anterior portion of the brain, between the olfactory gyre and the Sylvian fissure. With stimuli

of moderate intensity, the effect of stimulation is produced on the opposite side; bilateral

symptoms are observed only in the case of movements in which there is a regular functional

connexion of the two halves of the body, e. g. in ocular movements, movements of chewing,

etc. With stronger stimuli, the effect is confined as a rule to the muscles of the same side of the

body. The stimulable areas are seldom more than a

few millimetres in extent, and excitation of points

lying between them is, if the stimuli are weak,

unaccompanied by any visible effect. If the stimulus

is made more intensive, or is frequently repeated,

contractions may, it is true, be set up from these

originally indifferent points; but it is possible that such

results are due to diffusion of currents (in electrical

stimulation) or to an enhancement of excitability

brought about by the preceding stimulation. There

can, indeed, be no doubt that repetition of stimulus is

able to induce this enhancement for it is often found

that, under such conditions, the excitation spreads to

other motor areas, so that the animal is finally thrown

into general spasms, the phenomena of what is

called cortical epilepsy.[30] For the rest, the

contractions set up by cortical stimulation are always

distinguished from those released by electrical

stimulation of the coronal fibres by a much longer

duration of their latent period, the expression of that

retardation of the stimulation processes which is of

universal occurrence in the central elements.[31]

The phenomena of abrogation, observed after

extirpation of definite portions of the cerebral cortex

of the dog, differ in two respects from the [p. 195] results of the experiments with stimulation. In

the first place, they show that the removal of a stimulable area is usually followed by

disturbances of movement in other groups of muscles, which were not excited by stimulation of

the same area. Thus, extirpation of the area d in Fig. 80 is likely to produce paralytic symptoms

in the fore leg as well as paralysis of the hind leg, and, conversely, extirpation of the area c is

attended by a partial paralysis of the hind leg; again, destruction of the centres of neck and

trunk aa' involves both the extremities; and so on. At the same time, the paralysis of the

stimulable areas is always more complete than that of the areas sympathetically affected. In the

second place, the extirpation of parts of the cortex that are irresponsive to stimulation may also

give rise to phenomena of paralysis; and this statement holds not only of points of the cortex

lying between the stimulable areas, within the zone of excitability, but also of more remote

regions. It can thus be demonstrated that the entire anterior portion of the parietal lobe, and

even the superior portion of the temporal region as well, are in the dog centromotor in function.

Only the occipital and the larger, inferior portion of the temporal region can be removed without

producing symptoms of abrogation on the motor side. Fig. 81 gives a graphic representation of

these facts. The sphere of centromotor abrogation is dotted over; the size and number of the

points in any given area indicate the intensity of the phenomena of abrogation appearing

(always on the opposite side of the body) after extirpation of that particular zone.[32] The

character of these disturbances, and more especially the regularity with which definite muscle

groups are affected by the extirpation of definite parts of the cortex, render it improbable that

the results obtained from non-stimulable areas are the outcome of transitory inhibitions,

propagated as simple sequelæ of the operation from the point of injury to other, uninjured parts.

We may more reasonably explain the differences between the phenomena of stimulation and

those of abrogation [p. 196] by supposing that the excitable zones stand in closer relation to the

peripheral conduction paths than do the others, whose centromotor influence can be

demonstrated only by way of the inhibition of function which follows upon their removal. For the

rest, it is a significant fact for the theory of these phenomena of centromotor abrogation that

they do not consist by any means in complete muscular paralyses. In general, there is inhibition

of voluntary movement only: the muscles involved will still contract reflexwise upon stimulation

of the appropriate points upon the skin, and may be thrown into sympathetic activity by the

movement of other muscle groups. Further, all symptoms of abrogation, save where very

considerable portions of the cortical investment of both hemispheres have been removed, are

impermanent and transitory; the animals will, as a rule, behave, after the lapse of days or

months, in a perfectly normal way, and the restoration occurs the more quickly, the smaller the

extent of the cortical area destroyed.[33]

The demonstration of the centrosensory areas, if it is to be accurate and reliable, must, as we

said above, be undertaken by help of the phenomena of abrogation. This limitation of method,

and more especially the uncertainty which attaches to sensory symptoms, place serious

obstacles in the path of investigation. There are, however, two points in which the disturbances

of sensation set up by extirpations of the cortex in the dog appear to resemble the motor

paralyses which we have already passed under review. First, the cortical regions correlated

with the various sense departments are, evidently, not well-marked and circumscribed; they

always cover large areas of the brain surface, and even seem to overlap. Secondly: the

disturbances, here as before, do not consist in any permanent abrogation of function. If the

injury is restricted to a comparatively small area, they may be entirely compensated. If it affects

a larger portion of the cortex, there will, it is true, be permanent sensory derangements, but

they will express themselves rather in an incorrect apprehension of sense impressions than in

absolute insensitivity to stimulus. Thus, dogs whose visual centre has been entirely removed

will still avoid obstructions, and others, whose auditory centre has been extirpated, will react to

sudden sound impressions, although they can no longer recognise familiar objects or the words

of their master. They take a piece of white paper, laid in their path, for an obstacle which they

must go round; or confuse bits of cork with pieces of meat, if the two have been mixed together.

[34] All these phenomena indicate that the functions of perception have in such [p. 197] cases

been abrogated or disturbed, but that the removal of the centrosensory areas is by no means

and in no sense the equivalent of destruction of the peripheral sense organs. There is, further,

one respect in which the terminations of the sensory conduction paths differ from those of the

motor: while the derangements of movement point to a total decussation of the motor nerves,

the disturbances of sensation, or at least of the special senses, are bilateral, and accordingly

suggest that the fibres of the sensory

paths undergo only a partial decussation

in their course from periphery to centre.

Figg. 82, 83 and 84 show roughly the

extent of the visual, auditory and olfactory

areas in the cortex of the dog, as

determined by the method of abrogation.

The frequency of the dots indicates,

again, the relative intensity of the

disturbances which follow upon

extirpation of the area in question; the

black dots correspond to crossed, the

hatched dots to uncrossed abrogation

symptoms. We notice that the visual

centre is situated for the most part in the

occipital lobe, though less marked

disturbances may [p. 198] caused from a

part of the parietal lobe and probably also

from the hippocampus; the temporal lobe,

on the other hand, is practically exempt.

The auditory area has its centre in the

temporal lobe, from which it appears to

extend over a portion of the parietal lobe,

as well as the callosal gyre and the

hippocampus. The olfactory area has its

principal centre in the olfactory gyre.

Besides this, it seems to occupy the

uncus and the hippocampus, while its

share in the parietal region is but small. In

the visual and auditory spheres, the

crossed fibres have an undoubted

preponderance; in the olfactory area, the

uncrossed appear to be in the majority.

The gustatory area cannot be made out

with certainty: it probably lies on the two

opposite surfaces of the intercerebral

fissure in the anterior region of the parietal lobe.[35] On the other hand the area whose

extirpation affects the sense of touch and the sensations of movement -- the two cannot be

distinguished in this group of symptoms -- occupies a broad space on the convex surface of the

brain. It has its centre, in the brain of the dog, in the anterior parietal region and extends from

that over the whole frontal portion, and downwards and backwards to the margins of the

temporal and occipital lobes. The centrosensory area for the sense of touch has, that is,

precisely the same extent as the centromotor area for the general muscular system of the

body; it can accordingly be illustrated from Fig. 81, which we have already employed in our

previous discussion. This coincidence suggests the hypothesis that a distribution into smaller,

overlapping centres for the various parts of the body will obtain in the case of sensations as we

have found it to obtain in the case of movements. For the rest, the phenomena of abrogation

which make their appearance after removal of the touch sphere run precisely parallel to the

disturbances of the special senses described above: the permanent symptom is always the

derangement of perception, and never the insensitivity sometimes observed as the direct

consequence of operation.

The brain of the monkey so closely resembles the brain of man (cf. Fig. 64, p. 144), that the

discovery of its cortical centres is a matter of peculiar interest. It was therefore natural that the

attempt should be made, soon after the establishment of the centromotor points on the brain of

the dog, to determine the corresponding points on that of the monkey. Experiments were

carried out by HITZIG [36] and FERRIER,[37] who found, in agreement with the course of the

pyramidal paths, that the stimulable centres lie for the most part in the region of the two central

gyres, whence [p. 199] they extend as far as the superior portion of the subfrontal and

medifrontal gyres. More exact determinations of the points were then made in further

investigations by HORSLEY and SCHÄFER,[38] and by HORSLEY and BEEVOR.[39] Fig. 85

gives the results obtained by the two latter writers on a Bonnet Monkey (Macasus). They show

in general that the cortical centres for the trunk and the hind limbs lie principally on the superior

surface; those for the fore limbs somewhat lower down; and lastly, those for the muscles of

face,

larynx and eyes still lower, towards the Sylvian fissure; while the centres for movements of

head and eyes are situated forwards from the main centromotor area, the two central gyres, in

the region of the frontal brain. It was further found in these observations, as it had been found

in the corresponding experiments on the dog, that weak electrical stimulation simply produces

movements of a circumscribed muscle group upon the opposite side of the body, whereas a

somewhat more intensive stimulation evokes movements on the same side as well,[p. 200]

together, more particularly, with concomitant movements of other, functionally co-ordinated

muscle groups upon the opposite side. Thus, stimulation of the cortical centre for the shoulder

is very apt to affect, besides the shoulder itself, the muscles of the arm and fingers; or

conversely, stimulation of the finger centres will set up, besides movements of the fingers,

movements of the upper and lower arm; and so on. On the brain of the orangutan, whose

structural development brings it still nearer the human brain, HORSLEY and BEEVOR found an

arrangement of centromotor points that precisely parallels that found in the macacus. The only

difference is, that they are more clearly separated by small inexcitable areas. In all cases, the

regions lying beyond the parts specified, i.e., in particular, the anterior portion of the frontal

brain and the temporal and occipital lobes, proved to be inexcitable on the motor side.

These results of stimulation are, on the whole, borne out by the results of extirpation of various

regions of the cortex, if we allow for the greater margin of uncertainty which the abrogation

method always leaves (p. 191). It is noteworthy, also, that the disturbances are apparently less

quickly compensated in the more highly organised brain of the monkey than they are in the dog,

so that the symptoms of abrogation and stimulation are here more nearly in accord.

Nevertheless, according to the observations of HORSLEY and SCHÄFER, it is impossible to

induce an approximately complete paralysis upon the opposite side of the body, even in the

interval immediately following the operation, unless the whole centromotor zone is extirpated. If

the area of injury is more limited, the muscles involved show only a weakening, not a total

abrogation of movement.

The determination of the centrosensory centres is, again, far less certain; the interpretation of

the symptoms presents very much greater difficulties. Hence for the brain of the monkey, as for

that of the dog, the results may he regarded as reliable and assured only in so far as they refer

to the general delimitation of the various sense departments. With this proviso, we may

conclude from the experiments of HERMANN MUNK, with which those of other observers

agree on these essential points, that the cortical surface of the occipital brain constitutes the

visual centre, and that of the temporal lobe the auditory centre. The area for touch, taken as

inclusive of all the organic sensations, coincides in position with the centromotor regions for the

same parts of the body, i.e. is situated in the neighbourhood of the two central gyres and of the

superfrontal gyre.[40]

We have, in the above discussion, left the question of the nature of the cortical functions

untouched, save in so far as it is connected with the problem of the termination of the paths of

conduction in the cerebral cortex. The [p. 201] question cannot come up for consideration in its

own right until the following Chapter, when we review the central functions in their entirety.

Even with this limitation, however, the experiments upon the terminations of the conduction

paths still leave room for differences of interpretation. At the same time, physiologists are on

the road to an agreement: it cannot be disputed that the ideas of the moderate party, ideas

which compromise between the hypothesis of a strictly circumscribed localisation, on the one

hand, and the denial of any local differences whatsoever, on the other, have gradually gained

the upper hand. It is this middle course that we have followed on the whole, in the preceding

paragraphs. It may be that the lines of the various motor areas will, in the future, be drawn

somewhat more closely or somewhat more widely; but the fundamental assumption that the

functional areas extend from definite and narrowly circumscribed centres, and that at the same

time they frequently overlap one another, has established itself more and more firmly, as the

most probable view, in the minds of impartial observers. GOLTZ has protested with great

energy against the hypothesis of sharply defined localisations. His work has done a great deal,

both by its positive contents and by the stimulus it has given to other investigators, to clear up

our ideas upon the subject.[41] But the results which GOLTZ has obtained in his later papers

do not differ in any essential respect from those of most other observers; and he himself has

now come to accept a certain dissimilarity of central representation, which in its general

features resembles the account given above. Cf. also Ch. VI , pp. 281 ff. below.

More serious are the differences of opinion regarding the functional significance of the various

regions of what is called the sensory sphere. As regards the position of the centrosensory

areas, Munk has concluded, on the basis of numerous experiments with dogs and monkeys,

that we must distinguish between cortical areas in which the fibres of the sensory nerves

directly terminate, and areas in which sensations are raised to the rank of perceptions. The

phenomena which make their appearance after destruction of the former, he names, in the

case of the two higher senses, cortical blindness and cortical deafness; the disturbances which

result from extirpation of the centres of the second order, he terms mental blindness and

mental deafness. According to Munk, the visual centre in the dog includes the portion of the

brain lying posteriorly to the Sylvian fissure, and covered by the parietal bones; in the monkey,

the whole surface of the occipital lobe (A Figg. 86, 87). This visual centre is then subdivided

into a central area (A' Fig. 86), and a peripheral area, which surrounds the central on all sides

(A). The centre is supposed, on the one hand, to correspond to the spot of dearest vision of the

dye of the opposite side, and, on the other, to contain the elements in which memory images

are deposited. Its destruction accordingly means loss of clear vision and, at the same time, of a

correct apprehension of sensations. The peripheral portion, A, according to the same author, is

on the contrary merely a retinal centre. Every point within it is correlated with corresponding

points upon the two retinas, each half of the brain representing the same-sided halves of the

retinas of the two eyes. Hence, if one occipital lobe is extirpated, the animal becomes

hemianopic; it is blind to all the images which fall upon the same-sided halves of its retinas.

Further, in dogs the correlation is symmetrical. The central visual area of [p. 202] each

hemisphere corresponds to the smaller, lateral division of the retina of the same side, and to

the larger, median division of that of the opposite side; so that, e.g., extirpation of the central

visual area of the right hemisphere produces blindness over the extreme edge of the right

retina, and

over the whole surface of the left retina with the exception of its extreme edge. This distribution

accords, as will be seen at once, with that which has already been shown to exist in the

mesencephalon in consequence of the partial decussations in the chiasma.[42] MUNK found,

also, that these results of extirpation were con-[p. 203]firmed by experiments with local

electrical stimulation, which regularly induced movements of the eyes, interpreted by him as

movements of fixation due to visual sensations. Thus, stimulation of the posterior portion of the

visual centre causes the eye to turn upwards; stimulation of the anterior portion causes it to

turn downwards; while stimulation of the central area A' leaves the eye unaffected or, at most,

occasions slight movements of convergence. It thus appears that stimulation of the posterior

portion of the visual centre is the equivalent of stimulation of the lower half of the retina, that of

the anterior portion the equivalent of stimulation of the upper half of the retina, and that of the

middle portion A' the equivalent of stimulation of the fovea centralis: for every light-excitation in

indirect vision produces a movement, whereby a correspondingly situated objective light-

stimulus is transferred to the centre of the retina.[43] Similar eye movements were also

observed by E. A. SCHÄFER as a result of stimulation of a particular point in the tactual area of

the cerebral cortex; and BAGINSKY found that stimulation of the auditory area of the dog

aroused movements of the ears and, sometimes, of the eyes.[44] Movements of this kind,

following upon stimulation of central sensory surfaces, may be interpreted in two ways. We may

regard them as reflexes, released in the nidi of the motor nerves or in the mesencephalic

centres; or we may look upon them as stimulus movements, running their course in the paths

which subserve the conduction of voluntary movements. The latter view finds support in the fact

that there exist in the mesencephalic centres special organs of reflex transmission, whose

function is left wholly unimpaired by removal of the brain cortex. The visual centre is bounded,

below and on the outside, by the central apparatus of the sense of hearing. The area, whose

extirpation in the dog is followed, according to MUNK, by abrogation of auditory sensations,

occupies the lateral border of the parietal and the entire temporal lobe; in the monkey, it is

confined to the temporal region, which in the primates is more strongly developed (B).

Destruction of a limited area B' lying at the centre of this larger area (Fig. 86, II.), with retention

of the surrounding parts, is said to abrogate only the perception of articulate sounds, i.e. to

occasion mental deafness; while removal of the whole region B induces total deafness. A

similar distinction between the different functional areas is carried through by MUNK for the

centres of the sense of touch. Thus, the ocular sensations of touch and movement are referred

by him to a region which forms the direct anterior boundary of the visual centre (F); and the

cutaneous centre for the region of the ear is situated in the same way with regard to the

auditory centre. We then have, anteriorly, the other central areas of the general sense of touch,

placed one after another in the order: fore leg, hind leg, head (C, D, E), the series ending with

neck and trunk (H, J). These regions, in agreement with the results of other observers, coincide

with those described above as the centromotor areas for the same parts of the body. The

relation of the two is shown on the right half of the dorsal aspect of the brain of the dog

represented in Fig. 86, I., to which the motor areas of Fig. 80 (p. 194) have been transferred.

MUNK'S statements, and more especially those that relate to the distinction between direct

sensory centres and what he calls mental centres, have, however, been challenged from many

quarters. The physiological and psycho-[p. 204]logical assumptions which underlie this division

of functions are hazardous in the extreme. That apart, there are two principal points in which

MUNK'S conclusions are negatived by the facts as otherwise ascertained. In the first place, it is

evidently incorrect to assert that the removal of any cortical area of the animal brain is followed

by total blindness or absolute insensitivity to sound stimuli. There are many observations which

show that rabbits, and even dogs, will react appropriately to impressions of light and sound

after removal of the entire cerebral cortex. They avoid obstacles placed in their path, perform

complex expressive movements, and so on.[45] In the second place, the symptoms consequent

upon lesions of the cortex correspond in all cases to what MUNK terms mental blindness and

deafness; they are, as GOLTZ puts it, symptoms of cerebral weakness. The removal of a

cortical area is never the equivalent of destruction of the peripheral organ, or of a part of it.[46]

LUCIANI conjectures, further, that the more profound sensory disturbances noticed by MUNK

some time after the operation may perhaps be due to a propagation of descending

degeneration to the lower centres of the thalami and quadrigemina. Only the relations of

definite parts of the visual centre to definite regions of the binocular held of vision have found

confirmation in the experiments of other observers:[47] a result which, as we shall see below, is

also in agreement with the defects of the field of vision observed in man, after partial

destruction of the visual cortex.

(d) -- Motor and Sensory Cortical Centres in Man

The disturbances observed in man, as a result of lesion of the cerebral cortex, may take the

form either of stimulation phenomena or of symptoms of abrogation. The former, which appear

sometimes as epileptiform contractions, sometimes as hallucinatory excitations, hardly come

into account for the question of localisation of functions, since they rarely accompany local and

circumscribed injury of the cortex. We have, therefore, to rely upon the symptoms of

abrogation; and these are the more valuable, the more limited the range of function which they

involve. Nevertheless, it requires great care to separate them from the affections of surrounding

parts, which are seldom absent at the beginning of the disturbance, and from the phenomena

of restitution of function, which make their appearance after the lapse of time.[48] The

observations which have been brought together, with due regard to these precautions, lead to

results, more especially as regards the centromotor areas of the human cortex, which agree in

their principal features with the experimental results obtained on the brain of the monkey. This

will be seen at once from a comparison of Figg. 88 and 85 [p. 205] (p. 199), the former of which

gives a schema of the localisation areas in man, based on pathological observations, while the

latter shows the centromotor points of the brain of Macacus. It is evident that, in the cortex of

man as in that of the monkey, the areas whose lesion produces motor paralysis are grouped in

comparatively small region of the cortex, viz., in the two central gyres and the adjoining

superior divisions of the three frontal gyres. Here, as before, that is, they lie within the region

which corresponds to the pyramidal path. The centres for trunk and lower limbs, situated in the

highest part of the central gyres, extend further into the portions of these gyres that bound the

intercerebral fissure.[49] On the other hand, destruction of the cortex of the temporal and

occipital lobes, or of the anterior portion of the frontal lobes, occasions no impairment of bodily

movement. The paralyses appear always on the opposite side of the body, and consist in

abrogation or derangement of the voluntary [p. 206] movements, to which may be added, later

on, contractures due to the action of muscles not affected by the lesion.[50] It is clear, from the

relative positions of the centres, as shown in Fig. 88, that on the one hand paralyses of arm

and leg, and, on the other, paralyses of arm and face may very easily occur together, but that

leg and face cannot well be involved while the arm remains free: a conclusion that is fully borne

out by pathological observation.

The phenomena of abrogation observed in pathological cases of partial destruction of the

cortex inform us, further, of the position of the principal centrosensory areas in the human

brain. First and foremost, the central terminations of the optic paths,

constituting what is called the visual centre, have been definitively localised in the cortex of the

occipital lobe. The visual centre covers the whole inner surface of this lobe, and includes as

well a narrow marginal zone on its outer surface (Figg. 89, 90). The phenomena indicate, at the

same time, that each half of the brain is correlated with the nasal half of the opposite and the

temporal half of the same-sided retina, in accordance with the decussations of the optic fibres

in the chiasma mentioned above (Fig. 79, p. 188). Further evidence is given in support of this

arrangement by cases in which a partial atrophy of both halves of the occipital brain has been

observed after a long-standing blindness of the one eye, and by others in which a partial

degeneration [p. 207] of the pregeminum and geniculum of the opposite side has followed upon

destruction of the one occipital lobe.[51] The course of the degenerated fibres in the latter

instance shows, in accordance with the anatomical facts, that all the optic fibres pass through

these mesencephalic ganglia before they reach the central visual areas.[52] The main

difference between visual disturbances of this kind and those due to peripheral causes, e.g.

destruction of a retina, lies in the fact that they always affect both eyes. In still other cases,

lesions of the same cortical area give rise to symptoms that speak yet more decisively for the

central character of the derangement: sensitivity to light may remain intact, at all points of the

field of vision,

while the discrimination of colours, or the apprehension of forms, or the perception of the third

dimension is seriously impaired. In some such cases, however, it is found that other parts of the

brain, more especially the frontal and parietal lobes, are involved; occasionally, indeed, the seat

of injury resides in these alone, while the posterior portions of the cerebral cortex are left

comparatively unaffected.[53] We may therefore suppose that symptoms [p. 208] of the kind

are always to be referred to disturbances of a more complex order, implicating more than one

region of the brain. A like judgment must, undoubtedly, he passed upon the word-blindness

which we consider in the following Chapter. It must always be remembered, in a discussion of

these phenomena, that the formation of visual ideas is an extremely complicated process, not

confined to the region of the direct terminations of the optic paths, but involving; the co-

operation of numerous other cortical areas as well.[54]

A similar complication obtains in the case of the auditory area of the human cortex. Pathological

affection of the terminal areas of the acoustic nerve is shown, primarily, by abrogation or

impairment of the power of hearing; but this is invariably accompanied by a profound

modification of the faculty of speech. The connexion of the two is not surprising, since the

motor and sensory areas of the sense of hearing lie side by side in the cortex (see Fig. 88, and

Figg. 89, 90), and may therefore easily be involved in the same lesion. There is, however, a

further factor that introduces a peculiar complication into the phenomena: in all affections of

what we suppose to be the cortical terminations of the acoustic nerve, derangement of the

direct motor and sensory terminals is, apparently, always accompanied by disturbances of

connective paths or centres. Hence, most of the derangements of the faculty of speech -- those

ordinarily distinguished by their symptoms as aphasia, word-deafness, agraphia, word

blindness, etc. -- are of extremely complex character. And we must accordingly assume that,

besides the direct auditory centre, there are always involved other central areas which, like the

corresponding motor centre, lie in its near neighbourhood. We shall recur to the probable

conditions of these derangements of speech in our consideration of the complex functions of

the central organ (Ch. VII.). As regards the boundaries of the direct auditory centre, we cannot

speak with complete assurance; its connexion with other central areas, which co-operate in the

auditory functions, leave a margin of uncertainty. There can, however, be no doubt that the

principal terminus of the acoustic path (Fig. 89) is the posterior section of the supertemporal

gyre (T1 Fig. 88), the part that borders the end of the Sylvian fissure. That is, this region lies

directly opposite to the motor areas of the subfrontal gyre, which are brought into activity in the

movements of speech (Fig. 88).[55]

The demonstration of the centres for the fibres of the gustatory and olfactory nerves presents

considerable difficulty, though for other reasons.[p. 209] The method of abrogation here fails us:

the ambiguity of the symptoms, whether in man or in the animals, renders it practically

impossible to determine the effects of cortical lesion. In this instance, therefore, we must still

rely entirely upon the results of direct anatomical investigation of the course of the conduction

paths. These results indicate that the olfactory area occupies the space marked in Fig. 90 by

small open circles; i.e., that it extends on the one hand over a narrow strip on the posterior

margin of the frontal lobe and over the callosal gyre, and on the other over the superior and

inner margin of the temporal lobe, adjoining the posterior extremity of the callosal gyre. The

parts in question, and especially the callosal gyre, are, as we know, much more strongly

developed in certain animal brains, e.g. in the carnivores (Fig. 63, p. 143); so that the area of

distribution accords with the relatively low development of the sense of smell in man. The taste

area is supposed to lie somewhere in the neighbourhood of this olfactory centre. So far,

however, it has not been definitely localised, either by anatomical or by functional methods.[56]

We return to safer ground when we seek to determine the central areas for the sensations of

the general sense, i.e. more particularly for tactual and common sensations. Numerous

observations go to show, in agreement with the results of operation on animals, that the

centrosensory regions of the sense of touch coincide with the centromotor regions for the same

parts of the body. Disturbances of tactual and muscular sensation are found to follow upon

injury to the posterior portion of the three frontal gyres, the two central gyres, the paracentral

gyre, and the parietal and subparietal gyres; i.e. to the whole region indicated in Figg. 89 and

90. The reference of special areas to the different parts of the body, and the separation of touch

from common sensation, are matters of less certainty. On the former point, we can only say

that, despite the general coincidence of the sensory and motor regions, it is still possible that

the two kinds of centres are not wholly identical, but simply bound together by a close

relationship of structure and function. On the latter, we have a few observations that point to a

central differentiation of internal and external tactual sensations. Cases occur in which the

sensation of movement is abrogated, while cutaneous sensation and motor innervation remain

intact; and these isolated disturbances of articular and muscular sensations seem to be

induced more especially by affections of the parietal and subparietal gyres.[57] But, after all has

been said, the results so far obtained with regard to the localisations of the general sense leave

us still in doubt upon many [p. 210] points of detail. In particular, all statements concerning the

relation of these centrosensory cortical areas to the centromotor must be regarded, at present,

as altogether hypothetical. They rest, not upon reliable observations, but for the most part upon

some foregone psychological or physiological assumption.

If, in conclusion, we compare the whole group of results derived from pathological observation

of the relations of the cerebral cortex to the several conduction systems with the outcome of the

experiments made upon animals, we see at once that, where the facts are at all securely

established, there is a large measure of agreement between the two methods. Thus, the

position assigned to the centromotor areas in man and the animals is practically the same. In

particular, the motor points of the central gyres are arranged in a similar order upon the human

and monkey cortex. The same thing holds of the localisation of visual excitations in the occipital

lobe. In the acoustic area, it is true, we find differences. The development in man of the cortical

region connected with speech is offset, in most of the animals, by the greater bulk of the

olfactory centres. There is thus a more pronounced dissimilarity in the structure of the anterior

portion of the brain, and a consequent lack of correspondence of the cortical areas. According

to the observations of FERRIER, MUNK and LUCIANI, the auditory centre of the dog, e.g., is

forced, by the development of the olfactory gyre, relatively far back, into the posterior part of

the temporal lobe. This apart, however, the auditory area appears, to all intents and purposes,

to occupy an analogous position in the human and animal brain. And the same statement may

be made, finally, with still greater confidence, of the cortical areas for tactual and common

sensations, whose localisation refers us, in all cases, to regions which either coincide or

interfere with the corresponding centromotor areas. So far, then, there is a general agreement

between the results. The only difference of any considerable moment is that the derangements

of function resulting from cortical lesions are as a rule more serious in man than they are in the

animals. And this difference itself has only a relative significance, since it appears in the same

way between various classes of animals, e.g. between dog and rabbit, or still more markedly

between monkey and dog. It would seem, then, that the phenomena in point are simply

illustrations of the general fact that the subcortical centres have a higher value, as centres of

independent function, the lower the organisation of the brain to which they belong.[58] Lastly,

having allowed this difference its due weight, we have again to say that the character of the

disturbances produced by local lesions of the cortex is the same for man and for the animals, in

so far as the derangement never amounts to an absolute abrogation of function, and is

therefore by no [p. 211] means equivalent to the interruption of a peripheral conduction path.

The nearest approach to such a result is given in the paralyses which follow upon destruction of

the centromotor zones. Even these, however, are definitely distinguished by the possibility of

comparatively rapid restoration of function.

We have made mention, in the preceding paragraphs, of all the areas of the human cortex that

can lay claim, chiefly on the ground of pathological observations, to be considered as the

termini of motor and sensory conduction paths. The motor area, shown in Fig. 88, and the

visual centre of the occipital lobe were the earliest of these 'cortical centres' to be discovered,

and are at the present day the two whose lines can be most sharply drawn. Much more

precarious, for the reasons given in the text, is the status of the acoustic area: and this despite

the fact that the derangements of speech, which stand in intimate connexion with it, have been

under observation for a long period of time.[59] Finally, the correlation of the central gyres and

the adjoining region (as mapped out in Figg. 89 and 90) with the sensations of the general

sense (external and internal sensations of touch, pain, and organic sensations) may be

regarded as sufficiently well established. The localisation was first suggested by TÜRCK, who

noticed that lesions of these coronal fibres and of the crural fibres in the region of the capsula

of the lenticula produced unilateral sensory disability.[60] In these cases, and still more in cases

of destruction of the central gyres themselves, the symptoms are, however, invariably

complicated by the simultaneous appearance of motor paralysis in the corresponding regions of

the body. For the rest, the hemianæsthetic disturbances are usually distinguished from such

hemiplegic accompaniments by their more irregular character; they may be confined to certain

factors of the general sensitivity -- muscular sensations, pain, sensations of temperature, etc. --

or they may be combined with other sensory disturbances in the departments of special sense,

and more especially with amblyopia.[61]

All these sensations of the general sense, sensations derived from the organ of external touch,

as well as from joints, muscles, tendons and other bodily organs, have been grouped together

by certain authors under the indefinite name of 'bodily feeling.' In accordance with this usage,

the area ascribed to the general sense in Figg. 89 and 90 was termed by H. MUNK the 'area for

bodily feeling.' The title has become current; and its employment is generally connected with

various psychological hypotheses, which play an important part in the interpretation of the

centromotor symptoms induced by lesions of this region. Thus, SCHIFF propounded the theory,

[62] which has been accepted by MEYNERT,[63] H. MUNK,[64] and many other anatomists

and pathologists, that the centromotor innervations are direct concomitants of the ideas of the

respective movements. This means that the cortical region assigned to the 'area for bodily

feeling' is to be regarded as a sensory centre, analogous to [p. 212] the centre of sight or

hearing. The volitional process is then explained as a reflex transference, occurring, possibly, in

the cortex itself, or, perhaps, in deeper-lying parts. This aspect of the theory is rendered

especially plausible by the belief that 'will' is nothing else than an 'idea of movement,' and that

consequently the 'cortical function' underlying voluntary action consists simply and solely in the

excitation of a movement idea, i.e. in a sensory process. Now the assumption that 'will' is

equivalent to idea of movement is, of course, a purely psychological hypothesis; it can be

demonstrated or refuted, not by anatomical and physiological facts, but only by a psychological

analysis of the voluntary processes themselves. Hence we cannot enter upon its examination in

this place, though we shall take it up in due course. The physiological investigation of the

conduction paths is properly concerned with the single question, whether the cortical areas

under discussion are exclusively centrosensory in function, or whether they also evince

centromotor symptoms. If the question is put in this way, and we repeat that this is the sole way

in which, from the physiological standpoint, it can be put, then the only answer possible, in the

light of the observations, is the answer given in the text. But it need hardly be said that that

answer gives us no warrant for speaking of a 'localisation of the will' in the brain cortex. To do

so would be as absurd as to say, e.g., that the subfrontal gyre and its surroundings parts are

the seat of the 'faculty of speech.' The removal of a screw may stop a clock; but no one will be

found to assert that the screw is what keeps the clockwork going. The will in abstracto is not a

real process at all, hut a general concept, gained by abstraction from a large number of

concrete facts. And the concrete individual volition, which alone has actual existence, is itself a

complex process, made up in every case of numerous sensations and feelings. There can be

no doubt, therefore, that it involves a number of different physiological processes. The

hypothesis that a complex function, like speech or volition, is conditioned solely upon certain

individual elements may accordingly be pronounced a priori as improbable in the extreme.

Resides, all that follows from the observations is that those parts of the brain cortex which we

claim as centromotor contain transmitting stations, which are indispensable for the transference

of voluntary impulses to the motor nerve paths: the anatomical facts making it further probable

that the regions in question contain the proximate stations of transmission from brain cortex to

central conduction paths.[65]

There is one other fact that we may mention, in conclusion, as of importance for the

psychogenetic interrelations of the different sense departments. According to the investigations

of FLECHSIG, the fibre systems that radiate from the mesencephalon to the various cortical

centres obtain their myelinic sheath at very different stages of embryonic (partly also of

postembryonic) development, and therefore, we may suppose, assume the functions of

conduction at these same intervals. In man, the fibres that ascend to the tactual centre from the

sensory dorsal columns of the myel, together with a few others that enter the optic radiation,

are the earliest of the coronal bundles to attain to full development. They are followed, at a

somewhat later period, by fibres which in part supplement this pre-existing system, and in part

trend towards the olfactory and visual centres. The myelinisation of the fibre system of the

acoustic path is completed last of all, to some extent after birth. At the same [p. 213]

time, it does not appear that the animal series presents any thorough-going parallelism in this

regard; the investigations of EDINGER prove that the olfactory radiation is developed very early

in the lower vertebrates, while in man it belongs to the systems of later development.[66] And in

the human brain, this general course of development divides into a large number of separate

stages, each corresponding to the completion of some smaller fibre system. FLECHSIG himself

has thus been led to distinguish no less than forty fibre tracts, running in developmental

succession to definite regions of the cortex. He finds, in general, that the conduction paths of

the 'association centres,' discussed in the following Section, are the latest to reach maturity.[67]

It should be said, further, that FLECHSIG'S statements have been called in question from

many quarters. Some authorities altogether reject the idea that the myelinic sheath developes

system by system; others at least dispute the regularity of the development.[68] Moreover, it

cannot be denied that the greater the number of the cortical centres that we are called upon to

distinguish by the order of their completion in time, the smaller becomes the probability that

each single centre possesses a peculiar functional significance. Nevertheless, the general

result is noteworthy, that the conduction paths whose cortical centres receive special

elaboration in the human brain are apparently also the latest to attain to individual

development.[69]

§ 7. Association Systems of the Cerebral Cortex

The whole group of fibres that pass upwards in the myel and, reinforced by additions from the

posterior brain ganglia and the cerebellum, finally radiate into the corona of the cerebral cortex,

is ordinarily termed the projection system of the central organs. The name was first employed

by MEYNERT, and is intended to suggest the idea that the system in question represents the

various peripheral organs in determinate regions of the cerebral cortex. In metaphorical

language, the periphery is 'projected' upon the brain surface. The fibre masses of this projection

system, some of which enter the coronal radiations as direct continuations of the crura, while

others are derived from the mesencephalic ganglia, quadrigemina and thalami, and yet others

issue from the cerebellum, are crossed at every point of their path to the cerebral cortex by

foreign fibre masses, which connect various regions of the cortex with one another. This

second group is known (the term was again coined by MEYNERT) as the association system of

the cerebral cortex.[69] Both names, as employed here, have, of course, a purely anatomical

significance. The projection system has nothing at all to do with what, e.g., is called in

physiological optics the outward 'projection' of the retinal image, and the association system,

similarly,[p. 214] has nothing to do with the psychological 'association of ideas.' The point must

be sharply emphasised, because, as a matter of fact, confusions of this sort, due to obscurity in

psychological thinking, have often played -- nay, continue to play -- a part in discussions in

which the terms are employed. Now, as regards the projection system, the anatomical facts are

sufficient to prove that, if it represents a sort of projection of the peripheral sensory surfaces

upon the brain cortex, it can at best be accredited with but a partial performance of this duty.

For, on the one hand, the various sensitive areas of the bodily periphery appear, in most cases,

to be connected at the same time with several points upon the cortex; and, on the other, the

different fibre systems which terminate in a given area of the cortex may correspond to distinct

external organs. All this means that the projection system is at least as deeply concerned with

the central connexion of the bodily organs as it is with that central representation of them from

which it takes its name. As regards the association system, there is not the slightest reason for

bringing it into any kind of connexion with the associative processes of psychology. The only

hypothesis that we have the right to make about it, on the score of function, is that its fibres

serve in some manner to effect the functional unity of separate cortical areas.

The association system like the projection system, may be divided into various component

systems, distinguished in this case partly by the direction of connexion, partly by the distance

separating; the connected regions of the cortex. We thus obtain the following subordinate

systems of association fibres:

(1) The system of transverse commissures. This is

principally composed of the callosum or great

commissure, but is supplemented as regards the

temporal lobes by a portion of the precommissure,

which also contains the decussation of the olfactorius

fibres (Fig. 91; cf. above, p. 132, and Fig. 53, p. 127).

The callosum represents a strongly developed cross-

connexion; its fibre masses connect not only

symmetrical, but also, to some extent,

asymmetrically situated cortical regions of the two

hemispheres. The callosal fibres cut across the

coronal radiations at all points except in the occipital

region, where the two sets of fibres separate into [p.

215] distinct bundles (m', Fig. 58, p. 135; cf. also Fig.

57, p. 134). The connexion effected by the callosum

between symmetrical parts of the cortex is fullest, as

might be conjectured from its marked increase of

size in transverse section as we proceed from before

backwards, for the cortex of the occipital region. This

is why a defective development of the callosum, as

observed in cases of microcephaly, is accompanied

by a marked atrophy of the occipital lobes.

(2) The system of longitudinal connective fibres (Fig. 92). This system takes an opposite,

direction to the foregoing; its fibres connect remote cortical areas of the same hemisphere.

Dissection of the brain reveals several compact bundles of this kind,

devoted more especially to the connexion of the frontal with the temporal lobe (uncinate and

arcuate fascicles) and of this latter with the extremity of the occipital lobe (longitudinal fascicle).

(3) The system of intergyral fibres (fibræ propriæ, Fig. 92). This system serves to connect

adjoining cortical areas. The fibres are for the most part deflected round the depressions of

alba formed by the cerebral fissures (cf. also fa, Fig 58, p. 135).

The association systems that thus connect the various regions of the [p. 216] cerebral cortex

may be again divided into three classes, according to their mode of origin and termination. They

may (1) connect different areas of the projection system, i.e. centromotor or centrosensory

regions, with one another. They may further (2) connect determinate areas of the projection

system with other areas, in which no projection fibres directly terminate. Finally, (3) it is

probable that in certain parts of the cortex associative fibres of different origin run their course

together; so that these areas are connected with the projection system only indirectly, by way of

the association fibres that issue from them and terminate in other cortical regions. Areas of this

sort, which must be regarded exclusively as terminal stations of association fibres, have been

termed by FLECHSIG 'association centres.'[70] They occupy, according to this investigator,

practically the whole region of the cerebral cortex which is not taken up by the sensory centres:

i.e., in the human brain, the parts which are left unmarked in Figg. 89 and 90. If, then, we

consider every continuous surface of this sort as a separate central area, we shall have to

distinguish three association centres: an anterior or frontal centre, which covers the larger part

of the frontal brain; a middle or insular centre, which extends over the cortex of the insula and

its immediate neighbourhood; and a posterior, parietotemporal centre, of wide dimensions,

taking in a considerable portion of the parietal and temporal lobes. Between these association

centres and the projection centres there lie, still according to Flechsig, intermediary areas and

marginal zones, in which projection fibres, either intermixed with the others or grouped in

distinct bundles, terminate along with the association fibres. The validity of this distinction of

special centres, wholly deprived of direct connexion with the projection system, is disputed by

many authorities; and the statements made with regard to the boundary lines and dimensions

of the fields in question, and more especially with regard to the extent of the marginal zones

and mixed areas, are open to doubt on many points. Nevertheless, it appears to be an

established fact that certain areas of the human cortex are supplied for the most part by

association fibres, and that these are, in general, the areas whose destruction shows itself not

so much in direct centromotor or centrosensory symptoms as in more complicated anomalies of

function. On the other hand, it must not, of course, be forgotten that the 'direct' motor and

sensory centres themselves cannot possibly be regarded as simply projections of the peripheral

organs upon the brain surface. The symptoms of abrogation are decisive: the disturbances set

up are of a complicated nature, and their compensation may later be effected, within wide

limits, by vicarious functioning of other parts. This result is in harmony with the further fact that

there is no region of the brain surface [p. 217] that does not receive association fibres as well

as projection fibres: indeed, it is probable that the former constitute the large majority of the

coronal fibres in all parts of the human brain. On this count, therefore, it would seem that the

search for specific differences is altogether in vain. But if any and every derangement of

function due to central interference, in whatever part of the brain it may occur, is of a more or

less complicated character, it follows that there can be no question of contrast or antithesis as

between the functions of the various areas. The difference is always a difference of degree; we

have to do, in the particular case, with a closer and more direct or with a remoter and more

indirect relation between a given cortical area and certain peripheral functions. This fact should

be kept in mind, further, in all attempts to put a functional value upon the different distribution

and relative dimensions of the projection and association centres. It has been found, e.g., that

the association centres, together with the main bundles of association fibres that run between

the different parts of the brain (Figg. 91, 92), attain very much larger dimensions in the human

than in the animal brain: in many cases, indeed, their presence in the animal cortex cannot be

demonstrated at all. This statement holds more particularly of the frontal association centre,

whose high degree of development determines in large measure the peculiar conformation of

the primate, and more particularly of the human brain. Finally, it is worth remark that the cortical

area which contains the most extensive representation of peripheral organs, the region in the

neighbourhood of the central gyres correlated with bodily movements, the sense of touch and

organic sensations, also evinces the most extensive connexions with the association centres.

The existence of 'association centres,' as defined by FLECHSIG, has in recent years been the

subject of animated discussion among the students of brain anatomy and brain pathology.

RAMON Y CAJAL, EDINGER and HITZIG (the latter with certain reservations) declared

themselves in favour of the hypothesis; while DÉJERINE, VON MONAKOW, SIEMERLING, O.

VOGT and others pronounced the distinction altogether impracticable.[71] There are no cortical

areas, say these authorities, to which projection fibres cannot be traced; just as there are, by

general admission, none which are not supplied with association fibres. The question as such

is, of course, a question in anatomy pure and simple. We can here do no more than point out

that its settlement can hardly be of such importance, from the physiological point of view, as it

might, perhaps, appear to us from the anatomical. The occurrence of cortical areas which, in all

probability, are connected with peripheral organs only indirectly, by way of the conduction paths

that lead to other centres, may, no doubt, be con-[p. 218]sidered as evidence on the one hand

of the extremely complicated structure of the particular brain, and, on the other, of the

peculiarly complex function of these areas themselves. But the conditions do not, surely,

warrant us in ascribing to them a specific function, and setting them off, as 'psychical centres,'

from the 'projection' or 'sensory centres.' As a matter of fact, there is nothing to indicate that the

structure of the cerebral cortex conforms to any such simple design as that certain parts shall

be, so to say, reflections of the peripheral organs, while others shall be reserved as centres of a

higher order, serving to bring the direct centres into mutual connexion. On the contrary, it is an

essential characteristic of every part of the central organ that it brings together elements which,

while spatially separate at the periphery, nevertheless co-operate for the unitary discharge of

function. In the case before us, it is first of all the lower central parts, and then, in the last

resort, the whole body of the cerebral cortex, that mediate connexions of this kind. What is

called the 'visual centre,' e.g., is by no means a repetition of the retinal surface within the

cortex. The retina itself is, as we know, nothing else than a part of the cortex that has been

displaced far forwards; so that, in this instance, the central duplication would really be a piece

of quite needless self-indulgence on the part of Nature. It is because the visual centre contains,

along with the conduction paths that connect it with the retina, other paths, whereby it is able to

connect the retinal excitations with further functional areas, e.g. the motor, concerned in the act

of vision, -- it is for this reason that the visual centre is a true 'centre,' and not a mere duplicate

of the peripheral organ. If, now, there are portions of the cerebral cortex whose elements stand

in no sort of direct connexion with the conduction paths that run to the periphery, then we must

simply say that these areas are possessed, in an unusually high degree, of an attribute which

determines the character of the central organ at large, and which must therefore be predicated

in some measure of all the other areas, whose connexions with the periphery are more or less

direct. To speak of a number of 'psychical centres,' one must have made assumptions that are

equally impossible whether from the standpoint of physiology or of psychology. There is, in

reality, but one psychical centre; and that is the brain as a whole, with all its organs. For in any

at all complicated psychical process, these organs are brought into action, if not all together, at

any rate over so wide a range and in such various quarters as to forbid the delimitation of

special psychical centres within the functional whole.

§ 8. Structure of the Cerebral Cortex

The investigation of the fibre systems by physiological experiment, by pathological observation,

and by anatomical dissection comes to a natural end at the point where these systems pass

over into the cerebral cortex itself. If we wish to inquire further regarding their mode of

termination within the cortex, and more particularly regarding the mutual relations of the

different parts of the projection and association systems that run to one and the same cortical

area, we must gather up the results of the histological examination of the structure of the

cerebral cortex. Now our knowledge of this extraordinarily complicated formation has, it is true,

not advanced so far that we can establish, beyond the reach of doubt, the [p. 219] terminations

of all

the various paths of conduction that have been traced to it. Nevertheless, there is a certain

body of fact that may, without [p. 220] hesitation, be turned to account for the physiological and

psychological appreciation of the individual conducting systems.

We notice, first of all, that in certain structural outlines the cerebral cortex shows uniformity of

design over its whole extent. It consists throughout of several strata of nerve cells. In the

human cortex we can distinguish, according to the size, direction and position of the cells, eight

or nine distinct layers. The order in which these strata are arranged is, on the whole, the same

for all regions, though their relative thickness and the number of the elements characteristic of

each layer differ very considerably from part to part. Figg. 93 and 94 show the structural

relations obtaining in two typical cases. Fig. 93 gives a microscopical transsection through the

postcentral gyre, i.e. through a part of the centromotor region of the cerebral cortex; Fig. 94

gives a similar section through the occipital cortex of the human brain. In these, as in all other

sections, from whatever part of the brain they may be taken, the outermost and innermost

layers are practically identical in constitution: they are characterised by spindle-shaped cells, in

the former set crosswise in a fibrillar reticulum whose general trend is in the horizontal

direction, and in the latter placed lengthwise among longitudinally directed fibre masses. There

are, on the other hand, well-marked differences in the depth of the layers of large and small

pyramidal cells, and of those composed of large and small stellate cells. In the centromotor

regions, the pyramidal cells form the great majority of all the cell elements. This may be clearly

seen in the section from the postcentral gyre shown in Fig. 93, whose formation lies midway

between that of the typical 'motor' and the typical 'association' cortex (superfrontal,

subtemporal, etc. gyres); but it is still more apparent in the precentral gyre, where the pyramidal

cells ('giant' cells) are especially large and extend far down into the seventh layer of spindle

and triangular cells. In the visual cortex, on the contrary, the pyramidal cells, and particularly

those of the larger class, are greatly reduced both in bulk and in range of distribution, while

there are large accumulations of stellate cells, that send out dendrites in all directions (4 and 5,

Fig. 94).

These differences in the representation of characteristic cell forms are paralleled by differences

in the arrangement of the cortical fibre systems. Where the pyramidal cells are the prevailing

type, the fibres in general take a longitudinal course, ascending vertically from the alba to the

cerebral surface. A large number of these longitudinal fibres issue directly from the pyramidal

cells: the neurites of the larger cells (A Fig. 95), in particular, are continued without break to the

myelic columns. We have, then, in these longitudinal fibres of the centromotor region, the direct

point of departure of the pyramidal path. All the other processes of the pyramidal cells are

dendritic in character. The stoutest of them leaves the [p. 221] cell body on the side opposite to

the neurite and runs, still longitudinally, to the periphery of the cortex, where it is broken up in

the nervous reticulum of the outermost layer. We thus have, in all probability, a direct

centrifugal conduction, as indicated by the arrows in the

Fig., beginnings in the peripheral layer of the cortex and

continuing through the pyramidal cells into the motor

paths. This system is, however, cut across by other

longitudinal fibres, some of which issue from smaller

cells whose neurite runs a brief course and then splits up

into terminal fibrils, while others ascend in more

connected fashion from the alba, and again break up in

the fibrillar reticulum of the outermost cortical layer. The

former (B) constitute, it is supposed, links in the

association system, which, as we know, sends fibres to

every cortical area; the latter (D) are probably centripetal

neurites of deeper lying cells, situated in the

mesencephalon. Putting these facts together, we may

describe the structure of the motor cortex as follows. (1)

Its most characteristic constituent is the centrifugal path

(A), mediated by the pyramidal cells. It contains further

(2) a centripetal fibre system (D), probably connected

with the path (A) in the nervous reticulum of the

outermost cortical layer, and representing the terminal

sphere of a neurone territory which belongs to deeper

lying portions of the brain; and (3) an association path

(B) mediated by intercalary cells, which, like the other

two, takes in general a longitudinal course and

discharges into the fibrillar reticulum of the outermost

layer. The direction of conduction in this path is

indeterminate; it may possibly vary with the direction of

the incoming excitations. Finally, we must mention (4)

the plexus of stellate cells, which, while it plays but a

small part in the centromotor regions, is never entirely

wanting. This, if we may judge by the character that

attaches to it in the occipital cortex, is to be regarded as the [p. 222] terminal station of paths of

sensory conduction. Its position in the motor cortex is, as we have said, comparatively

insignificant. It may be distinguished from the other constituents by its plexiform structure, the

fibres running in all possible directions. The presence in the centromotor region of a formation

which is characteristic of the sensory centres, may, perhaps, be taken to mean that this region

is sensory too, as well as motor. Such an interpretation would be in accordance with the fact

that physiological experiment and pathological observation place the centre for the general

sense in this part of the cortex.

We now turn, by way of contrast, to the

'visual' cortex, as a typical illustration of a

pre-eminently sensory region. We are at

once struck by the marked difference in

the course of the fibres. Plexiform

formations, with fibres running in all

directions -- horizontally, therefore, as

well as vertically and obliquely -- are very

strongly preponderant, while the

longitudinal fibre masses characteristic of

the terminal areas of the pyramidal paths

find but scanty representation (Fig. 96).

These plexuses, C, are constituted of the

large and small stellate cells, sending out

processes in all directions, which appear

conspicuously in the section of Fig. 94; in

all probability, they consist simply of

interlocking neurones, of relatively limited

range. The characteristic systems of the

motor area also appear, only in lesser

numbers, in the visual cortex; just as the

formations which we connect with the

sensory functions are present, in some

degree, throughout the centromotor

region. We notice, in particular, the

longitudinal centrifugal fibres, connected

with the pyramidal cells (A). There are, [p.

223] further, the centripetal fibres, ascending to the cortex from deeper lying cell groups; and,

lastly, the supposed association fibres with their intercalary cells (B). We must accordingly infer

that the visual cortex discharges centromotor, as well as centrosensory functions. The muscles

that can be innervated from it are, we may suppose, more especially the muscles of the eye,

though it is possible that other motor organs, correlated with the ocular muscles, are also under

its control. H. MUNK has observed movements of the eyes, in animals, as a result of

stimulation of the visual cortex.[72]

Such are the differences that obtain between the two main types of cortical structure, the

sensory and the motor. Minor differences are found in the various parts of the motor cortex, and

again between the visual area and the other predominantly sensory regions. The former have

already been discussed. As regards the latter, we notice that in the olfactory cortex of man the

pyramidal cells are even rarer than in the visual; the smaller pyramidal cells are altogether

wanting. The auditory cortex is characterised, on the other hand, by its great wealth of stellate

cells and by the extent of its sensory fibrillar reticula. In the 'association cortex,' finally, these

plexuses become less conspicuous, and the granular layers, containing for the most part

intercalary cells of varying form, play the leading part.

Putting all this together, we may sum up as follows the general outcome of investigation into

the structural peculiarities of the cerebral cortex. Not only are the essential morphological

elements the same, for all divisions of the cortex, but their general arrangement also presents

no really significant differences. At the same time, there are several layers which, with their

characteristic elements, attain to very different degrees of development according to the special

functions of the various parts of the cortex. Two kinds of cellular elements, in particular, with

the arrangement of fibrillar processes that goes with them, appear to be of symptomatic

importance in this regard: the pyramidal cells, with their longitudinally directed fibres, and the

stellate cells, with their fibrillar reticula, -- the former characteristic of the centromotor regions,

and therefore, we may suppose, serving in the main as points of departure of the great

centrifugal conduction paths; the latter characteristic of the sensory regions, and therefore, in

all probability, serving in the main as terminal stations of paths of centripetal conduction. To

these we must apparently add, as a third characteristic constituent, varying greatly in extent of

development, certain [p. 224] cells with limited neurone territory, set longitudinally and

connected with longitudinal fibre systems, which may perhaps be looked upon as the substrate

of the 'association' paths. Lastly, in all regions of the cortex, the outermost layer with its

reticular fibre arborisations seems to form a meeting-place, in which conduction paths of the

most various kinds come into contact with one another. The question whether the different

regions of the cerebral cortex possess a specifically different structure, which may at the same

time serve as the basis for the discrimination of functions, or whether their constitution is, in

essential features, the same throughout, has often been under discussion in recent years.

MEYNERT, in his epoch-making studies of the human cortex, declared himself for uniformity of

structure;[73] and he has been followed by GOLGI [74] and KÖLLIKER.[75] Other investigators,

and more especially RAMON Y CAJAL,[76] to whom we are at the present time most deeply

indebted for our knowledge of the structural conditions here prevailing, uphold the hypothesis of

specific differences. This divergence of opinion seems, however, when closely examined, to be

much less radical than its phrasing indicates; it hinges, apparently, upon the different

interpretation put by the parties to the controversy upon the term 'specific,' as applied to

structural peculiarities. RAMON Y CAJAL'S enquiries have themselves furnished conclusive

proof of the extraordinary degree of similarity obtaining in the structure of the various regions,

and have shown that the differences are in every case merely relative differences in the

number of particular elements and in the development of the layers. Nay more: since they have

made it probable that the different centromotor, sensory and 'associative' functions are bound

up with definite cell and fibre systems which, as a general rule, are found in all parts of the

cerebral cortex, and that these functions are in the main conditioned simply upon differences in

direction of conduction, which in their turn depend upon differences in mode of connexion with

peripheral organs and with other cortical areas, they have really done away with any possibility

of the correlation of specific elementary substrates with the 'specific' functions of the various

departments of the cortex. They rather force us to the conclusion that the different modes of

cortical activity are founded not upon the specific character of the structural elements, but

solely upon their different modes of connexion. Nevertheless, as we shall see in what follows,

modern brain anatomy presents us with the very curious spectacle of a science that holds with

extreme tenacity to the hypothesis of specific functions, while its own results are constantly

rendering this hypothesis less and less practicable, and indeed, if the evidence of structural

relations is to count at all, bear striking testimony against the specific nature of the elementary

nervous functions.[p. 225]

§ 9. General Principles of the Processes of Central Conduction

(a) -- The Principle of Manifold Representation

It is almost inevitable that the student who is tracing the course of the conduction paths, and

their concurrence and interrelation in the various divisions of the central organs, should be led

into hypotheses concerning the functions of these different parts. Hence it is not surprising to

find that, as a matter of fact, physiological conclusions have often been based upon anatomical

data. The value of such inferences must, of course, always remain problematical, seeing that

they always need to be supplemented by direct physiological analysis of the functions

themselves. At the same time, it is evident that we may look to the conditions of conduction to

indicate points of departure for functional analysis. They will, at any rate, warrant us in ruling

out certain ideas, from the outset, as inadmissible, and in accepting others as more or less

probable; and they will do this altogether apart from any functional reference or physiological

knowledge. Thus, in view of the complicated character of the acoustic conduction, as

represented in Fig. 77 (p. 183), it will be granted without hesitation that an hypothesis which

should explain the process of tonal perception as due merely to sympathetic vibrations of some

sort of graded nervous structures within the brain must be pronounced wholly improbable. In

the same way, the idea that the act of spatial vision is effected by a direct projection of the

retinal image upon elements of the visual centre, arranged in mosaic on the analogy of the rods

and cones of the retina, could hardly be reconciled either with our knowledge of the relations of

the optic conduction to other, and more especially to motor paths, or with the observations

made upon the structure of the visual cortex. In this sense, then, the argument from anatomy to

physiology forms a useful preparation for the considerations of the following Chapter. Now that

we have concluded our discussion of the conduction paths which are of particular importance

for the psychological functions of the nerve centres, we may, accordingly, pause to point out

the main lines of interpretation suggested by the general results of the preceding enquiry, and

to attempt their formulation in certain laws or principles.

We head our list with the principle of manifold representation. This first and most general

principle was formulated long since by MEYNERT, himself the first to make a systematic study

of the microscopical structure of the brain. It declares that, as a general rule, every region of

the bodily periphery which is controlled by the central organ has not one but several means of

representation at the centre. In other words, if we have recourse to the analogy of a mirror and

its images -- an analogy, however, which, as we shall soon see, cannot really be carried

through -- it says that every [p. 226] sense organ and every organ of movement, together with

every least part of such an organ, every sensory or motor element, is reflected not once only,

but several times, in the central organ. Every muscle, e.g., has its proximate representation in

the myel, from which (under the right conditions) it may be stimulated, or its excitation inhibited,

without the interference of higher central parts. It is then represented a second time in the

regions of the mesencephalon, the quadrigemina or thalami; and again, a third time, in the

centromotor regions of the cerebral cortex. Finally, we must assume that it is indirectly

represented in the parts of the cerebellum, and in the association centres, with which these

regions stand in connexion. Now it is by no means necessary that the whole group of central

representatives shall co-operate in every discharge of function by the peripheral elements. On

the contrary, there can be no doubt that the lower centres are often able to exercise an

influence upon the peripheral organs, in which the higher representations are not involved at all.

When, however, the peripheral effect is the result of activity in the higher neurones, the

excitation, under ordinary circumstances, is either mediated directly by lower centres, or at

least arouses concomitant excitations in them. In this sense, therefore, the principle of manifold

representation appears as an immediate consequence of the complex nature of all central

functions. At the same time, it shows that there is an ascending progression in the co-operation

of the various central representatives of one and the same peripheral region, and thus leads at

once to the following second principle.

(b) -- Principle of the Ascending Complication of Conduction Paths

The central organs of the higher vertebrates are, very evidently, subject to a law of ascending

complication. The number of branch paths, and therefore of the relations mediated by them

between centres which, while functionally distinct, are still somehow interrelated by the needs

of the organism, increases rapidly as we pass from below upwards. In the myel, the main lines

of conduction are brought together compactly in the peripheral nerves; and the connection

between principal path and secondary paths is of a comparatively simple and limited character.

In the oblongata and the mesencephalon, these connexions begin to show a considerable

increase, both in number and complexity. In the mesencephalic portion of the acoustic and

optic paths, for instance, we find that the connexions with motor and with other sensory centres,

which in the myel were arranged on a relatively simple pattern, are repeated in a very much

more elaborate and complicated form. This ascending progression of conductive connexions

reaches its final term in the cerebral cortex. Every part of the cortex, however diverse its

proximate connexions and however distinct [p. 227] its proper function, is the meeting-place of

conductive systems of the most varied kinds; so that what we term a 'visual' centre, e.g.,

always possesses something of the 'motor,' and something even of the 'associative,' along with

its sensory character. It is, therefore, a corollary from this law of increasing complexity of

representation in the ascending direction, as bearing more particularly upon the functions of the

cortex, that every cortical area in the brain of man and of the higher animals is, in all probability,

itself the seat of a manifold representation. Every portion of the visual cortex, that is, will

contain, besides the representation of a part of the peripheral retina, further representations of

motor areas connected with the function of sight and, possibly, of other, functionally related

sensory areas; and finally, in all likelihood, indirect representations, mediated by the association

fibres, of more remote functional centres that are again in some way concerned in the act of

vision. Hence the idea that a sensory centre is, in essentials, nothing more than a central

projection of the peripheral sensory surface, -- the visual centre, e.g., a projection of the retina,

the auditory centre a projection of the 'resonance apparatus' of the labyrinth, -- even if it were

admissible on physiological and psychological grounds, could hardly be held in face of the

grave objections arising from the anatomical facts.

This principle of increasing differentiation in the ascending direction enables us to explain a

further fact, suggested by the results of the gross anatomy of the brain, but brought out with

especial clearness by histological examination of the conduction paths and by the phenomena

of function that we describe in the following Chapter. This is the fact that many, perhaps most

of the functions which in man and the higher mammals are finally integrated and co-ordinated in

the cerebral cortex, appear in the lower vertebrates to be completely centralised in the

mesencephalic ganglia: so, more especially, certain sensory functions, such as sight and

hearing. Even in the lower mammalian orders, e.g. the rodents, the cortical representations of

these organs do not attain anything like the extent and the functional importance that they

possess in man. That is to say, the central organ provides itself with new representations, only

in proportion as a more complicated co-ordination of functional units becomes necessary.

When this happens, there is a relative reduction of the existing central stations in the same

degree. This accounts for the comparative insignificance of the mesencephalic region in the

brain of the higher animals and of man.

At this point the question naturally arises, whether or not the investigation of nervous

conduction has furnished any evidence of specific differences [p. 228] in the functions of the

central elements and of their conductive processes. We answer it by saying that one, and only

one such difference may probably be inferred from the anatomical and physiological relations of

the conduction paths. This is the difference in direction of conduction, connected with the

twofold mode of origin of the nervous processes, which was first suggested from the anatomical

side by RAMON Y CAJAL, and which receives confirmation from certain elementary facts of

nerve mechanics (p. 99). In the older physiology, the establishment of determinate directions of

conduction was ascribed to the nerves themselves, though it could hardly be brought into

intelligible connexion with the properties of the nerve fibre. We are now able to refer it to a

peculiar process of differentiation in the nerve cell. The explanation has been given above, in

Ch. III. Every cell, as we there set forth, is the seat of excitatory and inhibitory processes, which

under the influence of this differentiation are distributed in different proportions to definite cell

regions. Originating in this way, however, the principle of different directions of conduction can

hardly be looked upon as a law of universal validity. It is rather a principle of development,

entirely compatible with the persistence of an undifferentiated condition in certain of the central

elements. We have, as a matter of fact, found this condition to obtain in various types of cells,

in which the twofold mode of origin of the conduction paths is neither anatomically proved nor

physiologically probable. And it is noteworthy that these cells always occur in situations where

the functional requirements do not include a differentiation of the directions of conduction, or

rather -- for this is really the more correct expression -- where there is no demand for inhibitions

of an excitation that has come in from a given direction. Thus, the differentiation is more or less

doubtful in many cells of the sensory system, from those of the spinal ganglia onwards; and

there are many intercalary cells, some lying within the central organs, some displaced far

outwards in peripheral sense organs, which physiologically give no ground whatever for the

assumption of a definite direction of conduction, and morphologically offer no sign of a twofold

mode of origin of their processes. Cf. above, pp. 158 f.

The differentiation of the directions of conduction is, then, the result of a process of

differentiation peculiar to the nerve cell, and apparently connected with an especial modification

of the cell structure. It is, at the same time, the sole form of functional difference that the

investigation of the conduction paths has brought to light; and, we must add, the sole form that

a simple determination of these paths can ever reveal to the investigator. For the enquiry has,

of course, its definite limits. It can tell us where, between what terminal stations, and (under

favourable conditions) in which directions the processes are conducted; but it cannot [p. 229]

tell us anything of the nature of the processes themselves. Nevertheless, it is a point of

importance for the physiology of the central functions that, apart from this differentiation of the

directions of conduction, no qualitative differences in the central elements can be demonstrated

by morphological methods or inferred from the mechanics of innervation.

(d) -- The Principle of the Central Colligation of Remote Functional Areas.

Theory of Decussations

We have yet to mention a circumstance from which, in very many cases, the principle of the

manifold representation of peripheral areas undoubtedly derives its peculiar significance: the

fact that bodily organs which lie more or less widely apart, but yet function in common, are

oftentimes brought into spatial as well as into functional connexion in their central

representations. This means, of course, that the integration of functions may be mediated with

the least possible circuity by conduction paths running directly between the central stations.

Thus, the nerve paths that are brought into action in the locomotor movements of man and the

animals issue from the myel at very different levels. But there are several places in the central

organ (mesencephalon and cerebral cortex) where the centres for these movements lie close

together; so that a suitable co-ordination can be effected, for voluntary as for involuntary, purely

reflex movements, along well-trodden paths of cross connexion between neighbouring centres.

In the same way, the intimate relation that obtains between the rhythm of auditory ideas and

the rhythm of bodily movements becomes intelligible when we remember the number and

variety of the connexions, at comparatively short distance, between the acoustic and the motor

centres. It would seem, then, to be one of the most important features of conduction within the

central organ at large that it serves, literally, to centralise: it unites the various functional areas,

and thus renders possible an unitary regulation of functions that are separated in space but

belong together in the service of the organism. And this means, further, that all the separate

functions distinguished by us, since they are known solely in this their centralised form, must in

reality themselves consist of an union of many functions, distributed over different, in many

cases over widely remote peripheral organs. We may, therefore, reject without discussion any

such view of the conductive connexions of the central organ as maintains, e.g., that there is a

central act of vision, independent of motor innervations and of the mutual relations of different

retinal elements: for the central organ of vision is not a mere projection of the retina upon the

cerebral surface, but an extremely complicated structure, in which all the partial functions

concerned in the visual function find representation. And we may reject, similarly, any theory

that pro-[p. 230]poses to isolate the rhythmical form of successive auditory impressions, as a

form of excitation peculiar to the auditory centre, and thus to separate it from the associated

motor impulses. Every psychophysical function that falls under our observation is already, in

point of fact, a centralised function, i.e. a synergic co-operation of a number of peripheral

functions. What the retina or the peripheral auditory organ could contribute of itself to the

formation of our perceptions, we do not know, and we can never find out: for the functions of

eye and ear and of all other organs come under our observation always and only in this

centralised form, i.e. as related to the activities of other functional areas.

Among these combinations of remote peripheral organs for unitarily centralised, synergic

functions, a place of special importance is taken by the connexions dependent upon

decussation of the paths of conduction. In their case, the separation and rearrangement of the

paths are carried, by their passage to the opposite half of the brain, to the highest conceivable

point; and, as a result of this, the functional significance of such central rearrangement is

shown with the greatest possible clearness. Among the decussations themselves, that of the

optic nerve, which appears in well-marked developmental sequence through the whole animal

kingdom, shows the most obvious relations to the visual function. Where the compound eye

occurs in its earliest stages of development, as in the facetted eyes of the insects, the retinal

image forms a rough mosaic whose spatial arrangement -- since every facet represents a

relatively independent dioptrical structure -- corresponds to that of the external object; what is

above and below, right and left, in the object has precisely the same position in its image. Such

an eye, therefore, if it has a muscular apparatus at its disposal, does not move about a point of

rotation situated within itself, but is seated upon a movable stalk, i.e. turns (like a tactual organ)

about a point lying behind it in the body of the animal. Under these conditions, it does not

appear that the optic paths undergo any appreciable decussation; indeed, it is characteristic of

the invertebrates at large that the great majority of the nerve paths remain upon the same side

of the body. When, on the other hand, we turn to the lowest vertebrates, we are met at once by

a complete reversal of the picture; the optic paths are now entirely crossed, so that the retina of

the right eye is represented exclusively in the left, that of the left eye in the right half of the

brain. RAMON Y CAJAL conjectures, with great acumen, that this arrangement may serve to

compensate the inversion of the retinal image effected by the dioptrical apparatus of the

vertebrate eye: he reminds us that the eyes of the lower vertebrates are, as a general rule, set

laterally in the head, and that they accordingly cannot furnish a common image of the objects

seen, although their two images may supplement each other in the sense that the one eye sees

parts [p. 231] of an extended object which remain invisible to the other. The hypothesis also

furnishes an explanation of the fact that the decussation changes from total to partial in

proportion as the eyes, placed in front of the head, acquire a common field of vision, -- as they

do in many of the higher mammals, and more especially in man. RAMON Y CAJAL, however,

bases his interpretation of these facts upon the assumption that the retinal image is directly

projected upon the visual cortex. Suppose, e.g., that the eyes are so situated, laterally, that the

right eye images precisely the half ab, the left the

half bc of the object abc (Fig. 97) It follows at

once, from the fact of inversion, that the two

retinal half-images aß and ßg are out of their

right positions: for if we regard ßg as the direct

continuation of aß, ß should be joined to ß, and

not g to a. If, now, the optic fibres undergo a

total decussation, this incongruity will disappear

in the projection on the central visual surface: the

two halves of the image can be put together as

half-images in just the same relation that they

sustain in the external object (a'ß', ß'g').

RAMON Y CAJAL believes that the decussation

of the optic nerves, thus necessitated by the

optical construction of the eye, has formed the

point of departure for all the further decussations

of conduction paths; that it was followed, first of

all, by the crossing of the motor paths of the

ocular muscles, and then by that of the other,

sensory and motor paths, correlated with these.

[77] The hypothesis is ingenious in the extreme;

and it is entirely probable that optic decussation

and binocular synergy are closely interrelated.

Nevertheless, the theory cannot be carried

through in its present form. It is based upon

assumptions that conflict not only with everything

else that we know of the nature of the act of

vision, but also, in the last resort, with everything

that we know of the character and course [p. 232]

of the conduction paths and of their terminations

in the brain cortex. The retina itself is, as we have remarked above, a part of the central organ

that has been pushed outwards to the periphery. It is, then, first and foremost, somewhat

surprising that the disorientation of the image on the retina should be of no consequence, and

its derangement on the cortex seriously disturbing. Those who hold such a view evidently rest it

upon the belief that consciousness resides directly in the cortex, and there takes cognisance of

an image of the external world, which must therefore, at this point, exactly repeat the real

position of the external objects. That certain difficulties arise from the presence of the cortical

gyres is admitted by RAMON Y CAJAL himself. To meet them, the further hypothesis must be

made, that the disorientation of the images produced in each individual brain by the convolution

of its outer surface is compensated by a remarkably accurate adaptation in the distribution of

the crossed fibres. But then there is still another difficulty. If the image on the central visual

surface corresponds exactly to the spatial properties of its object, then we must expect not only

that the asymmetry arising from inversion of the image is binocularly compensated by the

decussations in the chiasma, but also that, in each separate eye, there is an analogous

compensation with regard to the vertical dimension. What in the retinal image is above, must in

the visual centre be below, and conversely. The right and left decussation of the optic fibres

would then be accompanied, in every optic nerve, by a second, vertical decussation. But this

has not been demonstrated. Even in cases of what is called cortical hemianopsia in man, its

existence has never been suspected. On the other hand, it is noteworthy that this cortical

hemianopsia is much more obscure in its symptoms than the hemianopsia observed in cases of

interruption of the fibres in the optic tract and thalami; minor defects, in particular, may pass

without symptoms of any kind, or may be connected simply with a diminution, not with complete

abrogation of sensitivity to light.[78] This, however, is just the opposite of what we should

expect, if an undisturbed reproduction in the visual cortex of the space relations of the object

were the one thing necessary for the act of vision. Moreover, we must remember that there is a

very much simpler and more plausible explanation of the fact that eve see objects upright,

despite the optical inversion of their images, than would be given with this hypothesis of a

vertical decussation. Wherever the visual organ has become a dioptrical apparatus, involving

inversion of the image, the point of ocular rotation lies, not behind the eye in the body of the

animal, as it does in the stalked eyes of the invertebrates, but in a point d within the eye itself

(Fig. 98). Hence, when the central point of vision in the yellow spot moves in the retinal image

from below upwards [p. 233] in the direction aß, the external point of fixation in the object

moves from above downwards, in the

direction a b. That is to say, the displacement

of the point of rotation to the interior of the

eye brings with it a direct compensation of

the inversion of the image. For we estimate

the space relations of objects in terms of the

position and movement of the line of fixation

before the point of rotation, not behind it, and

not either in terms of the retinal image --

whose position is really just as little known to

us as are the space relations of the

hypothetical image in the visual centre: and

of that we do not even know whether or not it exists at all. Intrinsically, it is, without any

question, far more probable that we should assume in place of such an image a system of

excitations, corresponding to the various sensory, motor and associative functions

simultaneously concerned in the act of vision.

This compensation of the inversion of the retinal image by the motor mechanism of the single

eye evidently furnishes the nearest analogy for the reciprocal orientation of the right and left

retinal images as it occurs in binocular vision. Here, too, we must suppose, the motor

mechanism has not adapted itself, after the event, to the relation obtaining between the two

hypothetical images in the visual centre, but has from the first exercised a determining

influence as regards orientation within the common field of vision. Now for the visual organ with

laterally placed eyes, the fundamental characteristic of the field of vision is that it consists of

two entirely different halves; in the ideal form of such an organ, the separate fields will just meet

in the middle line. Vision of this sort may fitly be termed, with RAMON Y CAJAL, 'panoramic'

(as opposed to 'stereoscopic') vision. It covers a wide range; but it mediates only a superficial

image, and gives no direct idea of the third dimension. Further, a correct orientation of the two

halves of the panoramic image is possible only if an object that travels continuously from the

one half of the field of vision to the other shows no discontinuity in its movement; and this

condition, again, is fulfilled only if similarly situated eye muscles are symmetrically innervated

as the movement continues. If the object has been followed, e.g. by the line of regard of the

right eye in Fig. 97 from a to b,, then the movement must be carried on, without interruption,

from b to c, by innervation of the line of regard of the left eye; that is to say, the innervation of

the left rectus internus, whose direction of pull is indicated by [p. 234] the dotted line i2, must be

so co-ordinated with the innervation of the right rectus internus, i1, that it promptly relieves its

predecessor, to give way in its own turn to the innervation of the left externus, e2. Now there is,

as we have said, no ground for supposing that the visual centre is the scene of any kind of

pictorial projection, even remotely resembling that which we have on the retina. It is, however,

not improbable that the mechanisms of release, by means of which sensory are transformed

into motor impulses, are here arranged in a certain symmetry: in such a way, that is, that if e.g.

the road to the rectus internus is thrown open in the visual cortex of the right hemisphere at a

definite point e', it must in the left hemisphere be thrown open at a point e'', functionally co-

ordinated with e' and lying symmetrically with it to the median plane. This arrangement will,

naturally, come about by process of development. Let us take, as the first term of the series, a

state of affairs where the eyes of the two sides stand in no sort of functional relation to each

other: the condition seems, as a general rule, to be actually realised, e. g. in the visual organs

of the invertebrates. Here, we must imagine, these mechanisms of release, like all the other

central elements, will in each optic ganglion be arranged symmetrically to the median plane:

whatever lies farthest to the right on the right-hand side of the body will do the same on the left

side, and conversely. Suppose, again, that the development is carried a step further, and that

the eyes are to co-operate for a panoramic vision of the kind described above. The symmetrical

arrangement would now be insufficient; it would prevent the regular sequence of the

movements of the two eyes, required for an adequate apprehension of objects. The

arrangement in the visual centre of the hypothetical mechanisms of release is, of course,

unknown to us, apart from the probability of their symmetry with reference to the median plane;

fortunately, it is also, for our present question, a matter of indifference. We will assume, for

simplicity's sake, that the points of release lie inwards for the interni and outwards for the

externi. Then, if the object is followed in fixation from a to c, the externus e1, will first be

innervated from a central point e'. As the internus i1, comes into action, the central innervation

will move from e' to i'. Next, upon the entrance of the object into the visual field of the left eye, it

will pass without interruption to i2, i'', and thence, finally, to e2, e''. If there were no decussation,

the central point of release corresponding to a point upon the nasal half of the retina would, on

the contrary, lie inwards, and the point of release corresponding to a point upon the temporal

half lie outwards, on both sides of the median line. The arrangement of the points, from right to

left, would then be i' e' e'' i'', and the innervation would first of all travel on the right, from within

outwards, and then shoot across to the left visual centre, to execute a similar movement there.

We must add that the [p. 235] arrangement which controls movement naturally determines as

well the localisation of the resting eyes; so that bc is seen as the direct continuation of ab. The

reason, once more, is not that this is the order in which images of the separate points are

projected upon the brain, but that it is the order in which there is congruity of the sensory and

motor functions that work together in every instance of spatial perception. Presumably,

therefore, the total decussation of the visual organ with panoramic function does not represent

an arrangement which was first found good on the sensory side and later extended to the

motor. It must rather be regarded as an arrangement which, from the outset, applies to both

sensory and motor areas, and which mediates their co-operation.

We are, then, justified in positing an organisation that extends over the entire sensorimotor

system and is determined by the necessary co-ordination of particular sensory and motor

points. The hypothesis has the further advantage that it, and it alone, can adequately explain

the obvious connexion between the change from total to partial decussation of the optic fibres,

on the one hand, and the passage from panoramic to stereoscopic vision on the other.

Suppose that we have reached the stage at which the eyes are set so far forward that their

fields partially overlap, to form a common field of vision, embracing the same objects. The

conditions are now very different from those obtaining in panoramic vision. The synergy of the

eye movements, in so far as it is controlled by the nearer objects in the common field of vision,

is no longer laterally symmetrical, no longer such, i.e., that right corresponds to right and left to

left upon the two sides, but has become medianly symmetrical, so that those points are

homologous that lie on either side at the same distance from the median plane. Stated directly

in terms of eye movement, this means that the synergy in panoramic vision is that of parallel, in

stereoscopic vision that [p. 236] of convergent movement of the lines of sight. The difference

will be clear at once from a comparison of Fig. 99 with Fig. 97. In Fig. 97 where we have a

representation of the limiting case in which the two fields of the laterally placed eyes pass into

each other without break, the point of contact b is the sole point seen in common by both

halves of the visual organ. If the object is given the direction in the third dimension indicated in

Fig. 99, only this one point upon it remains visible to the two eyes. For the visual organ with a

common field of vision (Fig. 99) the case is different. Here the object, despite its position in the

third dimension, remains entirely visible to both eyes; the image ag upon each retina

corresponds to that side of it which, as it recedes in space, is turned towards the eye in

question. Along with this change in scope of vision goes, further, a change in the conditions

under which the eyes must move in order to bring

all parts of the object upon the yellow spot. The

movements are not touched off successively, as

in the previous instance, -- the left eye taking up

the movement at the point where the excursion of

the right eye reaches its limit, -- but have become

simultaneous: right and left eye range over the

object, from a to c and back again from c to a,

both at once. And the muscles co-ordinated for

the purposes of these movements are muscles

placed symmetrically not as regards external

space, but as regards the median plane of the

body and, consequently, as regards each

separate eye: internus and internus, externus and

externus. This means, once more, that there is a

radical change in the conditions under which the

movements are touched off centrally by light

stimuli. The farther inward, towards the median

plane, a retinal point lies in either eye, the farther

out, in the third dimension of space, lies the

corresponding point of the object situated in the

common field of vision. If, therefore, a point is

stimulated upon the nasal halves of the two

retinas, the interni are brought into action at i' and

i'', and a movement is produced by symmetrical

increase of convergence in the direction ca. If, on

the other hand, a retinal point is stimulated on

both sides that approximates to the near limit of

the field of vision, the binocular organ will

traverse the object with symmetrically decreasing

convergence; the two externi are brought into

synergic action at e' and e''. Recurring to our hypothesis that the elements in the central organs

are arranged upon a principle of median symmetry, we may say, therefore, that the

requirements of symmetrical convergence will be fulfilled if the mechanisms for the release of

motor impulses by light stimuli are distributed symmetrically to the median plane in the visual

centre of the same side. Since distant points in space correspond to retinal points that lie

nasalward, and near points in space to retinal points that lie temporalward, the arrangement of

the mechanisms of release will be in conformity with the medianly symmetrical disposition of

the parts of the brain if the musculi [p. 237] interni are represented in the brain, too, on the

inside, nearer the median plane, and the musculi externi on the outside. Let us assume that this

arrangement actually holds. Then the inversion of the image, due to the dioptrical structure of

the eye, is precisely the position that will satisfy the needs of stereoscopic vision and of the

mechanism of convergence. It is, accordingly, a condition of the adaptation of the visual organs

to these needs that the optic paths remain uncrossed for the whole extent of the common field

subserving stereoscopic vision. But the common field is only a part of the total field of vision. In

man himself and in the animals with a like visual endowment each eye has, besides, its own

particular field, represented by the inner portions of the retinas, and governed, of course, by the

conditions of panoramic vision and the special laws of eye movement that they bring in their

train. Here, then, the two visual organs are laterally symmetrical, and the necessary

decussation of the optic paths -- and, with them, of their centromotor releases -- is effected on

the pattern of Fig. 97. In this sense, therefore, the partial decussation of the optic paths gives

an accurate picture of the state of affairs resulting on the one hand from the co-operation of the

two eyes in binocular vision and on the other from the coordination of their independent

functions. We may add that, in all probability, the decussation in the chiasma possesses this

functional significance not only for the terminations of the optic paths in the visual cortex, which

we have had primarily in mind in the foregoing discussion, but also for the terminations in the

mesencephalic region. According to the plan of the opticus conduction laid down in Fig. 78 (p.

186), the two centres are similarly disposed in the essential matter of sensorimotor connexions.

There are many other instances of the decussation of conduction paths, recurring at all levels

from the myel upward. In no case, however, is the functional interpretation of the phenomenon

as obvious as it is in that of the opticus crossing. Nevertheless, we have no right to conclude

that all the other decussations are simple consequences of this transposition of the optic fibres.

On the contrary, the same synergy that is apparent here obtains also for the other organs of

sense and of movement, and more particularly for the relations between sensory excitation and

motor reaction, and may therefore lead independently at any point to analogous results --

though the results, once produced, may very well reinforce and support one another. This

relative independence of the different decussations is further suggested by the fact that in the

lower vertebrates, where the optic decussation is total, other decussations, as e.g. that of the

motor paths of the skeletal muscles, are much less complete than they are in man. Moreover,

in all the vertebrates up to man, the myel evidently retains the character of a central area in

which the conduction paths remain for [p. 238] the most part upon the same side of the body,

whereas the oblongata shows at once a large number of partial decussations, due to the

correspondingly large number of motor functions of bilaterally symmetrical nature -- movements

of respiration, of mastication, of swallowing; mimetic movements -- that have their centres in

this region. Similar decussations occur also in the olfactory and acoustic areas, where they are

again, in all probability, connected with motor synergies.

There is, finally, a further circumstance, affecting those cerebral areas which, as seats of the

more complex functions, receive association paths from various sensory and motor centres,

that presumably stands in intimate relation to the phenomenon of decussation: the

circumstance that certain centres, which exist potentially in both hemispheres, are

preponderantly developed upon one side. This applies especially to the 'speech centre,' which

we discuss in the following Chapter. In the great majority of cases, the speech centre has its

principal seat in certain frontal and temporal areas of the left hemisphere. Since this left side, in

consequence of the decussations of the motor paths, contains the centres for the motor

innervation of the right half of the body, we may suppose that the arrangement is connected

with the disproportionate development of the muscular system on the right and, more

particularly, with the right-handedness of the ordinary man. The latter phenomena may

themselves be referred partly to the character of human movements, which involve a

preference for the one side, and partly to the asymmetrical position of other bodily organs,

especially the heart.

The study of the conduction paths has been dominated, up to the present time, by the view,

natural to the adherents of a strict localisation theory, that every bodily organ must find its

representation at some point of the brain cortex. The other and at least equally tenable idea,

that all parts of the brain and, very especially, all parts of the brain cortex are intended to

mediate the interconnexion of different conduction paths, has, in consequence, been forced

unduly into the background. The strict localisation theory, as held by H. MUNK and by other

physiologists and pathologists, assumes that the surface of the brain is made up of a number of

sensory centres, which are in reality simple reflections or copies of the peripheral sensory

surfaces; so that e.g. every point upon the retina has a corresponding point in the visual cortex.

Secondarily, it is true, recourse is had to the subsidiary hypothesis (mentioned above, pp. 201

ff.) that the immediate neighbourhood of these direct sensory centres is taken up with special

ideational centres, to which the direct sensory impressions are in some way transferred: this

hypothesis is to explain the origination of memory ideas. The point of view persists, practically

unchanged, in FLECHSIG'S and RAMON Y CAJAL'S theory of specific 'association' centres.

For it appears to be the opinion of these authorities that, if the more complicated functions are

thus handed over to independent centres, the sensory centres proper are for that very reason

all the more securely established in their character as direct reflections of the bodily periphery.

As a matter of fact, however, this view is [p. 239] sufficiently refuted, even apart from the

complex nature of the disturbances consequent upon cortical lesion, by MEYNERT'S principle

of manifold representation. It is unjust to what is by far the most important aspect of central

organisation, the combination into an unitary resultant of component functions that are

oftentimes separate at the periphery. GOLTZ has been, from the first, its earnest opponent; and

in so far as it leaves altogether out of account the true character of the central functions, his

opposition is justified. On the other hand, he and his school have plainly gone too far in denying

localisation of function outright. Localisation, in a certain sense, is a direct corollary from the

fact that centralisation is never universal, but is always confined primarily to the integration of a

limited number of components. Within these limits, the course and distribution of the conduction

paths point, without any question, towards a certain localisation, though they and, in the last

resort, the structure of the cerebral cortex itself, point not less clearly to an interconnexion of

functions at all parts of the organ. If we look at the facts as a whole, we may safely say -- while

abstracting entirely from functional derangements, whose interpretation is oftentimes doubtful --

that modern brain anatomy has furnished overwhelming proof that the older idea of the brain

cortex, as a reflection or copy of the totality of the peripheral organs, supplemented at most by

a few special areas reserved for higher psychical needs, is altogether untenable.

The problem of the decussation of conduction paths, in the strict sense of the term, is less

difficult of solution than that of the unilateral representation of function, such as has been

demonstrated, in particular, for the functions involved in speech. Originally, it would seem, the

function in these cases was symmetrically disposed upon both halves of the brain, but practice

has for some reason been predominantly unilateral, and the one half has consequently gained

the ascendancy. It is natural to connect this difference of development with the right-

handedness of the majority of mankind. And the connexion is sustained by the fact that, in

various instances, derangements of speech have been observed in left-handed persons as a

result of apoplectic effusions in the right half of the brain.[79] Now it is not difficult to explain this

relation, if we once accept the possibility of unilateral development. In civilised man, who is

right-handed, writing is a function of the right hand; and the associations that colligate the

various speech functions are so many and so varied that the unilateral development of writing

alone might bring with it a corresponding unilateral localisation of the related factors. But since

men write, as a rule, with the right hand, and are in general more practised in the mechanical

control of the right hand than of the left, the left half of the brain must, by reason of the

decussation of the motor conduction paths, receive a larger measure of practice than the right.

This practice will, of course, be shared by the speech centre. The numerous instances of

restoration of the speech functions, with persistence of the central lesion, may then be referred,

along with all the other possible forms of vicarious function, to the substitution under special

conditions of the right for the left half of the brain. We have a similar substitution in the case of

the external organs: a patient who is paralysed on the right side is able, by practice, to use his

left hand for actions previously performed by the right, e.g. for writing; and the change of habit

at the periphery naturally carries with it a new course of practice at the centre. But when all this

is granted, the initial question still calls for answer; the question why the right-hand side of [p.

240] the body should ever have been preferred at all. In seeking to answer it, we must bear in

mind that most mechanical functions, to a certain extent even that of walking, make a heavier

demand upon the one side of the body than upon the other; and that, under this condition, the

right side is naturally marked out for special favours by the general asymmetry in the position of

the organs of nutrition in the higher animals. Here again, the plan of arrangement is, as we all

know, governed by the close interdependence of the individual organs. The placing of the liver

on the right means that the great reservoirs of venous blood also lie on the right, so that the

arterial system is necessarily relegated to the left. In the rare cases in which this disposition is

reversed (cases of what is called situs transversus viscerum), it is the rule that all the

asymmetrical organs are involved in the rearrangement of parts. Now the central organs that

stand in greatest need of protection are the circulatory organs. Consequently, most mammals

in combat with their enemies are apt to put their right side in the forefront of the battle; and this

habit must react favourably, by way of stronger development, upon the muscles of the right

side of the body. In man, the upright position brings with it a special need for the protection of

the central organs of circulation, and at the same time helps to render this protection easy and

efficient. On the other hand, it is probable that the left-handed situation of the circulatory organs

has furthered the development of the left-hand side of the brain.[80] Since, then, the more

developed half of the brain must correspond to the more developed half of the body, it is on the

whole intelligible that the peripheral paths of the right side should in the main be represented in

the left half of the central organ, and those of the left side in the right. On this assumption, it is

possible that the decussation of the pyramidal paths in man and the mammals may itself be a

simple consequence of the asymmetrical practice imposed by outside conditions upon the

bodily organs and their central representatives.

Notes

[1] Cf. the general discussion of reflex excitations in Chap. iii., pp. 85 ff. above.

[2] The most fruitful of these methods may find mention here. They are: GOLGI'S method of

impregnation by a metal, and more particularly by silver; and the methods of staining by

haematoxylin and methylene blue, introduced respectively by WEIGERT and EHRLICH. For

details regarding these and other methods, see OBERSTEINER, Anleitung beim Studium des

Baues der nervösen Centralorgane, 3te Aufl., 1896, 7 ff., and EDINGER, Vorlesungen über

den Bau der nervösen Centralorgane, 6te Aufl., 1900, 3 ff.

[3] FLECHSIG, Die Leitungsbahnen im Gehirn und Rückemark des Menschen, 1876. C. VOGT,

Etude sur la myélisation des hémisphéres cérébraux, 1900. Cf. §§ 5, 6, below.

[4] L. TÜRCK, Sitzungsber. d. Wiener Akad., nth. math.-naturw. Cl., vi., 1851, 288; xi. 1853, 93.

CHARCOT, Ueber die Localisationen der Gehirnkrankheiten, trs. FETZER, i., 1878, 159

(Lecons sur les localisations dans les maladies du cerveau, 1875); FLECHSIG, Ueber

Systemerkrankungen im Rückenmark, 1878.

[5] GUDDEN, Arch. f. Psychiatrie, ii., 1870, 693. FOREL, ibid., xviii., 1887, 162. VON

MONAKOW, ibid., xxvii., 1895, I, 386.

[6] MEYNERT, art. Gehirn in STRICKER'S Gewebelehre, 1871, 694 ff. (Buck's trans., 650 ff.);

Psychiatrie, Pt. I. 1884. LUYS, Recherches sur le systéme nerveux cérébro-spinal, 1865. The

Brain and its Functions, 1877 and later (Internat. Sci. Series).

[7] The olfactory, optic and acoustic nerves are purely sensory: the oculomotor, trochlear and

abducent ocular, the facial and the hypoglossal are purely motor; and the trigeminal,

glossopharyngeal and pneumogastric and accessory resemble the myelic nerves, i.e. become

mixed at a short distance from their place of origin. The sensory root of these latter has a

ganglion, which is wanting in the sensory nerves proper.

[8] C. BELL, An Idea of a New Anatomy of the Brain, 1811; An Exposition of the Natural

System of Nerves, 1824; The Nervous System of the Human Body, 1830.

[9] This analogy appears quite clearly as early as JOHANNES MÜLLER. Cf. his Lehrbuch d.

Physiol., 4te Aufl., i., 1844, 623. MÜLLER, however, leaves the question undecided. The first

physiologist who expressed the decided opinion that the function of the nerves is determined

solely by the organs with which they are connected was, as MÜLLER tells us, J. W. ARNOLD.

[10] LUDWIG and WOROSCHILOFF, Ber. d. sächs. Ges. d. Wiss., math.-phys. Cl., 1874, 296.

MOTT, Philos. Transact., clxxxiii. 1892, I.

[11] VAN DEEN, in MOLESCHOTT'S Untersuchungen zur Naturlehre des Menschen, vi., 1859,

279. SCHIFF, Lehrbuch der Physiol., 1858, i. 238. PFLÜGER'S Arch. f. d. ges. Physiol., xxviii.;

xxix. 537 ff.; xxx. 199 ff.

[12] LUDWIG and WOROSCHILOFF, op. cit., 297.

[13] H. E. HERING and C. S. SHERRINGTON, in PFLÜGER'S Arch. f. d. ges. Physiol., lxiii.

1897, 222. Cf. p. 93, above.

[14] Concomitant sensations with pain stimuli are described by KOWALEWSKY (trans. from the

Russian in HOFFMANN and SCHWALBE, Jahresber. f. Physiol., 1884. 26); 'conjugate'

sensations, with arousal of tickling, by E. STRANSKY (Wiener klin. Rundshau, 1901, os. 24-

26). In my own experience, the larynx and the external auditory meatus are especially liable to

concomitant sensations.

[15] PFLÜGER, Die sensorischen Functionen des Rückenmarks, 1853, 67 ff.

[16] FLECHSIG, Ueber Systemerkrankungen im Rückenmark, 30 ff. BECTHEREW, Die

Leitungsbahnen im Gehirn und Rückenmark, 1899, 17 ff.

[17] CHARCOT, Leçons sur les localisations, etc., 145 ff. FLECHSIG, Ueber

Systemerkrankungen, 42 ff. EDINGER, Vorlesungen, 6te Aufl., 86, 358

[18] EDINGER, op. cit., 317. RAMON Y CAJAL, Beitrag zum Studium der Medulla oblongata,

1896, 52 ff.

[19] Cf. RAMON Y CAJAL, Medulla oblongata, 43, 122 ff. EDINGER, op. cit., 86 ff.

[20] EDINGER, op. cit., 272. BECHTEREW, Leitungsbahnen im Gehirn und Rückenmark, 439.

[21] This exposition follows HELD, Arch. f. Anatomie, 1893, 201 ff.

[22] BECHTEREV, Die Leitungsbahnen, 169 ff.

[23] For a detailed account of the terminal nervous apparatus of the retina, cf. below, Ch. VIII §

[24] RAMON Y CAJAL, Die Structur des chiasma opticum. Trans. J. BRESLER. 1899.

[25] GUDDEN, Arch. f. Opthalmologie, xxv., I. VON MONAKOW, Arch. f. Psychiatrie, xxiii.,

1892, 619; xxiv., 229. GILLET and VIALET, Les centres cévébraux de la vision, 1893.

[26] MONAKOW, Arch. f. Psychiatrie, xx., 1889, 714 ff. BECHTEREW, Die Leitungsbahnen,

199 ff. RAMON Y CAJAL, Les nouvellas idées sur la structure du systèmenerveux, 1894;

Studien über die Hirnrinde des Menschen, i. Die Sehrinde, I 00. Besides the reflex path rr to the

nidi of the oculomotor nerves, there is another, which mediates the reaction of the pupil to light

stimulation. Its course has not yet been fully made out; but it seems to be altogether divergent

from the other, since the pupillary reaction is not abrogated by destruction either of the

pregemina or of the genicula. Cf. BECHTEREW, op. cit., 215.

[27] I avoid the use of the simpler terms 'motor' and 'sensory,' in order to indicate -- from the

outset, the essential difference that obtains between the conditions of conduction here and in

the peripheral nerves. The other terms in current use, 'psychomotor' and 'psychosensory,'

seem to me to be objectionable, for the reason that they suggest a participation of

consciousness or of mental functions which, to say the least, is hypothetical. It must also be

remembered that there are many central parts besides the cerebral cortex, e.g. the brain

ganglia, that are also endowed in certain measure with the properties under discussion.

[28] FRITSCH and HITZIG, Arch. f. Anat. u. Physiol., 1870, 300 ff. HITZIG, Untersuchungen

über das Gehirn, 1874, 42 ff. FERRIER, The Functions of the Brain, 2nd ed., 1886.

[29] LUCIAN, Arch. ital. de biologie, ix., 268.

[30] FRANCK, Les centres motrices du cerveau, 1887.

[31] FRANCK and PITRES, Arch. de physiol., 1885, 7, 149. BUBNOFF and HEIDENHAIN,

Pflüger's Arch. f. d. ges. Physiol., xxvi., 137.

[32] LUCIANI and SEPPILLI, Die Functionslocalisation auf der Grosshirnrinde, 289 ff, (Le

localizzazioni funzionali del cervello, 1885). HITZIG, Berliner klin. Wochenschrift, 1886, 663.

[33] On this point, cf. in particular GOLTZ, Ueber die Verrichtungen des Grosshirns, 1881, 36.

119 ff.

[34] GOLTZ, PFLÜGER'S Arch. f. d. ges. Physiol., xxvi., 170 ff.; xxxiv., 487 ff. LUCIANI and

SEPPILLI, op. cit. (German), 50 ff.

[35] SCHITSCHERBACK, Physiol. Centralblatt, v., 1891, 289 ff.

[36] HITZIG, Untersuchungen über das Gehirn, 126 ff.

[37] FERRIER, The Functions of the Brain, 1886, 235 ff.

[38] SCHÄFER, Beiträge zur Physiologie. C. LUDWIG gewidmet, 1887, 269 ff.

[39] HORSLEY and BEEVOR, Philos. Transactions. 1890, clxxix., 205; clxxxi., 129.

[40] H. MUNK, Ueber die Functionen der Grosshirnrinde, 2te Aufl., 1890. Berichte der Berliner

Akademie. 1892, 679; 1893, 759; 1895, 564; 1896, 1131; 1899, 936.

[41] Ueber die Verrichtungen des Grosshirns, Abth. i.-vii. In PFLÜGER'S Arch. f. d ges.

Physiol., 1876-1892.

[42] Cf. p. 188, above.

[43] On the relation of eye movement and retinal sensation, see below, Ch. XIV § 2.

[44] E. A. SCHÄFER. Proc. of the Royal Soc., 1887, 408. BAGINSKY, Arch. f. Physiol., 1891,

227 ff.

[45] CHRISTIANI. Zur Physiologie des Gehirns. 1885, 31 ff. GOLTZ, in PFLÜGER'S Arch. f. d.

ges. Physiol., li., 570 ff.

[46] GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., xxxiv., 459, 487 ff. CHRISTIANI, op. cit.,

138 ff.

[47] FERRIER, in Brain, 1881, 456; 1884, 139. LOEB, in PFLÜGER'S Arch. f. d. ges. Physiol.,

xxxiv., 88ff. LUCIANI and SEPPILLI, op. cit. (German), 145.

[48] For the criteria to be applied, cf. NOTHNAGEL, Topische Diagnostik der

Gehirnkrankheiten, Einleitung.

[49] NOTHNAGEL, Topische Diagnostik, 438 ff. H. DE BOYER, Études cliniques sur les lésions

corticales, 1879. EXNER, Untersuchungen über die Localisation der Functionen in der

Grosshirnrinde des Menschen, 1881. VON MONAKOW, Gehirnpathologie, 1897, 282 ff.

[50] FERRIER, Localisation der Hirnkrankungen, 12 ff. (Localisation of Cerebral Disease,

1878). NOTHNAGEL, Topische Diagnostik, 549. VON MONAKOW, Gehirn. pathologie, 376 ff.

[51] NOTHNAGEL, Topische Diagnostik, 389. LUCIANI and SEPPILLI, op. cit. (German), 167

ff. VON MONAKOW, Gehirnpathologie, 445 ff.

[52] VON MONAKOW, Arch. f. Psychiatrie, xxiv., 229 ff. Cf. also the conditions found to obtain

in the brain of LAURA BRIDGMAN, a deaf-mute, who became blind in early childhood. It may

be remarked, further, that the whole development of this brain suggests an elaborate system of

vicarious functioning, especially in the sensory region. DONALDSON, Amer. Journal of Psych.,

iii., 1890, 293; iv., 1892. 503 ff.

[53] Cf. the cases described by FÜRSTNER (Arch. f. Psychiatrie, viii., 162; ix., 90) and

REINHARD (ibid., 147), and by VON MONAKOW, Gehirnpathologie, 468 ff.

[54] Cf. the following Chapter, and the account of visual ideas given in Ch. XIV.

[55] WERNICKE, Der aphasische Symptomencomplex, 1874. KAHLER and PICK, Beiträge zur

Pathologie und pathologischen Anatomie des Centralnervensystems, 1879, 24, 182. LUCIANI

and SEPPILLI, op. cit. (German). 217 ff. VON MONAKOW, Gehirnpathologie, 506 ff.

[56] FLECHSIG, Gehirn und Seele, 2te Aufl., 1896, 61; Die Localisation der geistigen

Vorgänge, 1896, 34.

[57] EXNER, op. cit., 63 ff. LUCIANI and SEPPILLI, op. cit. (German), 321 ff. On disturbances

of the sensations of movement, consult further NOTHNAGEL, Topische Diagnostik, 465 ff.

VON MONAKOW, Gehirnpathologie, 362 ff.

[58] On this point see below, Ch. VI., §§ 5 and 6.

[59] See below, Ch. VI., § 7.

[60] CHARCOT, Leçons sur les localisations, etc. (Vorlesungen über die Localisation der

Gehirnkrankheiten, 120 ff.). NOTHNAGEL, Topische Diagnostik, 581 f.

[61] VON MONAKOW, Gehirnpathologie, 364 ff.

[62] Arch. f. experimentelle Pathologie, iii., 1874, 171.

[63] MEYNERT, Psychiatrie, 1884, 145.

[64] Arch. f. Physiol., 1878, 171; Ueber die Functionen der Grosshirnrinde, 44.

[65] Cf. with this WUNDT, Zur Frage der Localisation der Grosshirnrinde, in Philos. Studien, vi.,

1891 1 ff.

[66] FLECHSIG, Die Localisation der geistigeVorgänge, 13 ff. EDINGER, Vorlesungen, 6te

Aufl., 161 ff.

[67] FLECHSIG, Neurol. Centralblatt, 1898, no. 21.

[68] Cf. DÉJERINE, Zeitsch. f. Hypnotismus, v., 1897, 343; O. VOGT, ibid., 347 ;

SIEMERLING, Berliner klin. Wochenschrift, xxxv., 1900, 1033. See also p. 217, below.

[69] MEYNERT, in STRICKNER'S Gewebelehre, 117 (POWER'S translation, ii., 481 f.);

Psychiatrie, 40.

[70] FLECHSIG, Gehirn und Seele, 2te Aufl., 1896; Neurol. Centralblatt, 1898, no. 21.

[71] RAMON Y CAJAL, Die Structur des chiasma opticum, 56. EDINGER, Vorlesungen, 6te

Aufl., 228. HITZIG, Les centres de projection et d'association, Rapport lu au xiii. Congrès

internat. de Med. à Paris, Le Nevraxe, i., 1900 (criticism by FLECHSIG, ibid., ii., and reply by

HITZIG, 1900). VON MONAKOW, Monatsschrift f. Psychiatrie. viii., 1900, 405. O. VOGT,

Journ. de physiol. at pathol. gén., 1900, 525. See above, p. 213.

[72] RAMON Y CAJAL, Studien über die Hirnrinde des Menschen. German trans, by

BRESLER. Heft 1: Die Sehrinde; Heft 2: Die Bewegungsrinde, 1900. Comparative Study of the

Sensory Areas of the Human Cortex, in Decennial Volume of the Clark University, 1899, 311 ff.

[73] MEYNERT, Vierteljahrsschrift f. Psychiatrie, i., 91, 198; ii., 88. Also in STRICKER'S

Gewebelehre, ii., 704 ff. (POWER'S trans., ii. 381.)

[74] GOLGI, Sulla fina anatomia degli organi centrali, 1886.

[75] KÖLLIKER, Gewebelehre, 6te Aufl., ii., 809 ff.

[76] RAMON Y CAJAL, Studien über die Hirnrinde des Menschen, Heft i., 5 ff. Cf. also

FLECHSIG, Die Localisation der geistigen Vorgänge, 82 ff.

[77] RAMON Y CAJAL, Die Structur des chiasma opticum, 22 ff.

[78] VON MONAKOW, Gehirnpathologie, 450.

[79] OGLE, Medico-chirurgical Transactions, liv., 1871, 279.

[80] According to GRATIOLET, the frontal gyres develope more rapidly on the left than on the

right; in the occipital brain, the reverse order appears to obtain (Anatomie comparée

dusystèmenerveux, ii., 242). GRATIOLET'S results are, however, questioned by ECKER (Arch.

f. Anthroplogie, iii., 215). W. BRAUNE (Arch. f. Anatomie, 1891, 253) has also failed to find

confirmation of OGLE'S statement that the left hemisphere is, almost without exception, heavier

than the right. On the other hand, it is a fact easily verified that, in all primates, the fissures are

more asymmetrically arranged in the anterior than they are in the posterior part of the brain.

Moreover, the left frontal gyres, according to BROCA, are usually more complicated than the

right. These observations accord with those made by BROCA and P. BERT upon the

differences in temperature found in man at different parts of the head; the left half of the frontal

region is on the average warmer than the right, and the frontal region as a whole warmer than

the occipital (P. BERT, Soc. de biologie, 19 Janv., 1879).

Footnotes

[1] It should be said, with regard to Fig. 79, that the schema there given simply shows the

relative positions of the optic fibres in the nerve and optic tract, with reference to the various

parts of the retina, as they are to be inferred from the investigations of HENSCHEN (Brain,

1893). VIALET (Les centres de la vision, 1893) and others. The central arrangement of the

fibres corresponds to this peripheral schema only in so far as the bundles proceeding from the

macula lutea, find representation in the occipital cortex of both hemispheres. Cf. the account of

the whole matter in BECHTEREW, Die Leitungsbahnen im Gehirn und Rückenmark, 209 ff. On

the other hand, the definitive position of the lateral and median bundles in the cortical centres

is, as we show below (pp. 206, 235) for the most part the precise opposite of their position in

these proximate conduction paths; experiments on animals and pathological observations on

man prove that the lateral portions of the retina are represented on the same, the median on

the opposite side of the brain. In seeking to explain this fact, we must bear in mind that the

proximate termination of the optic paths lies in the mesencephalic centres of the genicula, and

that it is accordingly from this point that the final assignment of fibres to the cortex is made. In

view of the complexity of the relations involved, and of the somewhat ambiguous symptoms

which follow from injury to the cortex, many investigators, like HENSCHEN, and, to some

extent, VON MONAKOW, have recently taken the position that the connexion of the various

parts of the retina with the cortical centres at large has not yet been finally settled; and

HENSCHEN is further inclined to restrict the visual centre to a limited area within the calcarine

fissure (O' Fig. 65, p. 145), instead of allowing it the fairly extended region in the occipital cortex

(Fig. 89, p. 206) to which it is usually referred. VON MONAKOW, however, while admitting that

he is in doubt as regards the definitive correlation of retina and cortex, does not hesitate to

express his conviction that the central representation of the various parts of the retina is, in any

event, based upon their relation to the centres of ocular movement (Ergebnisse der Physiol., 1

Jahrg., 1902, 2 Abth., 600). This statement is, as the reader will see, in full agreement with

what is said below (pp. 229 ff.) of the theory of decussations in general, and of the

decussations of the optic paths in particular, as against the simple copy-theory of RAMON Y

CAJAL. -- Later note by AUTHOR.

[p. 241] CHAPTER VI

The Physiological Function of the Central Parts

§ 1. Methods of Functional Analysis

EVEN if we knew the course and the interconnexions of all the paths of nervous conduction,

there would still be one thing needful for an understanding of the physiological function of the

central parts: a knowledge of the influence exerted upon the processes of innervation by the

central substance. And there is but one possible way of determining this influence: we must

attempt to ascertain the function of the central parts by means of direct observation.

Under this limitation, two roads are open to the investigator who would gain an insight into the

complicated functions of the nerve centres. He can arrange the phenomena in order of their

physiological significance; or, accepting the lines of division drawn by the anatomists, he can

examine into the separate function of each individual central region. It is obvious that the former

of these procedures is to be preferred: not only because it lays the chief emphasis upon the

physiological point of view, but also because, when the investigation of the conduction paths is

over and done with, a doubt must still remain whether every one of the principal parts

distinguished by anatomy represents a similarly well-defined functional area. In the present

state of our knowledge, however, it is impossible to carry out the physiological programme with

any sort of completeness. The method can be applied, with any hope of success, only to the

two lowest central organs, myel and oblongata, where the phenomena may be referred without

exception to two basal physiological functions, reflex and automatic excitations: the latter

oftentimes deriving directly from nutritive influences exercised by the blood. We may well

suppose that these same two basal functions are the source of the physiological activities of

the higher central parts. At the same time, the interrelation of the phenomena is here so

complicated, and their interpretation in many cases so uncertain, that it seems wiser, for the

present, to examine each individual central area for itself with a view of discovering its

physiological properties. We shall, accordingly, preface our enquiry by a general discussion of

reflex and automatic action, in the course of which we shall have opportunity fully to consider

the functions of the lower central regions; and we shall [p. 242] then proceed to investigate the

brain and its parts in regular sequence from below upwards. We may, however, omit structures

which, like pons, crura and corona, are intended in the main simply for the conduction of

processes of innervation, and have therefore been sufficiently dealt with in the preceding

Chapter.

The methods employed in the, functional examination of the central organs are, in general, the

same as those which find application in the study of the conduction paths, save that anatomical

investigation, which there holds the first place, must naturally play a merely subordinate part

now that we are concerned with the activities of the organs. We shall ask assistance, where

possible in combination, from physiological experiment and from pathological observation; and

we shall pay attention, under both headings, to symptoms of stimulation and symptoms of

abrogation. The special conditions of the phenomena are such that stimulation experiments

must for the most part be employed in the general study of the reflexes and of automatic

excitations, whereas the functional analysis of the various departments of the brain must rely

almost exclusively upon symptoms of abrogation.

§ 2. Reflex Functions

(a) -- Spinal Reflexes

The simplest mode of central function, and the mode that still approximates most nearly to a

simple conduction of processes of stimulation, is the reflex movement. In so far as the reflex

process is a special form of conduction, we have discussed it in the preceding Chapter. But it is

more; it is a form of conduction modified in various ways by the influence of the central

substance. In the first place, the reflexes are not, like the processes of stimulation, conducted

in both directions in the nerve fibres, but only in the one direction, from sensory to motor path: a

fact which, as we explained above, is in all probability connected with the twofold mode of origin

of the nervous processes in the motor cells.[1] Secondly, the reflexes clearly show, in their

dependence upon the stimuli that release them, the effects of the peculiar conditions of

excitability obtaining in the grey substance. Stimuli that are weak and of brief duration fail, as a

rule, to evoke a reflex movement: but the movement, once it appears, may far surpass in

intensity and duration the direct muscular contraction set up by the same stimulus. Lastly, the

central character of these processes is evinced by the dependence of the reflex centres upon

other central areas with which they stand in connexion. Thus, it has been observed that the

reflex excitability of the myel is enhanced by removal of the brain.[p. 243] It appears, therefore,

that inhibitory influences are continually proceeding from the higher central organs, and lessen

the irritability of the lower lying reflex centres. These are, in general, still more strongly inhibited

if other sensory central parts, with which they are connected, are stimulated along with them.

The reflex, e.g., released by excitation of a sensory myelic root or of its peripheral radiation is

inhibited by simultaneous stimulation of the dorsal myelic columns, of quadrigemina and

thalami, of another sensory root, or finally of peripheral organs within which sensory nerves are

distributed. It is not improbable that the influence of the cerebral hemispheres belongs to the

same group of phenomena; for this too proceeds, in all likelihood, from the terminations of the

sensory conduction paths in the cortex. It has been observed that the inhibition of reflexes in

mammals is especially strong if stimuli are applied directly to the centrosensory areas of the

cerebral cortex.[2] The mechanism of reflex inhibition would thus appear to be the same

throughout: reflexes are inhibited, when the sensory cells that should transfer their excitation to

motor cells are simultaneously excited with a certain degree of intensity from other sensory

areas. This inhibitory effect is, however, limited to the condition that the areas whose

stimulations interfere lie at a sufficient distance from one another in space. If adjacent sensory

parts, or the nerve paths corresponding to them, are stimulated, the result resembles that of

summation of stimulations within the same sensory area: that is to say, the interference gives

rise not to inhibition but to intensification of the excitatory processes. Lastly, reflex excitations

may also suffer inhibition from central elements interpolated in their own proper path. This is

the interpretation of the fact that stimuli applied to the cutaneous radiations of the sensory

nerves are more effective than stimuli applied to the nerve trunks, and that contrariwise the

nerve roots become more irritable after their passage through the spinal ganglion. We must

suppose, that is, that the subdivision of fibres in the sense organ serves, on the one hand, to

increase irritability, and that, on the other, the excitation which arrives at a spinal ganglion cell

there undergoes a certain inhibition: a combination of circumstances which, naturally, brings it

about that the nerve trunk possesses a relative minimum of reflex excitability.[3] The general

lines to be followed, in an attempt to explain all these phenomena, are laid down by the

principles of nerve mechanics that govern the reciprocal relations of excitatory and inhibitory

effects, and by that morphological differentiation of the central elements which, in all probability,

runs parallel with them.[4][p. 244]

There are, further, many phenomena which show, still in accordance with the general principles

of nervous activity, that the individual reflex excitation aroused by a sensory stimulus does not

by any means come upon the scene as interrupting a state of absolute non-excitation in the

nervous elements. On the contrary, the state which we term the 'state of rest' is really a state of

oscillation -- as a rule, of oscillation about a certain position of equilibrium -- in which the

excitatory and inhibitory forces counteract one another. It is a state in which, on the average,

there is a slight preponderance of permanent excitation, though this may be transformed, under

special conditions, and more particularly under the influence of antagonistic effects, into a

preponderance of permanent inhibition. In this way, the single transient reflex process is

superinduced upon a reflex tonus, whose effects become apparent whenever there is

interruption of the sensory paths in which the permanent innervation of reflex excitation is

conducted. Thus, transsection of the sensory roots of an extremity is followed, in animals, by an

atonic, quasi-paralytic state, which however neither abrogates the influence of voluntary

impulses upon the atonic muscles nor prevents their action by way of concomitant movements.

[5] As regards intensity and distribution, these tonic reflex excitations appear, further, to stand

under the regulative influence of all the manifold conditions imposed upon the organs by the

nature of their functions. Reflex stimuli, which release transient reflex movements, may

accordingly produce radically different results, according to the state of the pre-existing tonus

and of the relative distribution of excitatory and inhibitory forces. Thus SHERRINGTON found,

in observations upon animals whose myel was cut through in the cervical region, that extensor

reflexes appeared if the leg was in the position of flexion, and flexor reflexes, if it was extended.

We may also appeal to this influence of the variable conditions of permanent tonic excitation

upon the individual reflex movement for explanation of the fact that the law of diffusion of

reflexes with increasing intensity of stimulus (discussed above, p 162) admits of exception; the

general and relatively constant conditions of reflex conduction are cut across by the more

variable influences arising from the reciprocal regulation of sensations and movements.[6]

(b) -- Metencephalic (Oblongata) and Mesencephalic Reflexes

The reflexes that have their seat in the oblongata are, in general, of a more complicated

character than the spinal reflexes. This organ is, in particular, the seat of a number of

compound reflexes, which play an important part in various physiological functions. We may

mention the [p. 245] movements of inspiration and expiration, with the closely related processes

of coughing, sneezing, vomiting; the muscular changes involved in the act of swallowing; the

mimetic movements; vascular innervation and the movements of the heart. Many of these

reflexes stand in an intimate relation of interdependence, as is indicated by the fact that their

peripheral paths are oftentimes laid down in the same nerve trunks. Some of the above-named

processes, such as the movements of respiration and the heart beat, result from a plurality of

causes, and therefore continue after interruption of the reflex paths; in such cases, the reflex is

but one of several determinants, and its influence is correspondingly restricted. Others, again,

like the movements of swallowing, appear to be pure reflexes; they are abrogated by

interruption of the sensory conduction to the reflex centre, even though the motor conduction to

the muscles governing the movement have been left intact. All these reflexes alike, however,

differ from the spinal reflexes on the point that, as a general rule, their sensory stimuli pass at

once to a large number of motor paths. Many of them are essentially bilateral, and do not

require the action of strong stimuli for their extension from the one to the other side of the body.

Thus the respiratory movements, which are released by excitation of the pulmonary radiation of

the tenth cranial nerve, involve motor roots that issue on both sides from the oblongata and

from the cervical and thoracic portions of the myel. These movements furnish, at the same

time, an illustration of a self-regulating reflex, which contains within it the impulse to continued

rhythmical repetition. While the collapse of the lungs in expiration serves reflexly to start the

movement of inspiration, their distention with inspiration serves conversely to excite the

muscles of expiration. If the reflex impulse given to the expiratory muscles in inspiration is too

weak to bring them into active exercise, it simply inhibits the antagonistic inspiratory muscles.

This is the case in ordinary quiet breathing, in which inspiration alone, and not expiration, is

connected with active muscular exertion. In the movements of swallowing, the regular

sequence is apparently maintained by a different mode of self-regulation. The act of swallowing

consists of movements of the larynx, pharynx and oesophagus: movements that succeed one

another in regular order, on the application of a stimulus to the mucous membrane of the soft

palate. The succession of movements in this instance is, perhaps, regulated in the way that

stimulation of the soft palate releases, first of all, simply the movement of the palatal muscles,

and that this in turn acts as a stimulus for the reflex elevation of the larynx and contraction of

the pharyngeal muscles. In fine, then, it is probable that all these oblongata reflexes, whose

detailed description belongs, of course, to physiology, are characterised by the combination of

movements for the attainment of definite effects, -- the manner of combination being

determined, oftentimes, by a [p. 246] mechanism of self-regulation, itself conditioned upon the

reciprocal relation of a number of reflex mechanisms. A second noteworthy property of these

reflexes is the following. The motor path of a given reflex movement sometimes stands in

connexion with a second sensory path, from which, accordingly, the same movement may be

aroused. Secondary sensory paths of this kind are connected, in particular, with the respiratory

centres, so that the combined activity of the muscles of respiration becomes available for other

purposes than those of inflation and emptying of the lungs. There is, e.g., a connexion of the

sensory nerves of larynx and oesophageal mucous membrane (i.e. of the superior and, in part,

of the inferior laryngeal nerves), and of the branches of the fifth cranial nerve distributed to the

nose, with the centre of expiration. Stimulation of these sensory areas produces, first, inhibition

of inspiration and then violent expiration. The latter is, however, preceded by a strong

inspiration, the immediate consequence of the establishment of inhibition, due to the

persistence of the influence of automatic excitation which we discuss below. Coughing and

sneezing are, accordingly, reflexes of expiration; but they are not excited from the sensory area

of pulmonary radiation of the vagus, from which the impulse to expiration ordinarily proceeds.

They are distinguished by the fact that stimulation of the nasal branches of the trigeminus

arouses not only the respiratory muscles but also the motor nerve of the face, the facialis, to

reflex activity. The sneezing reflex consequently affords a direct transition to the mimetic

reflexes of laughing, crying, sobbing, etc., in which again the muscles of the face unite in

conjoint function with the muscles of respiration.[7] Further: the secondary sensory path from

the expiratory centre to the mucous membrane of the air passages is paralleled by a similar

path from the inspiratory centre to the cutaneous investment of the body. We are thus able to

account for the movements of inspiration produced by intensive stimulation, especially cold

stimulation, of the cutaneous surface.

It may, then, be taken as a matter of common occurrence that in the oblongata a given motor

reflex path is connected with a number of different sensory paths. But more than this: one and

the same sensory path may enter into connexion, conversely, with several reflex centres, so

that its stimulation arouses coincidently various kinds of reflex movements. Here belong, e.g.,

the mimetic reflexes, mentioned above, in which movements of respiration are combined with

facial movements. A similar arrangement of connexions is in part responsible for the interaction

of respiratory movements and heart beat. The heart is supplied by two sorts of nerve paths,

which affect the sequence of beats in opposite ways: accelerating nerves,[p. 247] which

increase the rapidity of heart beat, and inhibitory nerves, which diminish it, or bring the organ to

a complete standstill. Both may be reflexly excited; but the centre for the accelerating fibres is

intimately connected with certain sensory paths that run to the heart in the spinal nerves for the

last cervical and first thoracic ganglion of the sympathetic; and the centre for the inhibitory

fibres with others, that take their course for the most part in the cardiac branches of the vagus.

Hence stimulation of the great majority of sensory nerves, and in particular of the cutaneous,

laryngeal and intestinal nerves, produces inhibition, and stimulation of the sensory fibres that

enter the muscles produces acceleration of the heart beat: this latter fact explains the increased

action of the heart that accompanies general muscular exertion. Similar results follow from the

movements of the lungs: their inflation accelerates, their collapse reduces the frequency of

heart beat. The respiratory movements are therefore regularly accompanied by fluctuations of

the pulse, whose rapidity increases in inspiration and decreases in expiration. On the whole,

that is, the movement of the blood is accelerated by enhancement of the movements of

respiration. Again, we find the same kind of interaction between the reflex connexions of

cardiac and vascular innervation. The vessels are governed, like the heart, by motor and

inhibitory nerves, both of which may be reflexly excited. Stimulation of most of the sensory

nerves releases the motor reflex, i.e., acts upon the nerve fibres which constrict the small

arterial blood vessels and thus produce an increase of blood pressure in the larger arteries, and

which are therefore termed presser fibres. The only exception to this action is in the vessels of

the part of the skin to which the stimulus is applied: these vessels usually dilate, either

immediately or after a brief stage of constriction, and thus occasion the hyperæmia and

redness of the stimulated parts. There are, however, various sensory areas which stand,

conversely, in direct reflex connexion with the inhibitory or depressor fibres of the blood

vessels, and whose stimulation leads accordingly to a widespread dilatation of the smaller

vessels. Here belong, in particular, certain fibres of the vagus, which radiate within the heart

itself and form its sensory nerve supply: fibres which, in all probability, are exclusively devoted

to this reflexly mediated interaction between cardiac and vascular innervation. For their

stimulation must be effected, in the normal course of physiological function, by increased action

of the heart; this, in turn, is produced by increase of blood pressure and of the amount of blood

contained in the arterial system; and this, once more, can be compensated only by a dilatation

of the small arteries, which permits the outflow of the blood into the veins, and thus at the same

time reduces the arterial blood pressure. We see, in fine, that all these reflexes of the

oblongata stand in relations of interdependence, such that the functions discharged by this

central organ mutually regulate and [p. 248] support one another. An intensive cold stimulus

applied to the surface of the skin produces, reflexly, a spasm of inspiration and an arrest of

heart beat. But the danger which thus threatens the life of the organism is avoided, since the

expansion of the lungs serves, again reflexly, to excite expiration and acceleration of cardiac

movement; while at the same time the stimulation of the skin brings about, by way of yet

another reflex, a constriction of the smaller arteries, and so prevents any excessive emptying of

the arrested heart. In many of these cases, as in a certain number of the reflexes proceeding

from the myel, the central transmissions have simply a regulatory significance. The peripheral

organs are the seat of direct innervation effects, due perhaps to special ganglion cells lying

within them, perhaps to the excitomotor properties of the muscle fibres themselves; and the

addition of the system of spinal and oblongata reflexes can do no more than modify these

effects by way of excitation or of inhibition.[8]

In all probability it is the nerve nidi of the oblongata, with their intercurrent central fibres, that we

must consider as the principal reflex centres of this organ. The complicated character of the

metencephalic reflexes appears to find its sufficient explanation in the anatomical conditions of

these nerve nidi. They are, upon the whole, more strictly isolated than are the centres of origin

of the spinal nerves. But, as an offset, certain nidi are closely connected by special central

fibres both with one another and with continuations of the myelic columns. These two facts,

taken together, explain the relative independence and singleness of aim of the oblongata

reflexes. Myelic fibres are involved in these reflexes to a very considerable extent; and it is

therefore probable that they are brought together, first of all, in some cinereal formation, and

only after leaving this enter into connexion with the nerve nidi to which they are assigned. Thus,

the respiratory motor fibres are, perhaps, collected in a special ganglionic nidus, which stands

in connexion with the nidus of the vagus nerve. We may fairly suppose that this significance

attaches to several of the grey masses scattered in the reticular substance. On the other hand,

it is not probable that movements so complicated as the mimetic movements, or the

movements of respiration and swallowing, possess each a single ganglionic nidus as their

special reflex centre. Apart from the fact that centres of this sort, for complicated reflexes, have

never been demonstrated, their existence is negatived by the nature of the movements

themselves. The respiratory movements, e.g., evidently require us to posit two reflex centres,

the one for inspiration, the other for expiration. Various mimetic reflexes, like laughing and

crying, can he much more easily explained on [p. 249] the assumption of a reflex connexion,

joining certain sensory paths at one and the same time with the respiratory centres and with

determinate parts of the nidus of the facialis, than on that of an especial auxiliary ganglion,

serving directly to initiate the above complex group of movements. In the same way, the

movements of swallowing must be derived, like the respiratory movements, from the principle

of self-regulation; we must suppose that the first movement of the entire process gives, as it is

made, the reflex stimulus to the second, this the stimulus to the next following, and so on.

Of the four 'specific' sensory stimuli, two only are concerned, to any great extent, in the arousal

of reflexes by way of sensory nerves: impressions of taste, and light stimuli. The former stand

in reflex relation to the mimetic movements of expression; to reflexes, i.e., some of which (as

we remarked above) readily combine with reflex respiratory movements, and thus lead us to

infer a close connexion of the corresponding reflex centres. Light stimuli regularly evoke a

twofold reflex response: first, closure of the eyelid, with a direction of the two eyes inward and

upward, and secondly contraction of the pupil. Both reflexes are bilateral, though with weak

excitations the movement is more pronounced upon the stimulated side. The reflexes released

by way of the auditory and olfactory nerves appear in the neighbourhood of the external sense

organs; if the stimuli are extensive, appropriate movements of the head may also be induced.

In man, the proximate auditory reflexes are for the most part confined to contractions of the

tenser tympani, which presumably accompany every sound stimulation: but in many animals

reflex movements of the external ear are clearly observable.

If the stimulus is extensive, or the degree of irritability unusually high, the sphere of reflex

activity may be extended beyond the limits of the direct reflex connexions. This phenomenon of

diffusion is more definite and uniform for the cranial than it is for the spinal nerves. In the case

of the optic nerve, e.g., the reflex to the muscles that move the eye-ball is connected, in

extensive stimulation, with contraction of the corresponding muscles for movement of the head;

and the facialis reflex to the orbicularis palpebrarum may be accompanied by concomitant

movements of the othes mimetic muscles of the face. Reflexes touched off from the gustatory

nerve fibres may cover a wider territory; they are apt to involve, not only the facial nerves, but

the vagus centre as well. Stimulation of the sensory nerves of respiration is confined, as a rule,

within the limits of its original reflex area. The strongest excitation of the central trunks of the

pulmonary branch of the vagus has no reflex effect beyond the tetanus of inspiration. The reflex

connexions of the expiratory fibres are more far-reaching. Stimulation of the sensory laryngeal

nerves, and especially of their peripheral ends, is [p. 250] likely to involve the muscles of the

face and of the upper extremity. But the fullest and most extended reflex relations are those of

the trigeminus, the largest of the sensory cranial nerves. Stimulation of the trigeminus affects,

first of all, its own motor root, which supplies the masseter muscles; and passes from this to the

nerves of the face, the respiratory nerves, and finally to the whole muscular system of the body.

There are two evident reasons for this range of reflex effect. First and generally, the trigeminus

controls the largest sensory surface of all the sensory nerves, so that its nerve nidi also occupy

a wide area, and opportunity is thus given for manifold connexions with motor centres of origin.

Secondly and particularly, the position of its nidi is favourable. The superior nidi are situated,

above the oblongata proper, in the pons; i.e. in the organ in which the ascending columns of

alba are grouped together, by the interpolation of cinerea, to form the various bundles of the

crus. We see, therefore, why it is that lesion of the oblongata and pons in the neighbourhood of

the nidi of the fifth cranial nerve is followed by general reflex spasms. The result need not, of

course, be attributed solely to these nidi: the stimulation in such cases may affect other sensory

roots of the oblongata as well.[9]

The reflex phenomena bear upon them the mark of purposiveness. As regards the oblongata

reflexes, this characteristic appears at once from the above description of their conditions and

of their orderly co-operation. But the spinal reflexes also show, for the most part, a certain

degree of the same quality. Thus, if a stimulus be applied to the skin, the animal makes a

movement of arm or leg that is obviously directed upon the removal of the stimulus. If the reflex

becomes stronger, the arm or leg of the opposite side will make a similar movement, or the

animal will jump away, apparently to escape the action of the stimulus. Only when the

movements take on a convulsive character, as they do with extremely intensive stimuli or in

states of unusual excitability, do they lose this expression of purposiveness. These facts have

suggested the question whether the reflexes may properly he regarded as mechanical

consequences of stimulation and of its diffusion in the central organ, or whether they are

actions of a psychical kind, and as such presuppose, like voluntary movements, a certain

amount of consciousness. Worded in this way, however, the question is evidently misleading.

There can be no doubt that the arrangements in the central organ can produce purposive

results with mechanical necessity; we have the same phenomenon in any perfected form of

self-regulating machinery. Moreover, the oblongata reflexes are highly purposive, and

nevertheless [p. 251] dependent upon definite mechanical conditions Again, there is no reason

whatever why a sensory stimulus should not release a reflex movement and arouse a

sensation or idea at one and the same time: so that we cannot take the absence of all

conscious process as the direct criterion of a reflex movement. On the other hand, the definition

of the reflex would, it is true, be indefinitely extended, and the term would cover practically the

whole range of organic movement, were we to apply it to any and every movement released in

the central organ by the action of sensory stimuli. Suppose, e.g., that I make a voluntary

movement, in order to grasp some object that I see before me. this, which is indubitably an act

of will, still falls under the general heading of a movement released by sensory stimulation. It

lacks, however, and conspicuously lacks, an attribute which is specifically characteristic of the

reflex; the attribute, indeed, that first gave rise to the distinction between reflex and voluntary

action, and without which the distinction loses all meaning. A movement mediated in the central

organ by way of response to sensory stimulation, if it is to be denominated a reflex movement,

may not bear upon it the marks of psychical causation; i.e., the idea aroused by the stimulus

may not constitute, for the agent's own consciousness, the motive to the external movement.

My involuntary reaction to a sensed stimulation of the skin is, therefore, a reflex, so long as the

sensation remains a mere accidental concomitant of the movement, so long, that is, as the

movement would be made in precisely the same way without such a concomitant sensation. On

the other hand, reaction is not a reflex, if I voluntarily put out my hand to seize the stimulating

object that is pressing upon the skin; for in this instance the movement is conditioned, for the

agent, upon the conscious process. In the individual case it may, naturally, be difficult to

decide, especially if the observations are made from the outside, whether a given movement is

or is not a reflex. But this practical difficulty does not justify our setting aside altogether the

criterion that distinguishes the reflexes from other forms of action, and leaving out of account

the fact that, while related by their purposiveness to psychically conditioned movements, they

differ from them, clearly and definitely, in the lack of conscious intermediaries. It is precisely this

criterion that makes the reflexes an easily distinguishable and characteristic class of organic

movements. We may also mention a further aspect of reflex action, closely connected with the

criterion just discussed, though naturally of less universal application: the fact that reflexes

follow immediately upon the operation of sensory stimuli, while psychically conditioned

movements admit of a longer or shorter interval between stimulus and movement. What holds

of this holds also of other objective characteristics, as e.g. that of the possibility of choice

between different means. Such criteria are not always applicable: partly, because [p. 252] the

characteristics do not attach at all generally to psychically mediated movements, but partly, too,

because the purposive nature of the reflexes leaves a certain amount of room for difference of

interpretation.

If we admit that these criteria are adequate to the empirical delimitation of the reflexes, as a

readily distinguishable group of organic movements, we must also accept the conclusion that

the central reflex area, in man and in the higher animals that resemble him, probably does not

extend higher up than the mesencephalic region. In all cases where a sensory stimulus is

conducted to the cerebral cortex, and there for the first time transformed into a motor impulse,

the central transference appears, without exception, to involve the interpolation of

psychophysical intermediaries; so that the action is presented to the agent's own

consciousness as psychically conditioned. Many authors, it is true, speak of 'cortical reflexes'

as of an established fact. But they are using the term reflex in a wider sense, in which any and

every movement that results from sensory stimulation is denominated a reflex, whether

psychical intermediaries are brought into play or not. From this point of view, the voluntary

action is sometimes defined as a 'cortical reflex.' It is clear that such an expression deprives the

word 'reflex' of all special significance. It is also clear that the retention of the term in its stricter

meaning is extremely important; for the origin of a class of movements that are at once purely

physiological and yet purposive in character is a real and distinct problem. We cannot, of

course, enter upon this question of origin at the present time; we can answer it only when we

come to examine the various forms of animal movements. We may, however, point out, in view

of the following discussion of the functions of the different central regions, that what holds of

man in this connexion does not necessarily hold of the animals. We may lay it down as a

general proposition that, in man, the centre at which the idea of the reflex gives way to the idea

of the psychically conditioned action is the cerebral cortex. But the law is not universally valid;

not even valid for all the vertebrates. It is a result of that progressive centralisation in the

ascending direction, of which we have spoken in the preceding Chapter, that the

mesencephalic areas which, in man, function simply and solely as reflex centres, appear in the

lower vertebrates still to be centres for psychically conditioned movements. Indeed, the facts

suggest that in the lowest vertebrates, where the cerebrum as a whole is of very minor

importance, even the oblongata and the myel may possibly, up to a certain point, mediate

movements of this psychical kind. Lower yet, in the invertebrates, they may proceed from any

one of the peripheral ganglia; and in the protozoa they evidently have their seat in the general

sensorimotor protoplasmic substance of the body. The centralisation of the psychical functions

in the brain, that is, goes pari passu with their decen-[p. 253]tralisation in the bodily organs; and

this decentralisation corresponds to an extension of the reflex functions. Hence, in the lowest

animals, all movements possess the character, not -- as is sometimes maintained in the

interest of certain ingrained dogmas -- of reflexes, but rather of psychically conditioned

movements.[10]

§ 3. Automatic Excitations

(a) -- Automatic Excitations in Myel and Oblongata

The phenomena of 'automatic function' are in so far parallel to the phenomena of reflex action

that they are processes of a purely physiological character, and accordingly have nothing in

common with processes which, like voluntary actions, recollections, etc., present themselves to

us in direct experience as 'psychically conditioned.' In this purely physiological sense, the

automatic functions are therefore nearly allied to the reflexes. But they differ from them in the

point that the automatic stimulation processes take their origin in the nerve centres themselves,

and are not released by a stimulus conducted to the centre from without. As a general rule, the

motor areas that evince reflex phenomena are also susceptible of automatic excitation. The

results of these automatic stimulations need not be muscular movements, or inhibitions of

particular movements, but may also take the form of sensations. Hence it is not always easy to

discriminate them from reflex excitations, or from the direct effects of external stimuli. For all

our senses are continually affected by weak stimuli, which have their ground in the structural

conditions of the sense organs themselves, and, so far as the sensory centres are concerned,

these weak excitations, such e.g. as are aroused by the pressure exerted in the eye upon the

retina, in the labyrinth of the ear upon the sensitive membranes, are, of course, the equivalent

of stimulation from the outside. If we rule out cases of this kind; it appears that the sole source

of automatic excitation is to be looked for in sudden changes in the chemical constitution of the

nervous substance, caused for the most part by alteration of the blood.

As regards the myel, the effects of automatic excitation are shown most clearly by the muscles

of certain organs of the nutritive system: e.g. the circular muscles of the blood vessels, whose

lumen becomes enlarged after transsection of the myel,[11] and the sphincter muscles of

bladder and intestine, where similar results have been observed.[12] The tonic excitations of

the skeletal muscles appear, on the other hand, to be exclusively reflex in character (cf. p. 93,

above), since transsection of the muscle nerves produces [p. 254] no change in muscular

tension, apart from the concomitant twitch and its elastic after-effects.[13] Automatic excitations

seem, however, to occur, alongside of reflex excitations, in the peripheral organs that are

separated from the central organs proper and provided with independent centres, e.g. in the

heart and intestinal muscles (cf. p. 248, above).

The automatic excitations that proceed from the oblongata are of especial importance. Here,

too, the reflex centres appear, without exception, to be automatic centres as well. The

movements that arise in them are consequently continued, after the sensory portion of the

reflex path has been interrupted. Here belong the movements of respiration and heart beat, and

the innervation of the blood vessels. All of these processes are connected with two centres,

distinct not only in function but also in locality: the respiratory movements with centres of

inspiration and expiration, the cardiac movements with centres for acceleration and inhibition of

the heart beat, the vascular innervation with centres for constriction and dilatation of the blood

vessels. Under such circumstances it seems to be the rule that the one centre acts reflexly

while the other combines automatic with reflex functions, or even gives the preference to

automatic stimuli: so the inspiratory centre in the case of respiratory movements, the centre for

inhibition of heart beat in that of cardiac movements, and the centre for vaso-constriction in that

of vascular innervation. It may be that the position of these nerve nidi, and the way in which

their blood supply is distributed, render them especially liable to automatic excitations. The

normal physiological stimulus to the production of such excitations is, in all probability, that

state of the blood which is induced by arrest of breathing or, indeed, by any circumstance that

prevents the elimination of the oxidised constituents of the tissues. The presence in the

dyspnoeic blood of oxidation products in general, whether of the final product of combustion,

carbonic acid, or of lower stages of oxidation as yet unnamed, appears accordingly to

constitute it a source of nervous stimulation. The accumulation of these materials excites the

inspiratory centre: an inspiration is made, which causes the lungs to distend and thus, in its

turn, serves reflexly to excite the centre of expiration (p. 245). This automatic stimulation

completes the circle of self-regulating functions, whereby the process of respiration is kept in

perpetual activity. The first impulse is given by the change in the constitution of the blood: this

acts as an internal stimulus to excite inspiration. The beginning once made, the further periodic

course of the whole process continues of its own accord. The expiratory reflex excited by

distention of the lung is followed, as the organ collapses, by the inspiratory reflex and at the

same time, in consequence of the renewed accumulation [p. 255] of products of oxidation, by

renewed automatic stimulation of the inspiratory centre.

We may suppose that the same changes in the composition of the blood condition the

automatic innervation of the inhibitory centre for the heart and of the presser centre for the

blood vessels. It is ordinarily assumed that the excitations in these two cases are not, as they

are in the case of respiration; subject to a rhythmical rise and fall, in consequence of the self-

regulation of the process of stimulation, but hold throughout to a uniform level of intensity. This

is inferred from the facts that severance of the inhibitory nerves of the heart, the vagus trunks,

produces a persistent acceleration of the heart beat, and that severance of the vascular nerves

effects a permanent dilatation of the small arteries. But these facts are not incompatible with

the theory that the automatic excitation in both cases oscillates between certain upper and

lower limits. There are, in reality, numerous phenomena that tell in favour of such a theory: e.g.,

the alternate constrictions and dilatations that may be observed in the arteries, and that usually

disappear after transsection of the nerves; or the connexion between rapidity of pulse and

respiration, a connexion which, as we have seen, depends in part upon the changes of volume

in the lung and is therefore explicable in reflex terms, but in part also suggests a different

origin, seeing that a long-continued arrest of breathing, whether it occur in the position of

inspiration or in that of expiration, arrests the heart as well. Moreover, in death by suffocation

we always find, besides intensive excitation of the inspiratory muscles, constriction of the blood-

vessels and inhibition of the heart beat. We may accordingly conjecture that the automatic

excitation of all these oblongata centres depends upon analogous changes in the constitution

of the blood. The observed differences may very well have their ground in the relations of the

peripheral nerve terminations; for the inspiratory centre stands in connexion with ordinary motor

nerves, whereas heart and blood vessels are characterised by the independence of their

peripheral innervations. The heart continues to pulsate, though with change of rhythm when

separated from all nerves whatsoever; and the vascular wall remains capable, under the same

conditions, of alternate constrictions and dilatations. The causes which determine these

peripheral excitations are, in all probability, similar to those which regulate the respiratory

innervation in the myel and, like the latter, are compounded of automatic and reflex processes

while the rhythmical function of the heart and the equilibrium between excitation and inhibition

in the vessels are also maintained by some self-regulative mechanism. That is to say, the

innervations of lungs, heart and blood vessels are, probably, in so far related to one another

that the automatic excitations from which they spring may be referred to one and the same

source of origin. The centres for these [p. 256] movements appear to offer especially

favourable conditions for the action of the internal stimuli; for no other central area reacts so

sensitively to fluctuations in the composition of the blood. In other quarters of the central

nervous system, we may suppose, the influence of the blood becomes effective only if the

blood supply has been modified from these centres of respiratory, cardiac and vascular

innervation, and the changes thus set up form a source of central stimulation. Thus, excitations

of the vascular centre, which inhibit the circulation of blood in the brain, are probably, in many

instances, the cause of general muscular convulsions. Under such circumstances, the external

symptoms are, for the most part, initiated in the pons; sometimes, perhaps, in a more anteriorly

situated motor brain-region.[14] The dyspnoeic blood may, however, occasion muscular

convulsions of the same kind, though less widely diffused, by stimulation of the myel.[15]

(b) -- Automatic Excitations in the Brain Cortex

Of the parts lying beyond the pons, the centrosensory and centromotor regions of the brain

cortex seem to be the principal centres from which, under the appropriate conditions, automatic

excitations may proceed. In their case, however, we are never in presence of purely automatic

processes, in the physiological sense defined above. The relations of the cerebral cortex to the

psychical functions are such that the automatic excitations are connected, in every instance,

with conscious processes, -- processes that may: in general, be subsumed under the rubric of

psychical association, and that refer us to psychophysical conditions of a very complicated kind.

Nevertheless, the part played by automatic stimulation is far from unimportant. It serves to

modify the excitability of the cerebral cortex; and the state of cortical excitability largely

determines the appearance and course of these psychophysical processes. Among its results,

we must mention, in the first place, those phenomena of stimulation that may almost be termed

the normal accompaniments of sleep. They show themselves usually, and oftentimes

exclusively, as sensory excitations. So arises the customary, sensory form of the dream, in

which automatic enhancement of excitability in the sensory centres produces -- always,

probably, under the influence of external sense stimuli -- ideas of hallucinatory character.

Sometimes motor excitations are also involved: muscular movements occur, ordinarily in the

mechanisms of speech, more rarely in the locomotor apparatus, and combine with the

phenomena of sensory excitation to form a more or less coherent series of ideas and actions.

In all these phenomena, sensory and motor alike, the automatic change of excitability is simply

[p. 257] the foundation, upon which the complex psychophysical conditions of the dream

consciousness and its outward manifestations are built up. The point of departure of these

central changes, which follow the oncoming of sleep, is again to be sought, most probably, in

the innervation centres of the oblongata. MOSSO has shown, by observation of cases in which

a portion of the skull had been removed, that at the moment of falling asleep the flow of blood

to the brain is reduced; and, further, that the supply may, in most instances, be temporarily

increased by the application of external sense stimuli, even if these are too weak to arouse the

sleeper.[16] The general reduction of the blood flow is, in all probability, the cause of the

marked diminution in the excitability of the brain centres, and of the corresponding obscuration

of consciousness, that characterise the approach of sleep. Very soon, however, this inhibition

of the central functions spreads still further, involving to a certain extent the centres of

respiration and heart beat; so that the phenomena of dyspnoea not infrequently make their

appearance during sleep. The enhanced excitability of particular central elements of the brain

cortex, vouched for by the phantasms of dreaming, may accordingly be ascribed to the direct

excitatory influence upon the cortex of the dyspnoeic modification of the blood. It is also

possible, in view of the reciprocal relations sustained by the various central areas, that

stimulations accidentally set up in a given region of the cortex will produce the more intensive

result, the greater the degree of latent excitation in the adjacent parts.[17]

Similar excitations of the cerebral cortex may occur in the waking state; but they are then

invariably the result of pathological changes. Here, again, investigation frequently refers us to

an abnormal state of the circulation as their ultimate condition. The abnormality may be of local

origin, proceeding from the vessels of the meninges or of the brain itself. Local lesions, in

particular, set up in the neighbourhood of the sensory centres, are ordinarily attended by

corresponding hallucinations. These, however, may also be due to general disturbances of

circulation, which appear sometimes as the consequence, sometimes as the cause of psychical

derangement;[18] for changes in the innervation of heart and vessels are frequently observed in

cases of mental disease.[19] Now all the chronic forms of insanity are connected with more or

less serious modifications of [p. 258] the brain cortex; and diffuse affections of the vascular

membrane with which the cortex is invested are the most frequent causes of acute psychical

disorder. But the phenomena of stimulation accompanying such disorder closely resemble

those that normally appear in sleep. They belong, as the latter also belong, partly to the

sensory, partly to the motor sphere. The sensory excitation manifests itself in sensations and

ideas of the different senses, oftentimes equal in intensity to those that can be caused by

external impressions, and therefore indistinguishable from them. These hallucinations are

accompanied by changes in the subjective sensations, muscular and organic, upon which the

affective disposition largely depends. Motor stimulations show themselves in the form of

imperative actions, which are likely to impress the observer by their unwonted energy. Here too,

however, as in dreams and dream movements, the enhancement of excitability due to

automatic stimulation is combined with further psychophysical processes, which are

responsible for the specific contents of the phenomena.[20]

§ 4. Functions of the Mesencephalon and Diencephalon

(a) -- Functions of the Mesencephalon and Diencephalon in the Lower Vertebrates

It is evident from mere inspection, and without recourse to histological methods, that the

mesencephalic and diencephalic region, which in man and the higher mammals, more

especially in the nearly related primates, cannot compare with the mass of the overarching

cerebral hemispheres, forms in the lower vertebrates the most highly developed part of the

central organ. Even in the birds and the lower mammals, where the prosencephalon has

already attained a considerable size, its relative development is still greater than that of the

superior parts (cf. Fig. 54, p. 128). These salient facts of the gross anatomy of the brain are

paralleled throughout by functional differences; so that it is far more dangerous in the case of

mesencephalon and diencephalon than it is in that of myel and oblongata to argue from

symptoms observed in the lower animals to the organisation of the higher, and in particular of

man. Yet another difficulty in the way of a functional analysis of this region, whether in the

animals or in man, lies in the circumstance that experimental interference and pathological

disturbance rarely affect a definite and definitely circumscribed area, but are apt to spread to

adjacent parts, -- experimental interference, more especially, involving the crural and coronal

fibres that pass upward below and between the thalami and quadrigemina. Hence most of the

results of the earlier [p. 259] experiments upon the transsection of these centres leave us

uncertain whether the motor derangements observed were really the consequence of the

destruction of the parts themselves, or not rather of the interruption of the neighbouring

conduction paths.[21] Indeed, the whole method was at fault. The symptoms of stimulation and

abrogation do good service in the investigation of conduction paths, and especially of their

beginnings in the myel and of their terminations in the cerebral cortex. But in the present case,

where the separation of the parts under examination from their surroundings presents extreme

difficulty, they can hardly be employed with any prospect of success. As the stimulus method is

here, for obvious reasons, practically out of the question, physiology has accordingly come

more and more to substitute for the direct an indirect form of the method of abrogation. Instead

of asking what functions remain intact after removal of the mesencephalic and diencephalic

centres which he has under investigation, the modern physiologist inverts the question, and

asks what functions are still left when all the prosencephalic parts that lie above and beyond

them have been cut away. He then makes a series of similar observations upon animals of the

same species in which the entire central organ has been removed with the exception of

oblongata and myel; and, by recording the difference of result in the two cases, is able to reach

a conclusion with regard to the functional significance of the intermediate central region. This

method was employed long since by FLOURENS upon birds, -- though employed, at first,

rather with a view to the determination of the importance of the prosencephalon itself, whose

extirpation it involved.[22] It was then applied, systematically, by GOLTZ, in work upon the frog;

[23] and has been used by CHRISTIANI [24] and, still more recently by GOLTZ [25] again for

mammals, and finally by J. STEINER [26] for vertebrates of all classes. It evidently guarantees

a somewhat more reliable result, if not for each individual centre included in the mesencephalic

and diencephalic region, at least for this region as a whole.

The observations taken on the lines here laid down prove that the functional importance of the

mesencephalic and diencephalic centres through-[p. 260] out the vertebrate series keeps

practically even pace with the development of the parts as revealed by gross anatomy. This

development is not uniform for the two regions: in the lower orders, the mesencephalon

(bigemina or optic lobes) has the preponderance, and the diencephalon (thalamus) is relatively

insignificant. Thus, in the entire class of the fishes, with the exception of Amphioxus lanceolatus

which stops short at the myel, the mesencephalon appears as the dominant central organ. So

long as it remains uninjured, the essential psychical functions are hardly modified. In particular,

the animals react quite normally to optical and tactual impressions, and move spontaneously

and appropriately. Smell alone is abrogated: the olfactory nerves are, naturally, removed with

extirpation of the prosencephalon: and the inception of nourishment, in so far as it is governed

by impressions of smell, may in consequence be more or less seriously deranged.[27] Passing

to the amphibia, we find at once a marked difference of behaviour in animals whose cerebrum

has been removed. One function is, unquestionably, retained by them, and must therefore

depend for its effectiveness upon the integrity of the mesencephalon: the function of

progression, and the regulation of co-ordinated movements of the whole body. The

decerebrised frog sits upright, like the uninjured animal; if made to change its place by the

action of cutaneous stimuli, it avoids obstacles laid in its path; and so on. It presents but a

single abnormality: that, at first, it neither moves nor takes food of its own accord.[28] At the

same time, its behaviour shows two noteworthy features. On the one hand, the functional

separation of mesencephalon and diencephalon is becoming clearer; on the other hand, we

observe the influence of practice upon the formation of new habits. If the diencephalon is intact,

the frog, as SCHRADER remarked, slowly recovers: it begins to catch flies again of its own

accord, and continues to improve until at last it is altogether indistinguishable from a normal

animal.[29] A bird deprived of the prosencephalon behaves in very much the same way. It, too,

as FLOURENS observed many years ago, at first remains motionless: it stands upright,

breathes regularly, swallows if it is fed, and reacts to stimuli by co-ordinated movements of

flight; but it makes no movements of its own initiative. Here again, however, there is a gradual

change of behaviour, if the animal is kept alive for any length of time: it makes restless

movements from side to side, avoids obstacles as it moves, and so forth.[30] CHRISTIANI, who

was the first to make observations on mammals, found that the rabbit, after removal of all the

parts of the brain anterior to the mesencephalon and diencephalon, is similarly capable of

reacting appropriately to light stimuli, of avoiding [p. 261] obstacles when stimulated to

movements of escape, and of occasionally executing what appear to be spontaneous

movements.[31] Finally, a still more thorough-going restoration of function was seen by

GOLTZ, in the case of dogs that he had kept alive for a considerable period of time after

complete extirpation of the cerebrum.[32] As usual, the animals were entirely passive in the

interval immediately following the operation: only the vegetative functions (heart beat,

breathing, movements of swallowing upon the introduction of food into the gullet) went on

without disturbance from the beginning. The progress of time brought with it, however, a much

more complete recovery of active function; and at last the animals moved about in an almost

normal manner, reacted to tactual stimuli by barking, got on their feet again if they had fallen

down, alternated between sleep and waking, and could be aroused from sleep by sound

stimuli. Smell had, it is true, been entirely abrogated with the extirpation of the olfactorius;

nevertheless, the dogs fed of their own accord when food was held against their muzzles. Bad-

tasting morsels they spat out again. On the other hand, there was never any expression of

pleasurable feeling, of attachment, and never any act that could be interpreted as a sign of

personal recognition. These were permanently lost.

From these results we must conclude that the mesencephalic and diencephalic region plays a

very considerable part in the whole vertebrate series up to the carnivores. It contains a group of

important central stations for the colligation of sense impressions with their appropriate

movements, -- stations which, like the reflex mechanisms of the myel, continue to function after

their severance from the higher central parts. But more than this: its integrity is the condition of

the integrity of the simpler psychical functions. The mental loss that the animal suffers by

operation is twofold. It loses, on the one hand, the functions connected with determinate

sensory nerves that are involved in the lesion caused by removal of the prosencephalon: so the

reactions to smell impressions. It loses, on the other hand, the functions which presuppose a

manifold connexion of present impressions with past experiences: so the recognition of

persons, the feelings of attraction and repulsion, of joy, etc. Some authors, it is true,

disregarding the results obtained from decerebrised dogs, and relying on observations made

upon anencephalic monsters, localised the feelings and emotions, in man as well as in animals,

in the mesencephalic and diencephalic region. But they are guilty of an obvious error in

reasoning. They ascribe the response to gustatory stimuli -- mimetic reflexes, which in these

pathological cases are left intact -- to concomitant feelings. It is, of course, no more allowable

to argue in this way than it would be to interpret any other reflex movement,[p. 262] on account

of its apparent purposiveness, as of necessity a conscious and voluntary action.

We see, then, that these middle brain regions are, for the animals in general, something more

than centres of complicated reflexes. In the light of the phenomena described above, we must

consider them also as centres for the simpler psychically conditioned functions. A more detailed

comparison shows, now, that as regards the time of their appearance these phenomena

present very striking differences. In the lowest vertebrates, the fishes, removal of the

prosencephalon produces no marked change of any kind in the psychical behaviour of the

animals. At a somewhat higher level, in amphibia, reptiles and birds, there is, at first, an

interruption of the psychical functions; and those that remain intact, since there is no trace of

any lasting after-effect and but very slight indication of adaptation to new conditions, might if

needs were be interpreted as complicated reflexes. It is, however, evident that in course of time

the animal makes a fairly complete functional recovery. The same picture, only with its lines

more strongly drawn, may stand, finally, for the mammal. The lapse of psychical functions after

the operation is here more pronounced; a longer period is required for recovery; the permanent

mental defect is more clearly observable. Nevertheless, the injury is compensated within wide

limits. If we are rightly to interpret these phenomena, we must of course remember that all the

functions which are permanently lost in the higher mammals -- recognition, expressions of

pleasure and attachment -- do not exist at all in the, lower animals. Now this gradual return of

the expressions of mental life, in animals endowed with a fairly well developed prosencephalon,

admits, if looked at simply by itself, of two explanations. It may be that the operation gives rise

to some sort of inhibitory effects, perhaps conditioned upon the injury to the parts, which must

be gradually overcome; or it may be that the uninjured remnant of the brain gradually takes on

a share of the functions discharged in the normal organism by the prosencephalon. According

as we incline to the one or the other of these interpretations, will our estimate of the functional

significance of the mesencenphalic and diencephalic regions vary. If we accept the former,

these parts will be responsible, throughout the vertebrate series up to the carnivores, for a very

considerable proportion of the functions of the brain at large; the animal's recovery will mean,

for them, simply a restoration of their original rights. If we accept the latter, their functional

activity after recovery will be abnormally increased, because partly vicarious. Now it cannot be

denied that there are many facts which tell in favour of the effect of operation, as at least a joint

factor in the general result. Radical interferences by operation, and especially interferences

with the central organs, are known to affect the functions for a certain period of time. Still, it is

[p. 263] hardly probable that the effect of operation is the determining factor in the case before

us. The contrast between the proximate effects of the loss and the subsequent state of the

animal is too striking and too uniform. Besides, there would be nothing to explain the

graduation of phenomena in the animal kingdom: the fact, e.g., that in the frog -- to say nothing

of the mammals -- a considerable period of time elapses before complete compensation is

observable, whereas in the fishes recovery sets in at once. And lastly, there are numerous

phenomena, drawn from all kinds of sources, which prone that injury or loss to the central parts,

whether in man or in the animals, may within very wide limits be offset by the vicarious

functioning of uninjured organs. We shall see presently that this law of functional substitution is

indispensable, if we are to explain the reciprocal relations of the various cerebral areas. It is,

then, only natural, in the absence of evidence to the contrary, that we should posit its validity in

the present instance for the interrelations of the various parts of the brain. We may add that the

general possibility of such vicarious function is inherent in the nature of the quadrigemina and

thalami, as intermediate stations upon the direct lines of conduction between the peripheral

organs and the cerebral cortex, -- stations in which all the sensory paths, and a large proportion

of the motor, are interrupted by the interrelation of neurone chains. Putting all these facts

together, we arrive at the following genetic conclusions, as a general point of view from which

the various phenomena observable in the vertebrate series may be classified and explained. At

the lowest stages of brain development, the mesencephalic and diencephalic region --

especially the former, since the diencephalon is as yet comparatively insignificant -- appears as

the principal central organ. Subordinate to this, on the one side, are oblongata and myel.

Adjoining it, on the other, as an appendicular structure, is the prosencephalon; originally, we

must suppose, an outcome of the separate development of the olfactorius. As we proceed

upwards through the vertebrate series, further representations of the conduction paths brought

together in the bigemina gradually make their appearance, as superior centres, in the

prosencephalon. In proportion as the latter advances, the mesencephalon and the

diencephalon -- the latter conditioned in its own development upon the formation of the

prosencephalon -- take on the function of intermediate centres, where excitations from the

periphery touch off complicated reflexes, and excitations from the prosencephalon evoke

reactions depending upon a more extended colligation of impressions. Nevertheless, the

possibility remains, up to the higher stages of development, that on the removal of the superior

regulatory mechanisms the lower centres may gradually recover some measure of the

autonomy of which at a lower level they were in complete possession. Hence it may well

happen that in the normal interplay of the central organs, -- so long, that is, as they stand [p.

264] under the dominance of the prosencephalic region, -- these parts of the brain may have no

other function than that of complex reflex centres; but that in the absence of the higher

regulatory organs they may once more assume the character of independent centres, whose

co-operation involves the appearance of psychical functions.

We conclude, therefore, that this part of the brain possesses, under all circumstances, the

importance of a centre whose office it is to bring into connexion the principal organs of sense

and of movement. Such a view of its functions is, upon the whole, confirmed by the symptoms

which ordinarily follow upon its direct removal or impairment. The most striking of these, upon

the sensory side, is the blindness which, in mammals, in accordance with the course of the

opticus paths, is correlated in particular with the pregemina, including the pregeniculum.

Disturbances of movement appear, on the other hand, provided that the lesion does not extend

beyond the quadrigemina, to be confined, at least in the mammals, to the muscles of the eyes;

the general muscular system of the body is unaffected.[33] If, however, the diencephalon is

injured, the general motor derangement is very pronounced. It consists, where the injury is

unilateral, in peculiar imperative movements, in which the animal, instead of going straight

forwards, turns round in a circle. These circus movements (Reitbahnbewegung, mouvement de

manège) are also observed after injury to other parts of the brain, more especially the crura and

the cerebellar hemispheres, and after unilateral extirpation of the semicircular canals of the

internal ear. In the lower vertebrates, e.g. the frog, the circus movements are invariably made

towards the uninjured side. In the invertebrates, too, the principal ganglia behave, in this matter

of motor disturbance and its direction, in precisely the same way as the mesencephalon of the

lower vertebrates.[34] In the mammals, on the other hand, the rule is that movement is directed

towards the injured side, if the anterior portion of the thalamus has been divided, towards the

uninjured side, if the section has been made in its posterior portion. Abnormalities have also

been observed in the tonicity of the muscles of the body, so that the animal when at rest is not

extended, but bends upon itself, the direction of curvature corresponding to that of the circus

movements.[35] These movements may take on various forms, according to the special

conditions of the injury: they may appear as rolling movements about the longitudinal axis of

the body, as 'clock hand' movements, or finally as circus movements proper. They are, we may

suppose, occasioned in all cases by an asymmetrical innervation, which however may itself be

due to a number of causes: to unilateral increase [p. 265] or decrease of motor innervation, or

to the asymmetrical release of reflex movements connected with disturbances of sensitivity.

Which of these conditions, or what combination of them, is actually at work cannot, at present,

be certainly determined.

The operation never fails to set up this motor derangement. In the higher mammals, we find,

further, symptoms of abrogation or diminution of cutaneous sensitivity upon the uninjured side

of the body. Such symptoms are always ambiguous, and the results are correspondingly

doubtful.[36] On the whole, however, if we look at the phenomena in their entirety, and take into

consideration at the same time the defects observed shortly after removal of the

prosencephalon and the known facts regarding the course of the conduction paths, we may

conclude that the mesencephalic and diencephalic region constitutes, in all the higher

vertebrates, an important intermediate station on the road front the deeper lying centres to the

prosencephalon; a station for the release, on the one hand, of compound reflexes, more

especially of reflexes to visual and auditory stimuli, in which the prosencephalon is not

concerned; and, on the other, of centrifugal excitations from the cerebrum, whose components

are here co-ordinated in such a way as best to subserve the needs of the organism. In view of

the anatomical relations and of the results of experiments with partial extirpation, it is probable

that the postgemina represent, in the main, intermediate stations for the acoustic area; the

pregemina and pregenicula similar stations for the sense of sight; and the thalami proper

stations for the extensive area of the sense of touch. We thus have, in this whole region, a

group of nodal points for the function of all the sense departments (with the exception of smell)

and of the movements correlated with them. It follows that the mesencephalon and

diencephalon, in proportion to their degree of development as compared with that of the

prosencephalon, are able between them, after the lapse of the prosencephalic functions, to

undertake in their own right the unitary regulation of the processes of the animal life; although

certain functions, conditioned exclusively upon the cerebrum, are of course permanently lost.

This physiological status implies a concomitant development of psychical processes: the

persistence of impressions of sense for a certain time after the cessation of stimulus, the

formation of complex perceptions, mediated by associative processes, and the conduct of

movements in accordance with impressions received in the more remote past. In other words,

centres that originally subserved reflex action and the transmission of impulses have, under

pressure of novel conditions, become transformed into independent centres of direction. They

are still centres [p. 266] of the second order; but their lesser functional value, as compared with

the higher centres whose substitutes they are, depends essentially upon the degree of

development attained by these higher centres themselves.

For a long time, the physiology of the mesencephalic and diencephalic region suffered from a

misconception. It was insistently held that the functions of these parts were not only analogous,

but in the main actually equivalent throughout the vertebrate kingdom; so that, in particular,

what held of the animals must also hold of man. The older method, of experiments with direct

abrogation, was not competent to remove this error. The necessary change of view has been

brought about, gradually, by extirpation experiments on the prosencephalon itself; experiments

which, as we remarked above, have really attained in this way a different purpose from that

upon which they were originally directed. Physiologists, from FLOURENS to GOLTZ, made

these experiments with the primary intention of deriving from the resulting symptoms of

abrogation a more exact knowledge of the function of the prosencephalon. But it became more

and more evident -- especially, as it happened, in the course of investigations pursued by

GOLTZ and his pupils -- that this direct end could be accomplished but very imperfectly, if at

all, on account of the direct and indirect consequences that follow the operations, and that

make the comparison of the injured with the normal animal anything rather than a problem in

simple subtraction. At the same time, it became evident that all such experiments yield most

important information regarding the functions of which the uninjured brain remnant is capable.

Extirpation experiments are, therefore, still valued by the modern investigator, but they are

valued for a different reason. They are not expected to reveal anything of moment concerning

the functions of the parts destroyed, but rather to illustrate the possible functions of those that

are left intact. It need, however, hardly be said, after the discussion in the text, that these

functions are not to be identified forthright with those discharged by the same parts in the

normal interplay of the organs. In this connexion, the differences that we find throughout the

animal kingdom, the very differences that were formerly overlooked, are of great significance.

First and foremost, the differences between the various vertebrate classes, but secondarily and

within certain limits -- despite the radically divergent position of the central organs, from the

genetic point of view -- the differences among the invertebrates as well, have in many instances

shed light upon the far more complicated conditions obtaining in the brains of the higher

mammals. The impulse to such comparison came in the first instance from morphology. On the

side of physiology, it is the especial merit of J. STEINER to have shown, by his experiments on

fishes and on the frog, supplemented by later work on reptiles and invertebrates, how

extremely variable is the role assigned to the mesencephalon in the vertebrate series. Setting

out from the spinal functions of Amphioxus, which, as we know, has no other central organ than

the myel, STEINER has, further, attempted a general theory of the mesencephalic functions at

large. But here, unfortunately, his foundation is uncertain, and the structure erected upon it still

less secure. In the annelids, he says, the individual metameres and the corresponding terms in

the series of ventral ganglia are all equivalent, so that any portion of the worm is, in its own

right, just as capable of movement and, apparently, of sensation, as is the whole animal.

Amphioxus is, now, to be regarded in the same way: its myel consists simply of a series of [p.

267] equivalent terms, not subordinated to any higher centre. Then, at the next stage,

represented by the primitive fish, the shark, and at stages of progressive advance, represented

by the other fishes, the myel is brought under the central superintendence of the

mesencephalon, which henceforth remains, throughout the vertebrate series, the real directing

centre: the prosencephalon is to be considered as merely supplementary. It is true that, in the

mammals, the prosencephalon attains a marked preponderance; nevertheless, the

mesencephalon and diencephalon contain the centres for the regulation of the whole system of

bodily movements, and therefore still hold the part of the central organ proper. In this sense,

STEINER defines a 'brain' as "the universal centre of movement, in connexion with the

functions of at least one of the higher sensory nerves." The criterion of an 'universal centre of

movement' consists, for him, in the occurrence of unilateral forced movements after injury to

the one side of the organ. If, then, there is no part of the central organs at which these circus

movements can be released, there cannot either be any unitary centre of direction within the

nervous system; the entire central organ must consist of a number of equivalent metameres.

[37] Now it is plain that the point of departure for all these theoretical considerations is

furnished by the annelids. The bodily segments of these animals appear, when divided off, to

represent independent vital units, every whit as capable of continued spontaneous movement

as was the original, uninjured worm. The annelids, moreover, do not execute circus movements

after removal of the dorsal ganglion of the one side. But the only inference that can be drawn

from the latter fact is, surely, this: that the occurrence of forced movements is, in all probability,

not an universal criterion of the presence of a directing centre from which lower centres are

controlled. And further: we have no right to assume the complete functional independence and

equivalence of all the terms in the series of ventral ganglia, unless the individual segments

move just as independently while they are still connected with the total annelid body as they do

after their separation. This, however, is not the case; in the uninjured animal all the metameres

move in exact co-ordination with one another. We must, therefore, conclude that the whole

chain of ganglia normally functions as an unitary system, of which, if we may judge from the

anatomical relations, the dorsal ganglion is the directive centre. In just the same way the myel

in Amphioxus is apparently controlled by its most anterior portion as in some sort the equivalent

of the brain of the craniota (see p. 252 above). The word 'brain' is, in the first instance, an

expression taken over by science from popular parlance, and on that account is, as ordinarily

employed, extremely difficult of definition. If we are, nevertheless, to make the attempt, and are

not to break with the general application of the term, we must say that the vertebrate brain is

not a separate central organ, but rather the complex of all those central organs which share in

the direction of the animal functions. In this sense, oblongata, mesencephalon and

diencephalon, prosencephalon and cerebellum have equal claims to the name. If, on the other

hand, we are to restrict the term 'brain' specifically to the central parts that are able in their own

right to maintain the animal life -- though perhaps on a reduced footing -- after the removal of

the rest, then the oblongata still has, at any rate, as good a claim as the mesencephalon and

diencephalon. The point is that the brain, taken as a whole, is not a centre, but a complex of

centres, and of centres so related to one another that, if one of [p. 268] them is lost, a portion of

its functions can, as a general rule, be taken over by another.

Experiments on completely decerebrised animals, especially the higher mammals, are of

extreme importance, not only for the functions of the mesencephalic and diencephalic region,

but also for the more general question of the functional representation of higher by lower

centres. We therefore append here a somewhat detailed account of the phenomena observed

by GOLTZ in the decerebrised dog that, of all operated on by him, longest survived the

operation.[38] The animal was deprived of its left hemisphere in two experiments, performed on

the 27th of June and the 13th of November, 1889; the entire right hemisphere was removed on

the 17th of June, 1890. It was killed, with a view to post mortem examination, on the 31st of

December, 1891. The general result of the autopsy was confirmatory; the cerebral hemispheres

had been completely done away with, in part directly by the operations, in part indirectly by

subsequent softening of the tissue. The animal had thus lived for more than eighteen months

after the final operation. Immediately afterwards, it had been entirely motionless; but the

capacity of spontaneous movement returned as early as the third day. The dog moved to and

fro in the room, and was able to avoid obstacles laid in its way, without having first run against

them. Placed on a smooth floor, it would slip up, but recover itself at once and of its own

accord. If its toes were forced into an unnatural position, it corrected the displacement

immediately as it began to walk, and stepped with the sole of the foot in the normal way. It lifted

its leg, without falling in, from a hole that had been prepared for the purpose of the experiment.

It once sustained an accidental injury to one hind paw, and thereafter, until the wound was

healed, held up the injured leg in walking, precisely as a normal dog would do. The sense of

touch was blunted; but the animal reacted to tactual stimuli of some intensity, though the

localisation of the point stimulated remained, it is true, fairly uncertain. If, e.g., the left hind foot

were seized, it would snap to the left, but generally in the air, without reaching the hand that

held it. The auditory sensitivity was also greatly reduced; nevertheless, the animal could be

aroused from sleep by intensive sound impressions. Gustatory stimuli were sensed. Meat

dipped in milk and held before its mouth was seized and chewed up; meat dipped in a solution

of quinine was taken, but spat out again, with wry movements of the mouth. The sense of smell

was, of course, entirely abrogated: the olfactory nerves had been destroyed in the operations.

At first, therefore, the dog took nourishment only when food was placed in its mouth. Later on, it

became accustomed to seize and gulp down bits of meat, and to drink milk, as soon as its

muzzle was brought in contact with them. It ate and drank of the solid and liquid food thus

offered until its appetite was satisfied; it would then lie down and go to sleep. The functions of

the sense of sight were shown -- in addition to the avoidance of obstacles, mentioned above --

in the reaction of the pupils to light stimuli. On the other hand, the animal was wholly insensitive

to threatening gestures and movements, and to other animals presented for its notice.

Consistently with this behaviour, it remained till its last day dull and apathetic. There was no

question of any real 'cognition' and 'recognition' of the objects about it. The only expressions of

feeling were snarling and biting when intensive stimulation was applied to the skin, and a

tendency to restlessness under the influence of hunger. Nevertheless, the avoidance of [p. 269]

obstacles shows an adaptation of movement to the varying conditions of sense impressions;

and the same fact is brought out still more clearly in the following experiment. Two long boards

were put together to form a blind passage-way, about twice as long as the animal itself, and so

narrow that it could not turn round. When the dog was introduced into this passage-way, it first

walked to the farther end, and ran against the wall. For some time, it reared up vainly against

this obstacle; but presently it began to back out, and finally, by this crablike movement, reached

the open. Of all experiments on decerebrised animals, this is, without doubt, the experiment

whose result seems to approach most nearly to what is termed an 'expression of intelligence.'

Nevertheless, it is plain that in this case, as in the others, the adaptation of the reacting

movements to the sensory stimuli are still confined within limits where it is out of place to speak

of any real 'reflection' -- of a choice between different possibilities. The symptoms themselves,

considered solely by themselves, might, naturally, be interpreted as voluntary actions. But it is

another question whether the whole context in which the phenomena appeared permits of such

an interpretation. And this question must, surely, be answered in the negative, for the same

reason that we decline, e.g., to ascribe the avoidance of obstacles to a true 'cognition' of the

objects, -- the cognition in this instance being disproved by other symptoms. If, however, we

rule out expressions of intelligence and voluntary actions, in the strict meanings of those terms,

this attitude must not, of course, be construed as a denial that the actions of the decerebrised

animal are, in part, conscious processes. On the contrary, it must be regarded as, at the least,

extremely probable that they may be interpreted as conscious and, in this sense, not merely as

purely mechanical reflexes. We cannot, however, enter upon this question with any fulness until

we come to our psychological discussion of the idea of 'consciousness' (cf. Part V., Ch. xviii.,

below).

(b) -- Functions of the Mesencephalon and Diencephalon in Man

In man, and indeed in all the other primates, who in this regard stand upon practically the same

level as man, the preponderance of the prosencephalon, which becomes the more marked the

higher we ascend in the vertebrate series, has reached a limit where the centres of mid brain

and 'tween brain retain least of their original relative independence. This statement is justified

both by the relations of the conduction paths and by the nature of the disturbances produced by

pathological defects. We cannot, it is true, -- and the reasons are obvious, -- expect to find

human cases that shall reproduce the conditions of total extirpation of the prosencephalon, with

permanent retention of function in the middle brain regions. But in cases of restricted lesion of

the quadrigemina and thalami, it would seem that a restitution of functions by way of vicarious

representation in co-ordinated or superior parts may occur very extensively in the human brain.

At the same time, the close connexion of the pregemina with the visual functions is evidenced

by the derangement of ocular movements that accompanies injury to these parts; while

disturbances of visual sensitivity in man appear, for the most part, only when the geniculum is

involved. In individual cases -- and the [p. 270] result accords with what we know of the course

of the conduction paths -- auditory disturbances have been observed after injury to the

postgemina. Lesions of the thalami, as might, again, be expected from the anatomical facts and

from the results of experiments on animals, are followed by anæsthesia or by motor

disturbances or by both combined. Sometimes, it is true, affections of the thalami run their

course without any sign of disturbance whatsoever:[39] a fact that testifies to the wide range of

vicarious functioning possible, in this particular instance, within the human brain, and that

constitutes a marked quantitative difference between man and the animals, in which the

phenomena of abrogation are much more intensive. A second and still more striking difference

is this: that the symptoms which in experiments on animals, from the fishes up to the mammals,

are set up with the greatest uniformity by unilateral lesions of this area -- the imperative circular

movements -- are represented in man, at the best, only by such reduced and vestigial forms as

a permanent deflection of the eyes or an unilateral execution of mimetic movements.[40] The

determining factors in this result are apparently two: on the one hand, the voluntary

suppression of the symptoms, and, on the other, the greater scope of the automatic regulations

and functional substitutions that, in the human brain, counteract the disturbances in question.

Both factors indicate that, while the basal functions of this region of the human brain

correspond to those discharged by the same region throughout the animal series, still its

relative importance, as compared with the superior centres, has now become less. Compound

reflex centres for the principal sense departments, sight, hearing and touch; and

comprehensive regulatory centres for the motor excitations issuing from the higher parts of the

brain: these the mesencephalon and diencephalon have remained. But other regulatory

mechanisms, and the independent processes of release within the prosencephalon, have

increased in importance alongside of them: so that their assumption in man of such psychical

function as has been observed to persist in the dog alter removal of the prosencephalic parts

can hardly be regarded as probable.

Striatum and lenticula belong, morphologically, to the prosencephalon (pp. 128 f.). But little is

known of their function. They appear, however, to be cortical areas, sunk into the substance of

the hemisphere, and specifically correlated with the mesencephalic and diencephalic ganglia.

This view is suggested by the extent of their fibre connexions, more especially with the thalami

(Fig. 74, p. 179). It is borne out, further, by the phenomena observed in experiments on

animals, and in cases of lesion in [p. 271] man, in which these structures are involved. The

phenomena consist always of paralytic symptoms or, when excitatory influences are at work, of

exaggeration of movement. Here again, however, and particularly in the case of man, the

phenomena of abrogation are most pronounced when the lesion has been rapidly produced:

slow growing tumours may, under certain circumstances, run their course without giving rise to

any symptom whatever. NOTHNAGEL found, further, that mechanical or chemical stimulation

of the striatum of the rabbit occasioned hurried running movements.[41] MAGENDIE observed

the same result after complete removal of the striatum.[42] Anæsthesia, on the contrary, does

not appear to be a consequence of injury to these structures.[43] For the rest, the intensive

disturbances that ordinarily follow upon sudden lesions of the striatum are not beyond

suspicion; they may be due to implication of the pyramidal paths ascending in the capsula to

the cerebral cortex. Besides these relations to the mid brain and 'tween brain, the anatomical

facts indicate a further connexion with the cerebellum. As a matter of fact, atrophy of the

striatum, and especially of the lenticula, has been observed in cases of congenital failure of the

cerebellum.[44]

§ 5. Functions of the Cerebellum

The functions of the cerebellum form one of the most obscure chapters in the physiology of the

central organs. The obscurity is intelligible, when we remember the extensive connexions of the

cerebellum with numerous other central arts, -- with the oblongata, with the mesencephalon

and diencephalon, and above all with the cerebral cortex. For, on the one hand, these

connexions make it difficult to determine whether destruction of the other brain centres involves

abrogation of the corresponding cerebellar functions. And, on the other, we are left equally in

doubt whether the disturbances observed in cases of lesion or defect of the cerebellum are not

due, in part at least, to the indirect implication of other parts of the brain with which it stands in

connexion. To these is added the further difficulty, that the cerebellar derangements appear to

be peculiarly easy of compensation by the enhancement or substitution of function in other

central parts. We have, therefore, as many reasons to overestimate as we have to

underestimate the importance of this organ; and our uncertainty is not a little increased by the

ambiguity of symptoms, which characterises all the phenomena of central abrogation, but is

especially marked in this particular case.[p. 272]

These symptoms themselves consist, for the most part, in motor disturbances. Complete

extirpation of the cerebellum in animals renders all movements vacillating and uncertain,

staggering or tremulous, though the influence of the will upon the individual muscle groups is

not destroyed. Transsection of various parts of the cerebellum, as well as of the cerebellar

peduncles, -- whose radiations are, for that matter, involved in all deep-going injuries to the

organ, -- is ordinarily followed by unilateral motor derangement. If the section passes through

the most anterior portion of the vermis, the animals fall forwards; in spontaneous movements,

the body is bent over anteriorly, always ready to fall and fall again. If it passes through the

posterior portion of the vermis, the body is bent backwards, and there is a tendency to

backward movements. If the one pileum is injured or removed, the animal falls towards the

opposite side, owing to unilateral contraction of the corresponding muscles; violent movements

of rotation about the long axis of the body are apt to follow. There occur also, at the moment of

operation, convulsive movements of the eyes, usually succeeded by a permanent detection.

These abrogation symptoms agree, upon the whole, with the phenomena of stimulation

observed with electrical excitation of various parts of the cerebellar cortex. Both alike are,

without exception, same-sided, in contradistinction to the consequences of cerebral injury,

which appear upon the opposite side of the body. The stimulation phenomena consist in

spasmodic movements of the head, the vertebral column, and the eyes.[45]

As regards man, clinical experience is in accord with the results of the observations on animals.

Motor disturbances are, again, the most constant symptom. They consist, chiefly, in an

uncertain and vacillating gait, sometimes also in similar movements of the head and eyes. The

arms appear to be less seriously involved; and it is but seldom that we observe, in man, those

violent rotatory movements that, in the animals, accompany unilateral lesions of the pilea or the

medipeduncles. For the rest, the motor disturbances in man are most intensive when the

vermis is the seat of injury; while affections of the pileum of either side, especially if the change

is merely local, may run their course without symptoms of any kind. Serious derangement

occurs, seemingly, only with complete functional disability of the pilea, or in the rare cases of

atrophy of the entire organ. Under such circumstances, however, the symptoms are not

confined to motor dis-[p. 273]turbances; they become exceedingly complicated, and

interpretation is correspondingly difficult.[46] Disturbances of cutaneous sensibility do not

appear to result from affections that remain limited to the cerebellum, not even from total

atrophy of the organ.[47] On the other hand, a characteristic subjective symptom, more

frequently connected with disease of the human cerebellum than with other central disorders, is

the dizziness that accompanies the motor disturbances. It is therefore probable that the attacks

of dizziness induced in the healthy subject by the passage of a strong galvanic current through

the occiput are due, in part at least, to its influence upon the cerebellum.[48] And for the same

reason we may suspect that this organ is involved in the dizziness produced by certain toxic

agencies.[49] Now there are, in general, two conditions under which the phenomena of

dizziness may be manifested: first, the functional derangement of certain peripheral sensory

apparatus, whose impressions mediate the arousal of sensations that generate the idea of the

static equilibrium of the body during rest and motion; and, secondly, such functional disorders

of central areas as are in any way calculated to alter the normal relation subsisting between

sense impressions and movements or ideas of movement. We shall presently become familiar

with a sensory apparatus of the former kind in the ampullæ and canals of the labyrinth of the

ear.[50] On the other hand, we appear to have in the cerebellum not the sole, but certainly the

most frequent central seat of symptoms of dizziness. When we remember how near together

are the labyrinth of the ear and this central organ, we can readily understand that the two forms

of disturbance of equilibrium are difficult to discriminate. Besides, we have every reason to

believe that they are functionally connected: the vestibular nerve, that supplies the vestibule

and canals with sensory fibres, sends a large number of representatives to the cerebellum.[51]

These relations to the vestibular division of the labyrinth are, perhaps, our best means of

accounting for the influence of the cerebellum [p. 274] upon bodily movements. We know that

all the other sense departments, and more especially those that mediate our spatial

apprehension of sensory impressions, the senses of sight and touch, find abundant

representation in it. And we find that where dizziness is set up by the action of definitely

demonstrable subjective or objective causes, these may ordinarily be traced back to one

general condition: disturbance of the normal correlation of sense impressions and bodily

movements. Again, however, this disturbance may, in the individual case, be brought about,

centrally and peripherally, in a great variety of ways. A man may be made dizzy by walking on

the ice, if he is not accustomed to it. The uncertainty of vision that goes with amblyopia or

strabismus, or that may he induced in a normal-sighted person by covering the one eye, is not

infrequently attended by dizziness. The symptoms are still more evident in the walking

movements of patients whose tactual sensations are dulled or destroyed by a degeneration of

the dorsal columns of the myel. In such cases, the resistance of the ground is not sensed in the

accustomed way: the patients lose their equilibrium; they stagger, and try to save themselves

from a fall by balancing with the arms.[52] These phenomena show, at the same time, the

indispensableness of the co-ordination of sense impression and movement for the correct

execution not only of involuntary, but also of voluntary movements. In the latter, too, it is as a

rule only the end to be attained that is clearly conscious; the means whereby this end is

reached are entrusted to the automatic working of a motor mechanism, where movement

interlocks with movement in the right order and to the right purpose. Each separate act in a

compound voluntary action reveals, accordingly a precise adaptation to the impressions that we

receive from our own body and from external objects. But since the voluntary action is directed

exclusively upon the end to he attained, the sense impressions that regulate the movements do

not, ordinarily, take any part in the idea of movement. Even the sudden lapse of the regulatory

impression is, in most instances, perceived only indirectly, by way of the consequent motor

disturbance and the subjective phenomena dependent upon it.

Disturbances of movement due to central causes may now, in general, be brought about in four

different ways. They may (1) be paralytic phenomena, i.e. they may be occasioned by a partial

abrogation of voluntary movements. They may (2) appear as purely anæsthesic symptoms.

They may (3) consist of disturbances of motor co-ordination. Or they may (4) result from

disturbance of the normal relation obtaining between sensations and the movements depending

upon them. The first of these possibilities is ruled out at once, since paralytic symptoms do not

occur after removal [p. 275] of the cerebellum or of separate parts of it; besides, dizziness is

never observed in the train of purely motor disabilities. The second seems to promise better.

Indeed, it has to a certain extent found acceptance; some authors have conjectured that the

cerebellum is an organ of what is termed the 'muscular sense.'[53] But this view can hardly be

reconciled with the fact that in cases of atrophy of the cerebellum in man, and after total

extirpation of the organ in animals, the capacity of active movements of locomotion is still

retained; the movements may be vacillating and uncertain, but they nevertheless allow us to

posit a certain degree of sensation in the locomotor muscles. The abrogation of other

sensations is equally out of the question. The third interpretation of the cerebellum, as centre of

motor co-ordination, was first put forward by FLOURENS,[54] whose views have held their own,

down to the most recent times, among physiologists and clinicians. But, first, this definition is

too indeterminate to characterise the specific form of co-ordination mediated by the cerebellum.

There is no single central motor area, from the myel upwards, that is not the seat of some sort

of motor co-ordination. Secondly, the phenomena of dizziness also tell against FLOURENS'

interpretation. They indicate that some kind of sensory disturbance is always involved along

with the motor. We are thus forced to the conclusion that the fourth of the above hypotheses is

the most probable: the hypothesis that inhibition of function in the cerebellum interferes with the

action of those sensory impressions that exercise a direct regulatory influence upon the motor

innervation proceeding from the cerebrum.

The acceptance of this hypothesis removes various difficulties. Thus, we can explain at once

how it comes about that the disturbances produced by lesions of the cerebellum resemble the

symptoms due to partial anæsthesia, and yet differ from them on the important point that

abrogation of sensations never makes its appearance among the cerebellar phenomena.

Where all conscious sensations persist, the only impressions that can be supposed to lapse are

those that act upon movement directly and without previous translation into conscious

sensations. Voluntary movements as such are as little affected as sensations; even after

complete destruction of the cerebellum, the will retains its right of control over each individual

muscle. This explains, again, how it is that the disturbances set up by injury to the cerebellum

may gradually be compensated. Compensation takes place in this way, that the movements are

regulated afresh by the conscious sensations that persist unimpaired. But a certain clumsiness

and uncertainty never disappear. It is evident, as one watches, that the [p. 276] movements

must always proceed from a sort of reflection. The immediacy and certainty of movement

shown by the uninjured animal are either lost or, if they may in some measure be regained,

must be acquired slowly and gradually, as the result of a long continued course of renewed

practice. Here too, therefore, the principle of the manifold representation of the bodily organs in

the brain is seen in opposition. The cerebellum appears to be intended for the direct regulation

of voluntary movements by sense impressions. If this hypothesis be correct, it will, accordingly,

be the central organ in which the bodily movements incited from the cerebrum are brought into

harmony with the position of the animal body in space. This conception agrees sufficiently well

with our anatomical knowledge of the course of the lines of conduction, incoming and outgoing.

In the postpeduncles the cerebellum receives a representation of the general sensory path,

reinforced, in all probability, by fibres from the optic nerve and the most anterior sensory cranial

nerves which run in the valvula and the prepeduncles. Its connexion anteriorly is effected by the

prepeduncles and medipeduncles, by which it is united partly to the anterior brain ganglia,

partly to the most diverse regions of the cerebral cortex.[55] Finally, the extensive

representations of the auditory nerve in the cerebellum (Fig. 77, p. 183) may be brought under

the same point of view. For if the cerebellum deflects at all that sensory secondary path whose

office it is to conduct impressions that influence voluntary movement directly, and not indirectly,

by way of conscious sensations, then we shall certainly expect to find that this same path

contains a representation of the eighth cranial nerve. The acusticus is precisely the sensory

nerve that gives certain objective sense impressions a specific relation to movement; our

movements adapt themselves involuntarily, in a corresponding rhythm, to rhythmical

impressions of sound.

The question of the functions of the cerebellum cannot be answered, at the present time, with

any degree of finality. The one point upon which physiologists are fairly unanimous is that this

organ is set off in relative independence, anatomically and functionally, from the other parts of

the central organ, and more especially from the cerebrum: so that no single function -- in

particular, therefore, neither sensation nor movement -- is wholly abrogated even after its

complete elimination, though profound derangements are produced in the co-ordinations of

function. But this very fact of relative independence, which in man and the higher animals must

be connected with a position of high functional importance, -- a position attested, in any case,

by the structure and volume [p. 277] of the organ -- renders the exact determination of the

nature of the 'co-ordinations' or 'regulations' effected by the cerebellum a matter of extreme

difficulty; and it is not altogether surprising that a good many of the physiologies are still

satisfied to stop short at these indefinite terms, -- terms that apply more or less to every central

organ, and are therefore tolerably non-committal in the particular case. This position has been

attacked, and rightly attacked, by LUCIANI. Aiming from the first at a definiteness of statement

that should match the preceding indefiniteness, LUCIANI undertook to analyse the phenomena,

so far as possible, along all their various lines, and thus to refer them to distinct groups of

symptoms. He has thus been led to distinguish three principal symptoms of abrogation, which

he regards as characteristic of cerebellar lesions: asthenia, atony, and astasia. The movements

lack their normal energy (asthenia); the tonus of the muscles is lowered (atony); and the

movements are uncertain and incoherent (astasia).[56] It has been objected, with some justice,

to this characterisation, that the symptoms which it discriminates are, in part at least, closely

interconnected: atonia and asthenia, e.g., always occur together.[57] But if the three terms are

considered simply as collective expressions for certain partial states, they may be accepted as

really denoting the essential features of the cerebellar symptoms. For the interpretation of the

phenomena, however, the emphasis must fall, without any question, upon that member of the

triad which is at once the most characteristic and also, unfortunately, the most complicated,

upon 'astasia.' LUCIANI seems here, in some measure, to have missed the true perspective;

he lays most weight upon the first two symptoms, -- which, no doubt, admit of a simpler

interpretation -- asthenia and atony. As a result of this mistake, he is inclined to regard the

cerebellum as primarily an apparatus for the production of nervous force, an 'auxiliary' or

'intensificatory system' for the whole cerebrospinal organ, which is not the seat of any specific

or peculiar functions, but reinforces the functional activity of the entire nervous system. In

support of this view, he adduces the trophic disturbances that appear, in course of time, more

especially after complete extirpation of the cerebellum, and that ordinarily take the form of

muscular atrophy, cutaneous inflammations, decubitus, etc. Now these disturbances, as well as

the striking lack of motor energy that perhaps stands in a certain relation to them, are

unquestionably very important symptoms. But the possibility still remains that the 'atony' and

'astasia' of movement are interconnected phenomena, in which a part is played by the influence

of sensory impressions. We saw, when we were discussing the myel, that the phenomena of

tonus are straitly conditioned upon the continued effect of such impressions (p. 93). And trophic

disturbances, of the kind observed after extirpation of the cerebellum, appear in all cases of

permanent derangement of innervation; they result from the disability of sensory as well as of

motor nerves; and they appear always to involve the co-operation of direct trophic influences,

exerted by the nerve centres, and of indirect, which have their source in the abrogation of

functions. LUCIANI lays special stress upon the fact, established by his observations, that dogs

whose cerebellum has been destroyed are still able, when thrown into the water, to make the

normal movements of swimming. But this experiment merely confirms, in a very complete way,

the fact that all cutaneous impressions can be sensed, and all locomotor movements voluntarily

[p. 278] performed, without assistance from the cerebellum. Swimming is precisely the form of

movement that may, under certain circumstances, bring into action a continuous voluntary

regulation, compensating any inco-ordinations that have arisen involuntarily, for the reason that

an intermission of movement means in its case the danger of drowning. The animal that

constantly staggers as it attempts to walk or run is, in swimming, compelled at every movement

to maintain itself above water by a maximal effort of will.

The view here taken of the cerebellar functions is in all essential points the same as that

developed by the author in the first edition of this work.[58] It finds striking confirmation in the

statements made by KAHLER and PICK, from the pathological standpoint, concerning the

relation of other forms of 'ataxia,' as it is termed, to the cerebellar symptoms.[59] HITZIG, too,

in his interpretation of cerebellar dizziness, seems to take up a very similar position.[60] In any

attempt at explanation of this symptom, and, indeed, of the abrogation phenomena at large,

especial attention must, in the author's opinion, be paid to the two facts brought out just now:

that, in the case of voluntary impulses proceeding from the cerebrum, the individual terms in

the series of purposive co-ordinations and regulations of the movements always succeed one

another, under ordinary conditions, in independence of the will, i.e. automatically; and that they

must always, on the other hand, take their direction from the sensory impressions received by

the organism.

The impressions conveyed to the central organs may, according to circumstances, be clearly or

obscurely conscious, -- may, in many instances, fail to come to consciousness at all. But, at

any rate, it is not in consciousness that they are transformed into the motor impulses whose

direction they determine. From this point of view we might, perhaps, characterise the

cerebellum outright as an auxiliary organ which relieves the cerebrum of a large number of

secondary functions: functions that were originally practised under the continuous control of the

will, and that in consequence can always be partially resumed by the cerebrum itself. As for the

first stage of practice, it may have occurred here, as in many other cases, either in the course

of the individual lifetime or in the previous life history of the species, which has left its

permanent traces, if anywhere, certainly in the organisation of the central parts. To ascribe to

the cerebellum itself any share in conscious functions, or to endow it, as some have done, with

a separate consciousness of the second order, a 'subconsciousness' is, as in the light of these

arguments it seems to the author, entirely unwarranted. For the fact before us is that the

cerebellum has developed into a centre of sensorimotor regulation, and that in the course of

this development the individual co-ordinations of the separate acts of movement with the

impressions of sense, all purposive and all subordinate to the ultimate end of the voluntary

action, have gradually been withdrawn from consciousness. And there is, upon the whole, only

one way in which this process can be envisaged we must suppose that, under the influence of

definitely directed cerebral innervations, there has developed a central mechanism, automatic

in unction, whose office it is to transmit the first, and only the first, discharging impulses to an

auxiliary centre; end that this auxiliary centre is endowed with self-regulating apparatus, again

[p. 279] automatic in function, which adapt each several movement to the sense impressions

coming in at the particular moment. These impressions may, of course, either come to

consciousness by the way or remain unconscious: the former, if the conditions favour their

special conduction to the sensory centre, the latter, if they are against it, or if the conduction is

somewhere inhibited: for the self-regulations as such the matter is indifferent. On the other

hand, it may very well happen, as a consequence of the direct conveyance of sensations to the

cerebrum and of its response to them, that disturbances in the cerebellar mechanism of the

sensorimotor self-regulations are presently compensated. Such compensation will, in particular,

always be possible where the lesions are simply partial, so that a new course of practice may

be entered upon and novel co-ordinations established. Where, on the contrary, the entire

cerebellum is thrown out, a large draft upon the cerebral functions will suffice to hold the

disturbances in check and so to mitigate the symptoms: but we can, it is true, expect nothing

more.

We suppose, then, that these self-regulations of the voluntary movements are in some way

mediated by the cerebellum. If, now, we are asked to give an account of them in detail, me

must reply that the question is very difficult to answer, all the more since there is still much

obscurity surrounding the directions and terminations of the conduction paths that meet within

the organ. The anatomical relations suggest, and we may accept the suggestion as a

provisional hypothesis, that the cerebellum, on the one hand, receives centripetal paths,

derived from every sensitive portion of the body, and, on the other, sends out intracentral (as

regards the organ itself, centrifugal) paths to every centromotor region of the cerebral cortex.

We may imagine, accordingly, that the sensory components functioning in a movement, more

especially sensations of touch and movement, are in the cerebellum united into a single

resultant; and that this is then conducted onwards to the cerebral cortex, and makes connexion

with the centromotor processes of discharge which are there in course. Thus, the regular

sequence of walking movements is at every stage dependent upon the condition that the

sensory impressions produced at each step by the movement itself are repeated in uniform

succession. Suppose, now, that such a rhythmical sequence is summated to form a resultant

which connects automatically with the voluntary impulses; and suppose that it remains

unchanged so long as its components persist without change, while it varies at once when and

as its components vary. We should then have, physiologically, a mechanism of self-regulation

which at one and the same time reinforces and relieves the centromotor functions of the

cerebral centres; and we should be able, psychologically, to explain by appeal to it the

automatic, unconscious character of these self-regulations of our movements, which still leaves

room for voluntary corrections and novel courses of practice.[61]

Over and above its influence upon the bodily movements, -- of whose reality there can be no

doubt, however various may be the interpretations put upon it, -- the cerebellum has at times

been accredited with functions of an entirely different order. Thus, the disturbances of

intelligence observed in cases where the organ is lacking, combined perhaps with the

anatomical fact that in the medipeduncles the cerebellum has extensive connexions with the

prosencephalon, has persuaded several authors to attribute to it a share in what are called [p.

280] the 'intellectual' functions. Apart, however, from these isolated observations, which may

very probably be explained by concomitant affections of other parts of the brain, the hypothesis

has no facts that are at all definite to support it. The view held by GALL and his pupils, that the

cerebellum stands in relation to the sexual functions, is hardly held by any physiologist at the

present day. The uncritical way in which GALL himself, and still more the phrenologists who

followed him, -- COMBE, for instance, -- heaped together quotations from older authors,

records of cases that had not been properly investigated, and observations in which the

suspicion of self-delusion forces itself irresistibly upon the reader, -- the whole forming a mass

of evidential matter that should be impressive solely by its bulk, -- would of itself forbid our

deviating any attention to their writings, even if we did not find upon every page the mark of

inveterate prepossession.[62] It should be mentioned, on the other side, that, now and again,

observers who cannot be accused of any similar prejudice, men like R. WAGNER [63] and

LUSSANA,[64] have regarded as possible this relation of the cerebellum to the sexual

functions; though their standpoint, in making this admission, has generally been that the

phrenological hypothesis cannot be certainly refuted. But this negative instance does not, of

course, furnish any valid argument; and the general uncertainty of our knowledge of the organ

necessarily implies that conjectures regarding its functions, of whatever nature they may be,

cannot easily be met by apodeictic proof of the contrary. This does not mean, however, that

they have become anything more than mere conjectures.[65] Moreover, the argument from the

indemonstrability of the opposite can be rebutted, in the present case, by a sufficient number of

positive instances, both experimental and pathological. LUCIANI was able entirely to extirpate

the cerebellum in dogs without producing a disturbance of the sexual impulse; in many cases

he observed an actual enhancement of the sexual phenomena.[66] The statistics of cerebellar

tumours in man have, also, failed to yield the slightest confirmation of the phrenologists' view.

[67] Finally, the symptomatology of cerebellar affections, so far as it is given objectively, on the

ground of observations, affords no hint of sexual reference.[68]

§ 6. Functions of the Cerebral Hemispheres

(a) -- Phenomena of Abrogation after Partial Destruction of the Prosencephalon

Our knowledge of the functions of mesencephalon and diencephalon is, it will be remembered,

mainly derived from observations of the psychophysical activities left intact after removal of the

prosencephalon (pp. 259 ff.). These same observations may, of course, be turned to account

for a [p. 281] theory of the functions of the cerebral hemispheres themselves. Indeed, the

results have, as a matter of fact, been applied more often to this than to the former purpose. It

is clear, however, that the positive judgment of the persistence of certain activities is more

definite and reliable than the negative judgment of their disappearance. Moreover, the

prosencephalon is, obviously, far more seriously affected than are the lower lying brain centres

by the indirect consequences of operation: whether these are the immediate disturbances

produced by the diffusion of excitatory or inhibitory effects, or phenomena of a more gradual

growth, changes wrought by compensation and substitution. In view of these facts, the

symptoms of abrogation observed with defects of certain parts of the cerebral hemispheres

cannot be accepted, without further examination, as the basis of inference regarding the

functions of the parts in question. Physiological experiment and pathological observation show,

both alike, that local lesions of the cerebrum are not necessarily followed by perceptible

alteration of functions. If any extensive portion of the tissue is removed the animals appear

heavy and stupid: but this change, too, disappears with time, very rapidly in the lower

vertebrates, gradually in the higher, and if some small remnant of the cerebrum has been left

uninjured may seemingly, as high up as the carnivores, give place to a complete restoration of

functional capacity. A pigeon, from whose brain considerable masses of the cerebral lobes

have been removed, is, after the lapse of days or weeks, indistinguishable from a normal

animal. In rabbits, and still more in dogs, the mental dullness and general motor inertia are

more evident than in birds. In man himself, textural changes of limited extent, if they are of

gradual growth, sometimes run their course without external symptom. More extensive injuries,

however, are, it is true, always accompanied in man by chronic disturbance of voluntary

movement, of sense, or of the psychical functions.[69]

These abrogation phenomena and their compensations are of peculiar interest when the

injuries from which they result are of definite character and considerable extent. Large portions

of the brain substance may be lost, and the animal, notwithstanding, make a complete

functional recovery. Thus GOLTZ found that dogs which he had deprived of the whole of one

cerebral hemisphere conducted themselves, some months after the operation, in very much the

same way as if they were normal animals.[70] There was a reduction of the cutaneous

sensitivity on the opposite side; and when the animal was free to choose between the

movements of its extremities, it preferred as a rule to use the muscles of the same side. The

per-[p. 281]ceptions of sight and hearing had also become uncertain, though they were by no

means destroyed.[71] GOLTZ afterwards made experiments, with like result, upon a monkey

(Rhesus), which was almost entirely deprived of one hemisphere.[72] Where a considerable

portion of both hemispheres is removed, the symptoms of disturbance are more acute, and at

the same time take a definite direction. Thus, dogs deprived of both frontal lobes gave marked

indications of motor derangement. The movements were awkward and clumsy, though the

capacity of movement was not abrogated. There was no change of sensation. Extirpation of the

two occipital lobes, on the other hand, produced disturbances of vision, which appeared,

however, to consist less in an abrogation of sensitivity to light than in a serious impairment of

the perceptual functions (cf. p. 196 above). In both cases, whether the frontal or the occipital

lobes were removed, the intelligence of the animals also seemed to be somewhat diminished,

though it was never wholly destroyed. As a general rule, emotive symptoms of like and dislike

were still manifested: in this respect, therefore, there is a radical difference between these

animals and the dog whose cerebrum was removed entire (cf. pp. 261, 268). At the same time,

there were signs of emotive disturbance, which varied characteristically according as the frontal

or occipital lobes were removed: in the former case, the animals appeared unusually irritable, --

a fact that may, perhaps, be brought into connexion with the coincident symptoms of

hyperæsthesia; in the latter, they became apathetic, probably in consequence of their partial

anæsthesia; quarrelsome dogs were rendered, to all appearance, good-tempered, though it is

true that they were also uninterested.[73]

The disturbances observed in man as the result of extensive cerebral deficiency appear, on the

whole, to resemble these set up by operation in animals. This is true, more especially, of cases

in which the one half of the cerebrum is wholly destroyed. Several instances are on record, in

the literature of pathology, in which this condition was induced by external injury or by changes

due to disease, and the patient nevertheless lived on for some period of time. Under such

circumstances, the opposite side of the body was, of course, completely paralysed, owing to

the decussation of the conduction paths. The intellectual functions, on the other hand, showed,

so the report declares, no noticeable alteration. The only points signalised are incapacity for

mental exertion, and an unusually rapid onset of mental fatigue.[74] Unfortunately, however, in

no one of these cases, which all belong to the older medical literature, was the patient

subjected to any [p. 283] accurate functional examination: so that we can draw from them

simply the general conclusion that there occurs in man a partial compensation of the

disturbances, similar to that observed in the corresponding experiments on animals. The

abrogation symptoms that appear when the anterior or posterior portion of the cerebrum is

wanting are also, it would seem, in agreement with the results of experiments on animals, both

in psychical regard and with respect to the derangement of motor and sensory capacity. At the

same time, the disturbances in this case are of a much more complicated kind, and their

psychological analysis is accordingly too defective to allow of any certainty of inference. All the

more prominent, on this account, is the position of certain territories of the cerebral cortex,

which are connected with definite psychophysical activities of a compound order. These we

shall discuss presently with some fullness: they are the only instances in which, in the present

state of our knowledge, detailed functional analysis is possible.[75]

All these observations, which refer to the consequence of more or less extensive cerebral

lesions, are, it is clear, of but comparatively slight importance for an appreciation of the

functions whether of the hemispheres as a whole or of particular regions of the cerebrum. Their

chief interest really lies in the evidence which they supply of the existence of very complete

arrangements for the compensation of the disturbances. We have seen that, in the lower

vertebrates, and even in many of the mammals, such compensation is rendered possible, after

removal of the entire prosencephalon, by the vicarious function of diencephalon and

mesencephalon, which thus become autonomous centres of greatly increased activity. Within

the cerebral hemispheres themselves, however, the possibility of substitution evidently goes

much further. Under favourable conditions, more or less complete adjustment may be made,

even in the human brain, to quite considerable defects. At the same time, we must accept the

corollary that the definitive phenomena of abrogation, in cases of partial lesion of the cerebral

hemispheres, may be turned to theoretical account only with the greatest reserve, and that our

study of the separate functional departments of the cerebrum must be based rather upon the

transitory than upon the chronic disturbances that follow in the train of cortical lesions. This

point will appear more clearly when we undertake the special analysis of the functions of vision

and speech.

(b) -- Phenomena of Abrogation after Total Loss of the Cerebral Hemispheres

The phenomena of abrogation that result from the loss of certain parts of the cerebrum are, as

we have seen, of doubtful significance; the in-[p. 284]fluences of compensation are

incalculable. On the other hand, the symptoms that follow upon total loss of the

prosencephalon have a definitive value, save only in the case of those animals in which this

defect also can be concealed by the vicarious functioning of mesencephalon and diencephalon.

Unfortunately, however, the effect of the operation is so extremely complex that here, again,

the symptoms of chronic abrogation admit only of very general and, therefore, indefinite

conclusions. In the first place, the psychophysical activities which continue after removal of the

prosencephalon do not afford a safe basis for judgment, since it remains uncertain whether and

how far they themselves owe their origin to some compensation of functions. But more than

this: it is exceedingly difficult, indeed, in many cases it is impossible, to distinguish between

complicated reflexes, that take place without any accompaniment of conscious sensations, and

reactions that occur at the incentive of sensations and sense perceptions. Hence the

investigator can never obtain more than a negative result. The functions that are permanently

abrogated by removal of the prosencephalon are, in all probability, conditioned exclusively

upon its integrity. But the activities that are temporarily deranged, and presently restored again,

must remain of doubtful significance, since there is no means of determining the extent of

possible compensations. Now we saw that birds, rabbits, and even dogs are not only capable,

in the decerebrised state, of purposive reaction to tactual and visual stimuli, but also adapt their

movements, like normal animals, to external impressions. They avoid obstacles, they recover

their equilibrium by balancing, etc.; nay more, they apparently execute spontaneous

movements, -- they run to and fro, seize and swallow the food that is offered them, and respond

by expressions of pain to intensive sensory stimuli. That is to say, they appear to be in full

possession of the sensory and motor functions. On the other hand, they give no signs of

intelligence, and never express joy or any other of the complex emotions. Moreover, their

spontaneous movements are more uniform and restricted than those of an uninjured animal

(See above, p. 262). In a word, these final abrogation phenomena lead us to the general, and,

we must also admit, indefinite conclusion that the intelligence, the higher affective processes,

and the compound voluntary actions are conditioned upon the integrity of the cerebral

hemispheres. We term this result indefinite, first, because the psychological terms that enter

into the functional determination require a more exact psychological definition before any

precise meaning can he attached to them, and secondly because it is clear, even without any

such definition, that an absolute delimitation of the intelligence and the complex affective and

volitional processes, as contrasted with the lower processes of the same kind that may possibly

continue after the removal of the prosencephalon, is a matter of extreme difficulty. In any event,

however, the [p. 285] distinction, so far as it is practicable at all, must also be left over for the

detailed psychological analysis of the processes involved.[76]

The general conclusion to be drawn from the abrogation phenomena, that the physiological

function of the cerebral hemispheres stands in intimate relation to the intellectual activities and

to the complex affective and volitional processes, is, upon the whole, confirmed by the results

of comparative anatomy, evolutionary biology and anthropology. Comparative anatomy shows

that the mass of the cerebral lobes, and more especially their superficial ridging by fissures and

gyres, increase with increasing

intelligence of the animal. This law is, however, limited by the condition that both factors, mass

and superficial folding, depend primarily upon the size of the body. In the largest animals the,

hemispheres are absolutely, in the smallest, relatively larger, i.e., larger as compared with the

weight of the whole body; and the ridging, as is natural from the relative decrease of surface

with increasing volume of an organ, increases with the size of the brain: in all very large

animals, therefore, the brain shows an abundance of fissures.[77] The physical organisation is

another factor of great [p. 286] importance. Of the terrestrial mammals, the insectivores have

the least convoluted, the herbivores the most richly convoluted brain, and the carnivores stand

midway; the marine mammals, although they are carnivorous, surpass the herbivores in the

number of their brain gyres. It thus comes about that there are, after all, only two connexions in

which the law quoted above has any claim to validity. It is valid if we take the widest possible

comparative survey of the development of the brain in the vertebrate kingdom; and it is valid if

we confine our comparison within the narrowest possible limits, and look at animals of related

organisation and similar bodily size. Only in the latter case, again, can the result properly be

termed striking. If we compare, e.g., the brains of different breeds of dogs, or the brains of man

and of the man-like apes, there can be no doubt that the more intelligent breeds or species

possess the larger and more highly convoluted hemispheres. By far the most significant of

these differences is that between man and the other primates The average brain weight of

males of Teutonic descent between the ages of thirty and forty may be set at 1,424 grammes,

and that of females of the same race and age at 1,273 grammes; the brain of a full-grown

orang-utan amounted, e. g., only to 79.7 grammes. Still greater is the discrepancy if we

consider, not the weight of the brain, but its superficial development, conditioned upon the

number of gyres. Thus H. WAGNER gives for man a surface of 2,106 to 1,877, for an orang-

utan a surface of 535.5 sq. cm.[78] the lower races of man the brain has also been found, as a

rule, to be both smaller and less convoluted.[79] And numerous observations go to show that,

within the same race and nationality, eminently gifted individuals possess large and richly

convoluted hemispheres.[80] Figg. 100 and 101 illustrate this point in two especially striking

cases. Fig. 100 is the dorsal view of the brain of a simple artisan of moderate, but not

subnormal mental capacity; Fig. 101 is the corresponding view of the brain of the famous

mathematician, C. Fr. GAUSS.[81][p. 287]

(d) -- The Hypotheses of Localisation and their Opponents. The Old and the New Phrenologies

These obvious differences in the superficial configuration of the cerebral hemispheres naturally

suggest the hypothesis that the general connexion between brain development and mental

endowment, which appears in them, is paralleled by specific relations between the relative

development of various parts of the brain surface and definite directions of mental capacity.

This hypothesis, which in itself is entirely justified, forms the point of departure of the system of

'phrenology' founded by FRANZ JOSEPH GALL. Unfortunately, however, the physiological and

psychological premisses upon which GALL worked out his ideas are untenable, and the

observations themselves and the conclusions drawn from them betray lack of accuracy and

scientific caution. GALL regarded the mental functions as the business of a number of internal

senses, to each of which, on the analogy of the external senses, be attributed a special organ.

Nearly all of these internal sense organs he localised on the outer surface of the brain,

assuming a parallelism of skull-form and brain-form which, as can easily be demonstrated,

does not obtain, at any rate to the extent required. GALL distinguished twenty seven 'internal

senses,' in naming which he makes use at need of the expressions sense, instinct, talent, and

even memory: we find, e.g., sense of place, sense of language, sense of colour, instinct of

propagation, instinct of self-defence, poetic talent, esprit caustique, esprit métaphysique,

memory of things, memory of words, sense of facts, sense of comparison, etc. It is useless to

repeat the statements of the phrenologists regarding these localisations. It may, however, be

mentioned that in one case -- and the fact shows that he possessed some gift of observation --

GALL made a lucky hit: he localised his 'sense of language' in a region of the cerebral cortex

approximately corresponding to the area whose lesions, as we shall see later on, have been

proved in modern times to constitute the most frequent cause of the syndrome of 'aphasia.'

Indeed, the discovery of the seat of aphasia is directly traceable to GALL'S suggestion, as has

been expressly acknowledged by BOUILLAUD, to whom it is due.[82] At the same time, we

must not forget that even in this instance, where a pronouncement of GALL'S has received a

certain measure of confirmation from the facts, there is really an essential difference between

what was actually discovered, viz., the anatomical seat of central derangements of speech, and

the phrenological 'organ of language.' The two can, in truth, be identified only if we force a

phrenological interpretation upon the phenomena of [p. 288] aphasia, which in the light of actual

analysis they will not fear. Cf. below, § 7b.

When physiology first took the field against the phrenological doctrine of localisation, it was

itself but poorly armed for the combat. It was inclined to lay a disproportionate weight upon the

indefinite or equivocal results of extirpation experiments on animals. It was easily influenced by

false analogies, and did not hesitate to accept psychological theories that at bottom were no

less questionable than the 'internal senses' of the phrenologists. Thus FLOURENS, whose

views long held undisputed sway in physiology, insisted strenuously upon the unity and

indivisibility of the cerebral functions, and argued from it that their organ must be similarly

indivisible. This opinion was largely determined by the analogy of other, unitarily functioning

organs. Cerebellum, oblongata and myel were each endowed by FLOURENS with an

independent and specific function, discharged by the organ as a whole. In the same way, the

mass of the cerebral hemispheres stood for him upon a single physiological level, like the

substance of a secreting gland, e.g. the kidney. He found confirmation of this view in the

observations on the results of partial and total extirpation of the prosencephalon in animals that

we have discussed above: for these experiments show, in general, that partial removal of the

cerebral lobes simply weakens the mental functions, as a whole, and does not, as on the

hypothesis of a localisation of functions might be expected, abrogate certain activities and leave

the rest unimpaired; while total extirpation of the cerebrum completely destroys all spontaneous

expressions of the mental life, i.e., as FLOURENS phrased it, all 'intelligence and will.'[83]

This doctrine, of the specifically psychical function of the prosencephalon and of the indivisibility

of that function, presently became untenable. It was overthrown partly by the pathological

observations on the consequences of local lesion in man, partly by the growth of knowledge

regarding the structure of the brain and the course of the conduction paths. Its place was taken

by the modern theories of localisation, which to a certain extent attempt a rapprochement with

the doctrine of phrenology. At the same time, they mark a twofold advance beyond GALL and

his disciples. First, on the side of physiology, GALL'S 'mental organs' are replaced by the idea

of the separate 'centres,' correlated with peripheral spheres of function that are, upon the

whole, definite and clearly distinguishable -- the sense organs, the various muscular territories,

etc. This change of view is evidently a reactive effect of the advance in knowledge of the

anatomy of the conduction paths. Secondly, on the side of psychology, such monstrous terms

as sense of facts, reverence, philoprogenitiveness, or -- to take instance of another sort --

sense of language, poetic talent, etc., are banished from [p. 289] the list of localisations, and

the terms 'sensation and idea,' terms which, as the framers of the theories believed, represent

the two fundamental forms of psychical process, and retained in their stead. 'Sensation' means,

in this connexion, any conscious reaction evoked by external sensory stimuli, while 'idea'

includes, in accordance with the nomenclature of the older psychology, all kinds of 'memory

image.' >From these general premisses, the doctrine of localisation or, as its close relations to

the older phrenological theories justify us in calling it, 'modern phrenology' has developed in

two directions. In both forms, it is based upon the assumption that the cerebral cortex is divided

up into a number of sensory centres, in which the excitations brought in along the sensory

conduction paths release the specific sensations. The centromotor regions are counted among

these sensory centres, on the further assumption that a voluntary action may be adequately

defined as the connexion of some reflex, arising either in lower centres or in the cortex itself,

with a concomitant sensation of touch or movement. As we leave this common ground of

general theory, however, opinions begin to diverge. The one form of the doctrine of localisation

asserts that the centres of sensation and idea are strictly connected, so that every sensory

centre covers both processes, and the entire cortical surface is therefore essentially composed

simply of a number of adjacent sensory centres. The distinction of sensational and ideational

functions within each centre is then conditioned solely upon the action of determinate --

functionally, not morphologically discriminable -- elements. It thus becomes necessary to posit

the existence of two sorts of cortical cells: sensation cells and idea cells. The former are

supposed to receive peripheral excitations by direct conduction; the latter take up excitations

proceeding from the sensation cells, and thereby acquire the capacity of renewing these

excitations, -- a process that, for brevity's sake, has also been termed the 'deposition' of ideas

in the specific memory cells. This form of localisation theory, which we may call the 'pure

sense-centre theory,' was first worked out by MEYNERT on a morphological basis, and has

since been employed by H. MUNK for the interpretation of his experiments on animals. It is

widely current at the present day, both in physiology and in pathology,. The second form of the

localisation doctrine differs from the first mainly by its assertion that the central areas which

subserve the colligation of sensations, and therefore also the retention of ideas, the centres, in

fact, which underlie the complex psychical functions at large, are spatially separate from the

sensory centres, though connected with them by manifold systems of association fibres. It

accordingly gives this second order of centres, whose office it is to colligate the various sense

departments, the special name of 'association centres'; and we may therefore designate the

second form of the localisation theory, briefly, as the 'theory of association centres.' According

to this view, [p. 290] the essential activity of the cerebrum consists in the function of the

association centres, while the sensory centres serve, on the whole, simply to take up the sense

impressions, in the order in which they affect the peripheral organs, and to raise them to

consciousness by their projection upon determinate cerebral surfaces. The expression

'association centre,' is used, in this connexion, both in a physiological and in a psychological

sense: physiologically, it is the 'association fibres,' characteristic of these centres as such, that

are connected only indirectly, viz., by way of the sensory centres with which they are correlated,

with the periphery of the body; psychologically, these association centres are looked upon as

the substrate of the associative processes upon which, as the psychology of association

teaches, and as our theorists believe, all the higher psychical functions depend. This second

form of the localisation doctrine is, unquestionably, superior to the first, in that it leaves a

somewhat freer scope to hypotheses of, the origin of the more complex psychical processes;

the schematic antithesis of sensation cells and idea cells, which naturally leads to a

corresponding and correspondingly untenable classification of the psychical processes

themselves, is replaced by the broader antithesis of direct sensory excitations and associations.

At the same time, however, the idea of association and of the association centre is left very

indefinite. It is, in the last resort, bound down to the belief that cortical areas which stand in

exclusive connexion with association fibre systems are the vehicle of the more complex

psychical activities; so that, from the functional point of view, the expression 'psychical centres'

would really be the more correct. But such a term shows very plainly that the second form of

the localisation doctrine brings us perilously near, once more, to the doctrines of the older

phrenology; and that, if it does not run altogether on the old lines, this is principally due to the

praiseworthy caution of its representatives, who have so far refrained from correlating the

different cortical areas of the association theory with complex psychical activities or

endowments of a definite kind.

These modern hypotheses of localisation, like the old-time phrenology of GALL, have not been

allowed to pass unchallenged from the side of physiological observation. GOLTZ and his pupils,

in particular, have disputed the strict localisation of the psychical functions, on the ground of the

defects observed after partial extirpations of the cortex. In opposition to the theory of sharply

circumscribed sensory centres, these investigators insist upon the complex character of the

disturbances following from local lesions, and upon the general reduction of the intellectual

functions after partial removal of the cerebral lobes. They thus come back to a view which

resembles that of FLOURENS: they emphasise the necessity of the co-operation of the

different cortical regions in the psychical functions, though they have given up the hypothesis of

the functional equivalence of all the parts of the cere-[p. 291]brum, as untenable in the present

state of our anatomical and physiological knowledge.

If, now, we attempt an appreciation of these different theories, standing opposed to one another

in the doctrine of the cerebral functions, we must, of course, try to do justice to all the

departments of experience that our judgment involves: to anatomy and pathology, that is, not

less than to physiology and psychology: and we must be the more careful, since the quarrels

between the older and newer localisation theories and their opponents have evidently been

due, in no small measure, to the all too common tendency to rely, exclusively and onesidedly,

either upon the anatomical facts or upon the results of experiments with animals. In both

events, we may add, psychology has usually been regarded as an unclaimed territory, with

which either side might deal as seemed best to it. We will ourselves, therefore, begin by setting

psychology in the foreground, though we shall, as was said just now, attempt an impartial

treatment of the other sciences also. >From the psychological point of view, then, the sense-

centre theory must be pronounced untenable; neither of its constituent hypotheses can be

accepted. In the first place, the 'sensory centres,' as is clear from the results of psychophysical

analysis of the functions of perception and from pathological observations on man, are not

simple repetitions of the peripheral sensory surfaces, but are in the strictest sense of the word

'centres,' that is, areas in which the different peripheral functions concerned in the activities of

sense are centralised. Thus, the visual centres bring together the functions of visual sensation,

of energy and synergy of movement in the visual organs, of the relations of these processes to

the visual reflexes that run their course in lower centres, and so on. The sensory centres would

not be centres at all, but superfluous duplications of the peripheral organs, if they possessed no

other significance than that of repeating the excitations touched off at the periphery. In this

regard the view embodied in the sense-centre theory is the result partly of an inadequate and

prejudiced reading of the anatomy of the conduction paths, partly of a wrong interpretation of

the experiments on animals, which, as we know, are pre-eminently ambiguous upon the point

at issue. In the second place, the theory opposes 'sense cells' to 'idea cells'; excitations are

supposed to flow from the former to the latter, and there to be deposited in the form of memory

images. It thus transforms a wholly inadmissible psychological distinction into an equally

inadmissible physiological hypothesis. The idea that sensations and ideas are absolutely

distinct conscious contents belongs to the older spiritualistic psychology, which taught that

'ideas,' as contradistinguished from the sensations evoked by physical stimuli, are purely

mental processes, the prerogative of the mind itself. This spiritualistic distinction is, of course, a

pure product of metaphysical speculation, and [p. 292] survives at the present time only in a

psychology of reflexion that has turned its back once and for all upon the facts of psychical

experience. No truly psychological observer will be found to assert to-day that ideas exist

independently of sensations, or that the sensations which enter into our ideas differ in any other

way than by their intensity, duration, and fragmentary character from the sensations aroused by

external sensory stimuli.[84] This distinction, derived as we have said from the spiritualistic

psychology, and then clothed about with the garb of materialism, stands in actual fact upon the

same level, in psychology as would, in anatomy, the dictum of some present day philosopher

that the mind has its seat in the epiphysis.

As compared with the sense-centre theory, the theory of association centres has certain

indisputable advantages. It has dissolved the unnatural alliance with the ideas of metaphysical

psychology, and has attempted instead to enter into relations with the psychology of

association, which agrees better with our modern conceptions. But, on the one hand, it still

holds to the erroneous view that the 'sensory centres ' are central repetitions of the peripheral

sensory surfaces, which latter must be projected upon the brain cortex solely that they may be

brought into touch with the consciousness which resides there; and, on the other, it confuses in

a very dangerous way the purely anatomical idea of 'association fibres,' that connect different

regions of the cerebral cortex with one another, and the psychological idea of association. We

are forbidden to suppose that the association fibres are, so to say, the vehicles for the

production of associations of ideas, by the simple fact that the commonest and most important

associations are those obtaining between the sensation elements of one and the same sense

department. Hence the paths that run between different sensory centres could, at most, be

taken as the conjectural substrate only of what are called 'complications,' i.e. of associations

between disparate ideational elements. But if the idea of 'association fibres,' understood in this

restricted sense, would possess a fairly definite meaning, the same thing can hardly be said of

the idea of an 'association centre.' Are we to imagine that the association fibres running

between the various sensory centres are inadequate to mediate complications, and that the

independent function of an organ that receives association fibres, and association fibres alone,

is further necessary to their production? This is the obvious meaning to be put upon the term

'association centre'; but it is scarcely the meaning that really attaches to it. It seems rather that

the theory is here influenced by reminiscence of the psychology of association, very much as

the sense-centre theory, in its distinction between sensations and ideas, is dominated by the

old spiritualistic view of mind. The psychology of association seeks, as we know, to derive

concepts, judgments, complex intellectual and affective processes all and sundry,[p. 293] from

associations of ideas. And the theory of association centres is evidently inspired, in the last

resort, by the idea that the various complex psychical products are originated in these centres,

-- always, of course, with the closest co-operation of the sensory centres with which they are

connected by association fibres. Here, then, we have a justification for the expression 'psychical

centres.' The further the distinction of such centres is carried, however, the more nearly do we

approach to the 'internal senses' of the older phrenology. For the psychical functions ascribed

to them must, naturally, become more special, and therefore more complicated, in proportion

as they themselves increase in number.

This extreme subdivision of the psychical functions is opposed by the anti-localisation school of

modern experimental physiologists. Its representatives are undoubtedly right in insisting upon

the multiplicity of relations in which these functions stand, and in affirming, as a consequence,

that we cannot speak of sensory centres, in the sense of a definitely circumscribed repetition of

the peripheral sensory surfaces, or of psychical centres, in the sense of circumscribed seats of

separate mental activities. Indeed, as the hypothesis of the functional equivalence of all parts of

the prosencephalon has gradually fallen into disrepute, -- an hypothesis, it will be remembered,

which derives in the first instance from FLOURENS, and was revived in the early stages of

reaction, -- this principle of functional interaction has come to be our most valuable guide in the

psychophysical analysis of the cerebral functions. The new anti-phrenological movement, like

its predecessor, sets out from the results of experiments on animals. These abrogation

experiments are, however, hardly qualified to lead up to any exact formulation of the principle:

their outcome is indefinite and ambiguous, and they are seriously complicated by the effects of

vicarious functioning, whose influence is in most cases greatly underestimated. What we rather

need is, evidently, an analysis of the individual central functions, in the light of observations of

pathological defects, carefully collected and compared. Hence instead of asking: What are the

consequences of the lack of a given cortical area, and what functions are accordingly to be

ascribed to it? we must now raise the question: What central changes do we find, when a given

function (language, the act of vision, etc.) is deranged, and what is the nature of the parallelism

between the functional and anatomical disturbances? The great advance that modern

pathology, in particular, has made in this field may be attributed, without hesitation, to the fact

that it has been forced, by the nature of its problems, to give up the first form of enquiry for the

second. And the significance of the advance, for our knowledge of the central functions, lies in

the further fact that the first form directs the attention onesidedly, from the very beginning, to a

fixed and definite central area, while the second points at once to connexions with other areas

and, in general,[p. 294] emphasises the principle of functional analysis as against the former

centre of interest, the correlation of determinate functions with determinate parts of the brain.

This change of standpoint means a breaking down of the barriers, not only between the

different regions of the cerebral hemispheres, but eve, to a certain extent, between the

prosencephalon and the posterior brain divisions (more especially the diencephalon and

mesencephalon) as well. For the complex functions prove, as a rule, to be functions in which all

these departments of the brain are variously involved; so that it is about as sensible to localise

a complex function in a restricted area of the cerebral cortex as it would be to throw the sole

responsiblity for the movements of walking upon the knee joint, because they cannot be duly

performed if that joint is ankylosed. In fine, the analysis of the complex functions themselves

comes up as a further problem, whose solution will effectively supplement, at the same time

that it transcends, the physiology of the central hemispheres. The solution, as things are, must,

it is true, remain imperfect; there are but few functions, at the present time, that admit at all of

this sort of analysis. Of those that do, the chief are the central act of vision, the functions of

speech, and the processes of apperception.

The problem of the localisation of the psychical functions begins with the great anatomists of

the sixteenth century. Among them, VASALIUS was especially instrumental in spreading the

opinion that the brain is the seat of the mental activities. For a long time, however, the old

doctrine of ARISTOTLE and GALEN, that made the heart the general centre of sensation, held

its own alongside of the newer teaching. DESCARTES was the first to regard the brain as an

organ subserving the interaction between mind and body. It is with DESCARTES,

consequently, that a question arises which was destined thenceforth to play a great part in the

discussions of physiologists and philosophers: the question of the seat of the mind.

DESCARTES himself, in answering it, made the curious mistake of selecting the epiphysis, a

structure which is probably a vestige of the old parietal eye of the vertebrates, and does not

properly belong to the brain at all.[85] At the same time, increasing efforts were made,

especially in the anatomy and physiology of the eighteenth century, to ascertain the

significance of the various parts of the brain. Interpretations were based, as a rule, upon the

results of anatomical dissection, though the psychological ideas prevailing at the moment were

also of some influence. Thus, at a later time, the mental faculties of WOLFF'S school,

perception, memory, imagination, etc., were commonly chosen for localisation, -- which was

arbitrary and, of course, very differently worked out by different authors.[86] It is the service of

HALLER, in particular, to have paved the way for a less artificial view, holding closely to the

data of physiological observation. The reform is intimately connected with his doctrine of

irritability, whose chief significance lay in the fact that it referred the capacities of sensation and

movement to different kinds of tissue; the former to the nerves, the latter to the muscles and

other contractile elements.[87][p. 295] The source of these capacities HALLER finds in the

brain. This organ is connected with the mind and the psychical functions only in so far as it is

the sensorium commune, or the place where all activities of sense are exercised and whence

all muscular movements take their origin. The sensorium extends over the whole substance of

cerebrum and cerebellum.[88] It is therefore certain that every nerve receives its physiological

properties from a definite central region; that. i.e., as is also attested by pathological

observation, sight, hearing, taste, etc., have their seat somewhere in the brain. At the same

time, the conditions of origin of the nerves seem to show that this seat is not sharply

circumscribed, but as a general rule is spread over a considerable area of the brain.[89] To the

commissural fibres HALLER assigns the function of mediating the vicarious function of sound

for diseased parts. He deduces the inexcitability of the brain substance from the fact that the

nerve fibres lose their sensitivity, in proportion as they split up within it into finer and finer

branches.[90]

The position thus won was never lost by physiology. Nevertheless, the endeavours after a

physiological localisation of the mental faculties were constantly renewed, the usual starting

point, now as before,, being furnished by anatomy. The views of the reactionaries were

systematised by GALL, and in this form long continued to exert an influence upon the science.

GALL, it should be remembered, did much real service in his investigations of the structure of

the brain.[91] The phrenology [92] which he founded proceeds upon the assumption that the

brain consists of internal organs, analogous to the external organs of sense. As the latter

mediate our perception of the outer world, so do the former mediate what may be called a

perception of the inner man. The individual capacities localised in the brain were, accordingly,

also termed 'internal' senses. GALL distinguished twenty-seven; his pupil SPURZHEIM

increased the number to thirty-five.[93] The mental faculties that are ordinarily recognised, such

as understanding, reason, will, etc., have no place in the phrenological list. These fundamental

forces of the mind are, in GALL'S opinion, not localised, but are uniformly operative in the

function of all the cerebral organs, and even in that of the external organs of sense. Every one

of these organs is, therefore, as he puts it, an "individual intelligence."[94] His main argument

for the analogy of the 'internal senses' with the external sense organs is derived from

anatomical investigations; as every sensory nerve is a bundle of nerve fibres, so is the whole

brain a collection of nerve bundles.[95] When, now, GALL and his followers came to put these

theories into practice, they substituted 'skull' for 'brain': the form of the skull was to yield up

information regarding the development of the individual organs. And this means, of course, that

they intended, so far as possible, to localise the organs on the surface of the brain. Here, then,

at the very outset, is evidence of a tendency to adapt observation [p. 296] to preconceived

opinions: a tendency that crops up again in all the special investigations, and robs their 'results'

of any sort of value. This apart, however, the monstrosity of the psychological and physiological

ideas that underlie the teaching of phrenology marks a long step backward from the more

enlightened position occupied by HALLER. The great Swiss already has an inkling of the true

principle, that the peripheral organs of the body must in some way be represented and brought

into mutual connexion in the central organs. The phrenologists make the brain an independent

group of organs, for which they posit specific energies of the most complicated kind.

The fact remains that in one of his localisations, that of the 'sense of language,' GALL hit upon

the right path, and in spite of all his errors, this fact has led certain authors in modern times, not

only to seek a just recognition of his services to anatomy, -- which are undeniable, -- but also to

attempt in some sort a rehabilitation of his phrenological doctrines.[96] It must, however, be

remembered, on the other side, that even as regards the syndrome of aphasia the expression

"organ of the sense of language" is inadmissible. We can imagine, if we are called upon to do

so, that the conduction paths concerned in the function of speech run their course in these

particular regions of the brain. We cannot possibly imagine, from what we know either of the

brain or of the psychical processes, that a definitely circumscribed brain area is the seat of

linguistic endowment, in the same sort of way that eye and ear are organs or the reception of

light and sound stimuli, or the pyramidal paths lines of motor conduction. And even so, the

faculty of speech is one of the comparatively simple 'internal' senses. How, then, are we to

conceive of the mechanism by which the fear of God, philoprogenitiveness, the sense of facts,

the impulse of self-preservation, and things like these are localised somewhere in the brain?

Granted that GALL was, in his day and generation, one of the highest authorities on brain

morphology: this honour is his, and is not to be taken from him. The phrenological system,

nevertheless, is and remains a scientific aberration, the joint product -- like its predecessor, the

physiognomics of LAVATER - -of charlatanism and unreasoning caprice. For this disease there

is no remedy. Hence it is a questionable experiment to rehabilitate any other of the 'mental

organs' that GALL pretends to have discovered, as P. J. MÖBIUS has undertaken to do in the

interests of the 'organ of mathematical ability.'[97] Looked at as it stands, 'mathematical talent'

comes perilously near the psychological monstrosity of GALL'S other localisations. But besides,

the unusually marked development of the superior external orbital angle, which MÖBIUS has

found confirmed in the case of three hundred mathematicians, admits of two different

interpretations. In the first place, it may be -- and this is, perhaps, the more probable hypothesis

-- a reactive effect of the mimetic tension of the muscles of the forehead, observable in

profound thought, upon the bony skeleton of the face. Or again, it may be due to the fact that,

in all highly developed brains alike, the frontal lobes are characterised by their mass and the

number of their fissures, quite apart from the question whether or not the intellectual

endowment of their owners takes the precise direction of a talent for mathematics. In other

words: it must first be shown that the protuberance under discussion is not to be discovered in

highly developed brains of poets, philosophers, philo-[p. 297]logians, etc., who at the same time

were conspicuously lacking in mathematical ability. So far, this proof is not forthcoming. But

suppose that it were given: what would follow? Certainly not, that there was a mathematical

organ, in the sense of the phrenologists, but at best this: that we were in presence of a fact,

which for the time being we could not explain, and which had about as much value for science

as the law that most great men possess unusually large skulls. The principal opponents of the

phrenology of GALL and his followers, in the first half of the nineteenth century, were the

French experimental physiologists, MAGENDIE and FLOURENS.[98] The views which these

investigators developed of the significance of the central organs plainly represent a reaction

against the phrenological doctrines. In MAGENDIE this spirit shows itself in the strict congruity

of theory with observed facts.[99] FLOURENS had the more decisive influence upon the

physiological ideas of the following period. His researches extended to the oblongata,

quadrigemina, cerebellum and cerebrum. The first of these he determined as the centre for

cardiac and respiratory movements: the quadrigemina as central organs for the sense of sight;

the cerebellum as the centre of conduction of voluntary movements, and the cerebral lobes as

the seat of intelligence and will.[100] He found, however, that different parts behaved

differently, as regards the functions dependent on them. The central properties of the oblongata

are confined within a small area, his noeud vital, destruction of which means instantaneous

death. The higher central regions, on the other hand, discharge the functions assigned to them

uniformly throughout their entire substance. This conclusion is an inference from the fact that

the disturbances set up by partial removal of the cerebral lobes, cerebellum or quadrigemina,

are gradually compensated as time goes on. It follows, therefore, that the smallest fragment of

these organs can function for the whole. In his view of the cerebral hemispheres as the properly

psychical centres, FLOURENS was evidently influenced, at the same time, by the traditions of

the Cartesian philosophy. DESCARTES had emphasised the indivisibility of the psychical

functions, more especially of the intelligence and will, and had on this very account demanded

an unitary 'seat of mind.' As DESCARTES' choice of the epiphysis could not be sustained, in

the face of more recent experience, FLOURENS substituted for it the total mass of the cerebral

hemispheres. FLOURENS doctrines thus came, in virtue of their spiritualistic prominence, into

sharp conflict with the more materialistically coloured ideas of the phrenologists: a

circumstance that was not without import for the issue of the struggle.

Their acceptance in scientific circles was, however, chiefly due to he fact that they gave a fairly

accurate representation of the observational data in experiments on animals. It was not realised

that the same psychological difficulties attach to them as to the phrenological theory of organs.

Nevertheless, intelligence and will are also complex capacities. That they should have their

seat in any the least fragment of the cerebral lobes is, after all, just as difficult of

comprehension as that memory for languages, sense of place, etc., should [p. 298] be

somewhere localised. Moreover, it remains an open question what significance is to be

ascribed to the separate parts distinguished by anatomical dissection of the cerebral

hemispheres, if they are throughout as uniform in functional regard as, say, the liver.

So it came about, even before the dawn of the new era of localisation, that the anatomists,

where they ventured at all upon speculation concerning the significance of the different parts of

the brain, were apt to recur, urged doubtless by arguments derived from their own science, to

the idea of localisation of particular mental capacities.[101] And as, in course of time, the

connexion of anatomical, physiological and pathological observations became more intricate,

the views introduced into physiology by FLOURENS gradually lost their hold upon men's minds.

The determining factors in the change of opinion were two: first, the investigations into the

elementary structure of the central organs, and, secondly, the physiological and pathological

experiences regarding the localisation of certain sensory functions and motor effects. Epoch-

making, in the latter connexion, was the renewal of interest by BROCA in the observations

made long since by BOUILLAUD on the anatomical substrate of aphasia.[102] Still, a certain

contradiction remained between these results and the consequences of partial removal of the

hemispheres by FLOURENS' method. The permanent symptoms produced by the latter

operation consisted, not in the abrogation of particular functions, but in the weakening of all: a

fact verified in the most recent times by GOLTZ.[103] So, in the controversy carried on

principally between H. MUNK and GOLTZ, we have repeated, in modern terms and within the

limits of experimental physiology itself, the older issue between FLOURENS and the

phrenologists. But the new phrenological view, as represented by the 'sense-centre theory' and

the 'theory of psychical centres' discussed in the text, is indefinitely better fitted to make terms

with science than was the old phrenology; and the newer antiphrenologists, in the same way,

have given up their original and impossible assumption of the functional equivalence of the

different parts of the brain, and lay increasing emphasis upon the principle of regional

interaction in the various complex functions. This principle, now, taken together with the

manifold phenomena of substitutive and auxiliary function, leads inevitably to the idea of a

relative localisation of functions. We say 'relative' for two reasons: first, because it is never the

complex functions themselves, but only their elements, that are localised; and, secondly,

because these elementary functions may also suffer all sorts of shifts and changes as a result

of the processes of vicarious function.

§ 7. Illustrations of the Psychophysical Analysis of Complex Cerebral Functions

(a) -- The Visual Centres

Of all the sensory nerve conductions, it is those of the acusticus and opticus, shown

schematiclly in Fig. 77 and 78 (pp. 183, 186),that, by the complicated course of their paths and

the number of central areas which these [p. 299] paths unite, make the most insistent demand

for physiological analysis. In the case of audition, however, we have not the data necessary for

the functional interpretation of the various areas involved in the nervous conduction. The

connexions with certain sensorimotor and regulatory centres, in particular, -- centres like the

pregemina, cerebellum, etc., -- can, in the present state of our knowledge, be referred only

quite generally to the interactions between auditory impressions and rhythmical movements.

The central mechanism of these movements themselves is still wrapped in so much obscurity

that the general notion of interaction cannot be replaced by any more definite ideas. In the case

of vision, the conditions are more favourable: not so much, perhaps, because the investigation

of its anatomical substrate has been brought to greater completion, as because the

physiological and psychological analysis of the functions as such has been carried further.

Anatomically and physiologically, the act of vision is, in the first instance, characterised by the

fact -- unparalleled, save in the one instance of olfactory sensations -- that it is, from the very

beginning, in some sort a central process. The retina, as we have seen, represents a brain area

displaced to the periphery of the body. Hence it is by no means to be regarded as a simple

receiving apparatus for external light stimuli, but rather as an organ of complicated structure,

containing not only nerve terminations, but also manifold ganglionic formations that mediate

connexion with other and higher nerve centres. The single elementary process that, in a certain

sense, reaches its conclusion in this peripheral organ is, we must suppose, that transformation

of the external light vibrations into some kind of photochemical process, the physical correlate

of which we find in the sensations of light and colour. In all probability, the vehicles of this

transformation are the specific sensory cells of the retina, the rods and cones (s, z Fig. 78).

[104]

It is at this point that the process of vision proper begins. The photochemical changes have

simply prepared the way for it, by impressing their peculiar qualitative differentia upon the

excitations conveyed along the opticus paths. The process of vision itself is compounded from

the manifold connexions into which these primary optical excitations enter, and by which they

obtain their concrete contents. This latter always carries with it a number of secondary

excitations, which are the principal factors in giving the particular optical stimulus its relations to

other sensations, and thus mediating the localisation and spatial arrangement of light

impressions. It is clear that the scheme of a simple and direct connexion between every point

upon the retina and a corresponding point in the visual centre of the occipital cortex -- a

schema still to be found here and there in the physiologies -- is not able to satisfy these

conditions. How inadequate it is will be [p. 300] seen at once from a consideration of Fig. 78 (p.

186), although the Figure does no more than indicate the course of the simplest conduction

paths those that admit of relatively certain interpretation, and altogether omits such others as

the pupillary reflex, which plays an important part in adaptation to brightness, and the branch

conduction to the cerebellum, the function of which has not yet been definitely determined. The

nearest approach to the original simple schema seems to be made by the direct optic radiation

ss. But even this path is interrupted, as it passes through the thalamic region (AK), by nodal

points, at which, in all probability, connexions are also made with other paths. Further: as we

said above (pp. 188, 230 ff.), the decussation of the optic nerves occasions a partial

transposition of the paths of the right and left sides: the rearrangement is adapted to the

functions of binocular vision, but at the same time suggests a relation to the centrifugal motor

innervations proceeding from the occipital cortex. This relation is also attested by the structure

of the visual cortex (p. 219), and by the probable existence of centrifugal conductions issuing

from it (c'f' Fig. 78). Again, a second main conduction of the opticus paths, which, if we may

judge from the position of the chiasma, is undoubtedly involved in the same plan of decussation

that includes the principal path, leads to the mesencephalic region, where it enters, in the

pregeminum (OV) into two connexions. The first of these is a reflex connexion to the nidi of the

oculomotor nerves (rr). It is, probably, at once same-sided and crossed, according to the way in

which the fibres are distributed in the chiasma, and the conditions laid down by the

requirements of binocular (panoramic or stereoscopic) vision. The second is a sensory

connexion; for the supposedly centrifugal terminations of the opticus in the retina (e f) also take

their origin from the mesencephalic region. The excitations carried by this path may be brought

in either along the centripetal path cp that ends in the same region of the brain, or along the

higher centrifugal path c'f' that reaches it from the visual centre. The functional arrangements

warrant the conjecture that, in both cases, the paths will take a crossed course; for it is but

natural to regard the centrifugal system cf, c'f' as the substrate of the coexcitations, which can

also be demonstrated psychologically as concomitant sensations, and which in all probability

have a part to play in the functions of binocular vision. In view of the motor synergies which

these functions engage, and of their dependence upon the sensory excitations of the visual

centre, the central portion c'f' of this centrifugal path will presumably make connexion, not only

with its sensory continuation cf, but also with the motor path rr, so that the regulation of ocular

movements by light impressions can be effected in two ways: the reflex, by direct release from

cp, and the centromotor, by the excitations brought up from the visual cortex in c'f'.

Now we must, of course, assume, as a general rule, that every excitation [p. 301] of the

peripheral organ of vision, whatever point it may strike, will discharge at one and the same time

into all the different paths of conduction opening up before it, provided always that none of

these paths have been rendered impassable by interruptions of conduction. In the entire

complex of organs that thus work together in the particular act of vision, the retina on the one

side and the visual centre of the occipital cortex on the other constitute the two principal

centres. Their functions are in some sort antithetical. The retina, lying farthest out towards the

periphery, plays the leading part in the origination of sensations; the visual cortex, lying farthest

in towards the centre, plays this part in the final combination of the separate functional

components of the act of vision. The transformation of light vibrations into photochemical

processes, which takes place in the elements of the retina, is at any rate indispensable for the

first origination of light sensations: for observations made on the congenitally blind prove that

the brain cannot mediate such sensations unless the retina has previously been in function. On

the other hand, the visual functions, once originated, may persist after removal of the external

sense organ; the victim of accidental blindness, despite the atrophy of his optic nerves, if he

originally possessed any vivid sense of colour at all, sees coloured memory images and, more

especially, can enjoy a wealth of colour in his dreams. We must accordingly suppose that the

excitatory processes in the central apparatus, particularly in those of the occipital visual centre,

come in course of time, through the influence which they exert upon the processes of external

stimulation, themselves to resemble those processes. The change is an illustration of the great

adapt ability of the central nervous substance to the varying conditions of excitation, vouched

for by many other facts. This capacity for adaptation is, indeed, evidenced by the central

process of vision in two different ways: intensively, in the change just mentioned, and

extensively, in the manifold functional substitutions that follow the loss of particular central

parts. Apart from minor phenomena of this kind, which appear in cases of central lesion and are

probably to be referred to the vicarious function of neighbouring parts of the same brain area,

e.g. of particular divisions of the visual cortex, one for another, we have here to consider two

principal substitutions, again of very different character, that occur between the two main

departments of the optic conduction, the mesencephalic and prosencephalic regions. Both of

these regions bring together sensory and motor conductions that belong to the same peripheral

organs; so that, in the nature of the case, loss of either centre is compatible with the retention

of certain essential visual functions, and a new course of practice may partially make good the

defect. Observation, as we saw above, proves that these possibilities are realised. The visual

centre in the mesencephalon is able, in particular, to discharge the most essential of the visual

functions, independently of the [p. 302] visual centre in the occipital cortex (pp. 260 ff.). True,

the defects that remain, even with the utmost extent of vicarious activity, demonstrate at the

same time that under normal circumstances the act of vision is a complex function, conditioned

upon the co-operation of all these centres, whose several functions are themselves of a

complex nature. In this analysis of the central functions of vision, we have still left out of

account two principal factors, whose significance cannot at the present time be estimated in

any sort of detail, though a rough guess may be made at their meaning. The first consists of the

relations to the motor regulatory mechanisms situated in the cerebellum; the second of the

connexions mediated by the association systems of the cerebral cortex both with other sensory

centres and with yet more central brain regions, which are not directly correlated with definite

sense departments, but themselves contain the junctions of various sensory and motor

conduction paths. Now we cannot conceive of an optical excitation that does not, to some

degree, release at any rate a certain proportion of these manifold excitations. Hence we might

conclude without hesitation, merely from the morphological relations of the optic conduction

and the physiological analysis of its processes, that the simplest act of vision is, physiologically,

an occurrence of great complexity, even if we were not constrained to posit this complication of

conditions by our psychological analysis of the visual processes.[105]

(b) -- The Speech Centres

The name 'speech region' is ordinarily applied to a cortical area, lesions of which, whether they

affect larger or smaller portions of its substance, are attended by disturbances of the functions

of speech, without setting up at the same time other psychical disturbances, especially those of

what is termed 'the intelligence,' of any noticeable kind. Where the intelligence is affected, there

is always good reason to refer its impairment to more extensive changes involving other cortical

areas. Psychical disturbances of this sort, due to diffuse cerebral disorder, may be

accompanied by derangement of speech, or even by complete abrogation of the speech

functions, without direct injury to the speech centre itself. In such cases, the disturbances of

speech are evidently secondary symptoms. Hence the only phenomena that are important for

the relations of speech to definite cortical areas are those observed when the seat of injury is

strictly confined to the speech centre proper. This region, in contradistinction to the sensory

centres, whose representation in the brain cortex is without exception bilateral, has the

peculiarity that its development is exclusively unilateral. Since the majority of mankind are right-

handed, it is situated, as a result of the decussation [p. 303] of the conduction paths, upon the

left hemisphere. The corresponding cortical area of the opposite side is not employed for any

other function. Under normal circumstances it serves, we may suppose, though for the most

part to a very limited extent, as an auxiliary to the principal speech centre. If, however, the

function of the latter is abrogated, it enters under stress of the new conditions upon a special

course of practice, and gradually takes the place of the lost organ. This appears, at any rate, to

be the only way in which we can explain the restoration of function observed in cases where the

speech centre of the left side has been destroyed over a wide extent of the cerebral surface.

According to pathological observations of the phenomena that result from its partial abrogation,

the entire speech region divides into several sub-regions or 'speech centres,' as they are called,

each of which would seem to preside over a definite phase of

the total functions of speech. The first of these to be discovered, owing to the striking character

of the symptoms produced by its injury, was the 'motor speech centre.' It occupies the posterior

third of the subfrontal gyre (BROCA'S convolution: M Fig. 102). Destruction of this area and of

its subcortical fibres gives rise to the phenomena of 'motor' or 'atactic aphasia,' which consists,

according to the extent of the lesion, either in complete abrogation of the movements of

speech, or simply in the abrogation or impairment of determinate articulations: the voluntary

innervation of the muscles of speech is left intact. Of the remaining centres, we can distinguish

most clearly the 'sensory' or 'acoustic speech centre,' by the characteristic form of the

symptoms which accompany its lesions. It occupies the supertemporal gyre, more [p. 304]

especially its posterior portion (about two-thirds of its whole extent: A Fig. 102); though this

area cannot be sharply separated from the general auditory centre (WERNICKE'S centre)

which lies in the same neighbourhood. Destruction of the sensory speech centre gives rise to

the phenomena of 'sensory' or 'amnestic aphasia'; words and phrases can, for the most part, be

perfectly articulated, read or repeated from dictation; but the patient has lost the power of

clothing his ideas in the corresponding phrases, and in extreme cases is able to hear but not to

understand what is said to him, i.e., to associate a meaning contents to the sound of the words.

This condition has been termed 'word deafness.' The disturbances again extend, according to

the extent of the injury, either to the patient's whole vocabulary or only to certain of its

constituents: the latter more especially in the minor degrees of 'amnesia,' which may range

through all possible gradations to the condition of ordinary weakness of memory. Thus,

sometimes it is merely certain classes of words, particularly proper names and names of

objects, or in more severe cases all words whatsoever, with the exception of the particles and

interjections most commonly employed, that are forgotten. It is therefore customary to

distinguish total and partial amnesia. A special sub-form of partial amnesia is found, finally, in

what is called 'paraphasia,' the symptoms of which, however, usually show a tendency to pass

over into the syndrome of atactic aphasia. In its amnestic form paraphasia consists in the

substitution of another, wrong word for the word required; in its atactic form, it consists in the

substitution of certain sounds for others, so that the word is wrongly pronounced.

Somewhat less assured is the localisation of two further functions, functions that do not

necessarily belong to speech in the stricter sense of the word, but that are intimately connected

with it: the functions of writing, which, as a predominantly motor activity, connects with the

articulation of sounds, and the functions of reading, which, as a more sensory process,

connects in the first instance with the auditory perception of words. It has often been observed

that abrogation of the movements of writing, with retention of the capacity of voluntary

contraction of the muscles concerned, may be produced by lesion of a cortical area lying

directly above the motor centre and belonging to the medifrontal gyre (S Fig. 102). This

syndrome has been termed 'agraphia.' It seldom occurs, apparently, in pure form, but either

accompanies motor aphasia or is connected with disturbances of the other voluntary

movements of hand and fingers. A comparison with the position of the general motor centres

(Fig. 88, p. 205) shows, also, that these and the centres for the special functions of speech and

writing are either entirely coincident or, where that is not the case, directly apposed. As regards

reading, we find that the relation of the optic to the acoustic speech centre corresponds to a

certain extent with that of the writing centre [p. 305] to the centre for articulation. It appears,

from numerous observations, that the optic speech centre belongs to a region of the subparietal

and the second occipital gyres (the 'angular gyre': O Fig. 102) situated between the general

centre for vision and the acoustic speech centre. Destruction of this region produces the

peculiar syndrome of 'alexia' or 'word blindness': words can be spoken, and can also be heard,

understood and remembered; but their written or printed symbols are not understood: they

appear as meaningless pictures: although in other respects the visual functions remain

unimpaired. All these disturbances, now, can not only occur in the most various combinations,

with each component developed in a different degree, but may also be accompanied by further

central disorders; so that it is but seldom that the syndromes of the typical forms of aphasia are

seen pure, and unmixed with other phenomena. At the same time, the lesions of the different

regions themselves produce somewhat divergent effects, according as the cortex proper or the

subcortical parts are more seriously affected. Hence it is customary to distinguish cortical and

subcortical disturbances. The latter are also termed intercortical or conductive disturbances, on

the assumption that, while the cortical lesions involve the speech centres themselves, the

subcortical injuries interrupt the connexions mediated between different centres by association

fibres.[106]

These manifold gradations and combinations must here be followed out in some little detail,

though only in so far as they furnish the necessary data for an appreciation of the

psychophysical aspect of the phenomena, and afford an insight into the peculiar significance of

that category of complex 'centres' to which the speech centres belong. An especial interest

attaches, in this regard, to the connexions in which the typical cases of 'sensory' and 'motor'

aphasia come under observation, and to the phenomena of mutual assistance and gradual

recovery that follow in their train. The association paths anatomically demonstrable at all points

between the centres marked out in Fig. 102 naturally suggest themselves as the substrate of

these connexions. Since, however, nothing more can at present be learned from the anatomical

maps than the general possibility of this synergy of the different centres, pathologists are

accustomed, in order to explain the connexions in individual cases, to base their discussions

upon a geometrical scheme, in which the centres themselves are represented by circles, and

the paths of conduction to and between them by single lines of connexion joining the circles. A

simple schema of this kind is shown,

e.g., in Fig 103. It adopts, in all

essential features, the arrangement

suggested by LICHTHEIM.[107] The

little circles M and S (A) denote the

primary, motor [p. 306] and sensory-

acoustic, speech centres; E and O

are the secondary centres co-

ordinated with them, E that of the

movements of writing, O that of visual

word-pictures. Besides these centres,

however, all schematic

representations of this sort are

compelled to introduce a 'concept

centre,' C, and lines of connexion

running to it from the primary speech

centres M and S (A), in order to

indicate the relations with the

ideational or conceptual contents of

the words. It is obvious that this

centre C, together with the name

attached to it, is in reality only an indefinite expression for the manifold relations in which the

various speech centres must stand with all the cortical areas that can claim a share in the

origination of the ideational and affective contents of the constituents of speech. In what

follows, we shall, for brevity's sake, include this contents under the single term 'meaning

contents': an expression which, in the present instance, recommends itself by its very

indefiniteness. It need, again, hardly be pointed out that such a meaning contents cannot

possibly be conditioned upon any sharply circumscribed central area, but presupposes the

combined activity of variously constituted groups of sensory centres and of many other of the

regions that belong to the indeterminate category of 'association centres.' The circle C,

therefore, can stand here simply as the indefinite symbol of these manifold relations. This

presupposed, the schema first of all explains the occurrence of two general forms of speech

derangement. Abrogation of particular functions will occur, Whenever certain of the speech

centres themselves, M, S (A), E, etc., are destroyed in whole or in part; interruption of

conduction or, in psychological language, abrogation of the associations normally subsisting

between the various phases of the speech function will occur, when the connexions between

the centres are broken, e.g. at 3, 6, 10, etc. In the latter event, the phenomena will take on a

different form according to the direction in which the processes are conducted or, in other

words, the associations made. It is, however, generally assumed that the conductions may [p.

307] take any direction as between the centres themselves, and that only the peripheral lines

running to the two principal centres M and S convey their impulses towards a predetermined

goal. The centre S receives its excitations centripetally from the direct auditory centres and, by

way of these, from the peripheral organs of audition. The centre M gives out centrifugal

impulses, first of all to the direct motor centres of the brain cortex, and then, from these again --

probably with the co-operation of the co-ordinating and regulatory centres of diencephalon,

mesencephalon and cerebellum -- to the organs of articulation. So far, the directions of

conduction are opposed. These articulatory movements are, however, accompanied by

sensations, sensations of extreme importance for the uninterrupted flow of articulate speech;

and we must accordingly posit, as their substrate, the existence of other, centripetal excitations,

issuing from the motor organs. The conditions are indicated in Fig. 103 by the two arrows at 4.

From a plan of the speech centres and their connexions, such as is given by this Fig., we can

read off without difficulty the different forms of speech derangement and their possible

combinations. Thus, destruction of M will produce the syndrome of motor, destruction of S that

of sensory aphasia; lesion of E will mean agraphia, lesion of O word-blindness. The schema

also shows the more complex symptoms that may result from interruptions of conduction.

Suppose, e.g., that connexion is broken at 3 between M and S. Words and phrases will still be

heard; and, provided that the conduction from S to C is unimpaired, will be rightly understood.

Further, if the conduction between C and M is intact, they may also, in contradistinction to the

symptoms of cortical motor aphasia, be spontaneously uttered, with their right meaning upon

them. On the other hand, heard words cannot be repeated, or can be repeated only with

difficulty, -- perhaps through the mediation of the idea centre, C, -- because the requisite

association path between M and S, the path which is supposed to run in the alba of the insula,

is now out of function. Similar consequences follow from an interruption of conduction between

M and E, S and O, O and E. A break at 12, e.g., would mean that the writing of words to

dictation has become impossible, though printed or written words can still be copied from sight,

provided that the conduction O E remains uninjured; etc., etc.

Adequate, however, as this schema appears to be, for an understanding of the manifold forms

of possible derangement, it nevertheless fails in two respects to do justice to the facts. In the

first place, it represents certain disturbances as probable, even as necessary, which in reality

do not occur at all, or, if they occur, do so only with very considerable limitations and

modifications. In the second place, it leaves a large number of phenomena -- more particularly

the qualitative peculiarities of the disturbances, and the compensations due to interaction of the

different functions -- alto-[p. 308]gether unexplained.[108] On the former count, it may suffice

here to mention the most striking example of incongruity between the anatomical plan and the

actual functional conditions. According to the Fig., there may be interruptions of conduction

between S and M that will prevent the translation of the heard into the spoken word, while the

patient's apprehension of the meaning of words and power of spontaneous word formation

remain unimpaired. But phenomena of this nature do not occur at all, in a pure form; the

symptoms usually ascribed to the disturbance in question are paraphasic, and appear to be of

more complex origin.[109] The incongruity thus facts is evidently due to an erroneous

assumption which underlies the construction of the Fig., -- erroneous, because it disregards the

actual psychological association of the speech functions: the assumption, namely, that the

central areas C, whose activity is necessary for the origination of the meaning contents of a

verbal idea, are connected in the same way both with the motor and with the sensory speech

centres. Viewed in the light of the normal phenomena of speech associations, this assumption

is wholly inadmissible. On the contrary, the possibility of a movement of articulation is so

intricately associated with the acoustic word symbol, and the acoustic word symbol itself so

frequently precedes articulation as a constituent of the total word complication, that the

association may be much more probably referred to the indirect path C S M than to the direct

connexion C M. But if the indirect road is really followed, then the syndrome to be expected

after interruption of conduction at 3 will, of course, be entirely different: spontaneous

articulation and the translation of the heard into the spoken word must always suffer together.

And if this conclusion, again, is not borne out in all cases, the result simply proves that in the

preparation of the schema functional moments of such weight and importance have been left

out of account that we may well doubt whether constructions of the mind can serve any useful

purpose.

That these functional moments exert a real influence is shown with especial clearness in two

groups of phenomena, whose whole character forbids the relegation of the aphasic symptoms,

whatever they may be, to any rigid scheme of localisation. The first group consists in certain

qualitative peculiarities, which attach to pathological amnesia in general precisely as they do to

the normal lapse of verbal memory in advanced age. They find expression in the law that those

words disappear most readily from memory which are associated in consciousness with

concrete sensible ideas. The amnesic symptom that sets in most easily, and that is therefore

the first to [p. 309] appear in old age, is, accordingly, forgetfulness of proper names. Next in

order come the ideas of concrete objects: chair, house, table, etc. Somewhat more durable are

the concrete verbs: go, stand, cut, strike, etc.; still more the abstract ideas, nominal and verbal:

virtue, love, hate, have, be, become, etc. Finally, as most permanent of all, come interjectory

words and the abstract particles: but, for, and, because, etc.[110] It is surely evident that no

anatomical schema, however complicated its construction, can do justice to this sequence of

phenomena. On the other hand, the order of impairment is explained at once by the

associations in which verbal ideas are uniformly involved in consciousness. The more directly a

verbal idea evokes a determinate object idea, the greater the converse possibility that the

object idea itself represent the word in our thought. We are perfectly able to remember our

acquaintances without at the same time reproducing their names. Concrete object ideas, like

chair, house, table, may also come to consciousness immediately, without the words that

denote them. But abstract ideas call be thought only by help of the corresponding words; and of

these, again, the particles that are oftenest used naturally have the advantage. We are thus

able, without difficulty, to explain the phenomena of progressive amnesia, in functional regard,

from the psychological associations, on the one side, and the general effects of functional

practice, on the other. Now these phenomena too have, of course, their physiological substrate.

Only, the substrate cannot be conceived as in any way stable, given with fixed centres and their

connexions, but must be thought of as labile, developing by function itself and continually

changing as function changes.

We are led to this conclusion yet more directly by the second of our two groups of phenomena:

the symptoms of auxiliary and vicarious function that regularly follow in the train of speech

disturbances, and that point again to the universal validity of associations and their gradual

establishment by practice. What is called amnestic aphasia still furnishes the most striking

illustrations; it shows the phenomena of associative compensation in extraordinary variety.

Thus, we not infrequently find that the word for an object is not directly at the patient's disposal,

but that it is immediately remembered if other verbal ideas, that often occur in connexion with it,

are intentionally reproduced. In one case, which has become classical, the patient, who

suffered from an almost total amnesia induced by an injury to the head, was always able to

discover the forgotten words by writing them down. Further, if he were required to name the

attributes of an object, he failed to do so, both when the object itself was named and when the

attributes in question were exhibited to him in other objects, but succeeded, after some

improvement in his condition had set in, if he saw the object before [p. 310] him.[111] Cases of

this kind, which cannot be included in any rigid schema of localisation, are referred by certain

authorities to a 'functional aphasia,' and thus distinguished from the typical forms of cortical and

subcortical aphasia. In reality, however, instances of associative compensation are extremely

common. We may unhesitatingly assume that if the phenomena are not recorded, and more

especially if they are not recorded with minor degrees of derangement, this is merely because

they have not been observed or not explicitly verified. All the aphasic disturbances are, indeed,

at once functional and anatomical: and the functional disorder does not exclude structural

defect as such, but only that rigid localisation which is presupposed in the schematisations of

the centres and their connexions. The error that has crept in is the error of inversion. The

anatomical conditions have been first laid down, and the functional symptoms that came under

observation have then been assigned, as accurately as might be, to determinate cortical areas.

But the first requirement evidently is that we analyse the functions themselves, and only then

turn to consider how, in the light of this analysis, the anatomical conditions are to be envisaged.

If we take our stand upon this principle, we must necessarily begin by examining these

psychological facts front which the associative relations may be ascertained that hold between

the individual constituents of a verbal idea or between the different word ideas themselves. For

the sake of brevity, these associative relations may he termed, in their entirety, the

psychological structure of verbal ideas. We may, further, limit our enquiry to the constitution of

consciousness that characterises the adult members of a civilised community, where the

artificial development of the capacities of reading and writing has been superinduced by

practice upon the natural function of speech. Proceeding on this basis to our task of a general

appreciation of these activities, and of the disturbances which occur amongst them, we note,

first of all, that every complete verbal idea is a complicative association of three constituents:

the phonetic utterance, L; the script form, S; and the meaning contents, B. Each of these is, in

turn, made up of two, more intimately compounded elements, Thus, the phonetic utterance, L,

is composed of auditory idea (a) and articulatory sensation (m). The script form S is composed

of visual idea (o) and movement sensations of writing (m'): these latter, we may suppose,

corresponding originally to the pantomimic movements with which primitive man accompanies

his talk, and then translated by civilisation later on into the specific form of writing [p. 311]

movements. Lastly, the meaning contents B may be analysed, in general, into an ideational

component (v), and a feeling (g) dependent both upon this and upon the whole configuration of

consciousness, but more especially upon its relations to other contents, past and present. Not

only B, however, but the other constituents of the verbal idea, L and S, as well, are

associatively connected in the most various ways with further meaning contents and word

ideas; so that a given combination L S B never really occurs by itself alone, but always as a

formation only relatively isolable from a more or less complicated tissue of associations. Now

all the phenomena of speech, and all its disturbances also, indicate that these associations with

other verbal and ideational contents vary within wide limits from case to case. But they indicate,

further, that the different constituents of one and the same verbal idea may be associated in

very different degrees of intimacy: this altogether apart from the fact that the vivacity and

activity of these constituents must vary very considerably with the constitution of the individual

consciousness with the stage of practice attained, and with the specific conditions under which

the ideas are formed. Thus, there are individuals who are but obscurely aware, in ordinary

speech, of the elements o, or m, m', of the complex L (a m) S (o m') B (v g); the auditory

impression of the word dominates in consciousness. There are others, for whom m is especially

prominent; and yet others, for whom even o is constantly noticeable. In reading, o naturally

stands in the foreground of consciousness. It is, however, so closely associated with the terms

a m that these may always be perceived, more or less clearly, along with it. We find the most

complete representation of all three constituents in the process of writing to dictation, where a

directly arouses m, m' and o. In this instance, just because the word constituents proper force

themselves all together upon consciousness, the meaning contents v g may very easily be

relegated to entire obscurity; it is a common experience that writing to dictation slips more

readily than any other form of speech function, more readily even than repeating from dictation,

into a meaningless routine. The degree of intimacy of the association between the meaning

elements v g is further conditioned, as we remarked above when discussing the phenomena of

partial amnesia, upon the logical and grammatical value of the verbal ideas. It is owing to the

influence of this factor that, in abstract word forms, the ideational constituent v entirely

disappears behind the word elements proper, a and o, and only the affective element g

remains. We shall treat later of this 'conceptual feeling'; here we have simply to note that it is

firmly associated to the word constituents L S.[112]

Finally, there are two further phenomena, connected with this variation [p. 312] in the intimacy

of associative combinations, that require a special mention. They bring out, in a striking way,

the quite extraordinary variety of tendency and disposition exhibited by the speech functions in

the individual case. The first is this: that the closeness of any particular association, of the

group here under discussion, depends not only upon the connected elements themselves, but

also upon the direction of their connexion. It is but rarely that a speech association is of

approximately equal strength on both sides. The principal instance of the kind is the connexion

between the two components of the first term L of the complete verbal idea; here, under normal

conditions, the auditory impression a is as powerful in arousing the tendency to articulatory

movement m as the movement is to evoke the auditory idea. The same sort of reciprocal

influence appears to be exerted, though on the whole with somewhat less of constancy, by the

auditory word idea a and the feeling g which attaches to the meaning idea. A heard word first of

all arouses a feeling of its meaning, before the meaning itself has come clearly to

consciousness. This order of events obtains more especially in the case of unfamiliar or entirely

unknown words; oftentimes, indeed, the whole process stops short with the arousal of the

conceptual feeling. But we also observe, conversely, that in cases where the word that

expresses a particular idea is for the moment beyond reach, in cases i.e. where we are "trying

to think of" a word, a strong associative influence is exercised by the feeling accompanying the

idea; so that it is clearly the association g a, and not (or, at any rate, only in a much slighter

degree) the association v a, that is primarily responsible for the success of the act of

recollection. In direct antithesis to these associations, in which the strength of the connexion is

approximately the same for both sides, are those in which the one direction has a decisive

advantage over the other. Here belongs, e. g., the association between o and m, where the

direction o m represents a much stronger associative tendency than the direction m o, -- the

script form arouses the movement of articulation, but this has very little power to call up the

script form; or the association between m and m', where the movements of writing easily

arouse the articulatory sensations of the organs of speech, but these require special conditions

if they are to touch off the movements; and so on.

As a rule, then, the speech association runs more smoothly in the one direction than in the

other. The second complicating condition is this. There is a continual fluctuation, not only in the

strength of the connexions at large, but also and more particularly in the preponderance of the

one or the other direction under the influence of practice. The effects of practice are seen most

clearly in cases where some change has been produced in the normal intensity of the individual

constituents of the idea, or where certain associations have lost their efficacy. In deaf-mutism,

e.g., the elements am [p. 313] are entirely wanting; This loss is compensated by the

development of the elements o m' and of the two-way association between them: m', at the

same time, appears in its original character as component of a mimetic movement. If the

patient learns to speak, the connexion o m develops as a two-way association of the most

pronounced type, and completely replaces the normal connexion a m; and so on.

We may now attempt, in the light of the preceding paragraphs, to construct a plan of the whole

number of associations that obtain between the elements of the complete word idea. Let the

elements be denoted by the symbols, a, m, o, m', v, g; the associations between them by

connecting lines; the direction of the associations by arrows; and their relative stability by the

thickness of the lines. The ordinary course of the associations, within a complete word idea, will

then be represented, in general

terms, by the schema shown in Fig. 104. The effects of other word ideas, past or present, are

indicated by the symbols v', g', and the interrupted lines proceeding from them.[113] This

schema, it must be remembered, is not a stable structure; it varies from individual to individual,

and from time to time. And the associations that enter into it are themselves variable

processes, largely dependent, in particular, upon foregone associations and all sorts of other

psychical conditions.

We began this discussion with a review of the 'speech centres.' From the point of view to which

we have now attained by our functional analysis of verbal ideas, these centres, mapped out in

Fig. 102 in accordance with pathological observations of the lesions underlying disturbances of

speech, naturally acquire a very different significance from that assigned them in the plan of

localisation and conduction laid down in Fig. 103. For it is clear that the constituents a, m, o, m'

of a word idea can be brought into settled relation with definite brain areas only on condition

that the associations between them may be regarded as relatively stable connexions. There [p.

313] are, however, two reasons why they may not be so regarded. In the first place, both the

intimacy and the direction of the associations are throughout dependent upon individual

practice. In the second place, the formation of at least one set of associations, those that

include the terms o and m', cannot be explained save as the outcome of a development which

is conditioned upon a certain level of civilisation, and which therefore began late in the history

of the race. We might accordingly expect to find, as in fact we do, that these associations are

subject to especially large individual variations. But variability in the conduction paths

necessarily brings with it a certain degree of variability in the centres themselves: a conclusion

which we need not hesitate to accept, since it is borne out, from another side, by the

phenomena of restitution of function. In fine, then, the expression 'speech centre' cannot, under

any circumstances, designate a central organ, in the sense in which this term is ordinarily

employed, -- an organ that presides, exclusively or even preponderantly, over a determinate

group of functions. It can mean nothing more than an area which contains the most important

nodal points of those conductions whose co-operation is indispensable for functions of this

particular kind. In other words, the significance of the centres is rather that they comprise the

points of connexion than that they include the points of departure of the elementary processes

concerned in a certain complex activity. Such a conception harmonises very well with the idea

suggested by the effects of practice and of restoration of function: the idea that it is not the

centres but, in a certain sense, the functions that are the original things. The functions make

their centres, and are constantly modifying them in accordance with the variable conditions of

function itself. Hence the localisation of the functions is not stable, but labile; the boundaries of

the functional areas are not fixed, but changing, -- subject to the functional influences that

modify the conditions of conduction, and with them the actual conduction paths. So it comes

about that functional analysis of the very phenomena which first inspired the modern doctrine of

strict localisations, the phenomena of derangement of speech, has at every stage thrown the

intrinsic impossibility of this doctrine into clear relief. Such a result, however, is, after all, but the

natural consequence of the extreme complexity of the speech functions, and of that many-

sidedness and variability of the psychophysical conditions which they are peculiarly fitted to

bring into open view. Understood in this way, it is also a result that must be generalised. What

holds of the speech centres holds, in reality, of the 'sensory centres' as well, despite the fact

that, even to-day, they are ordinarily interpreted as simple central projection surfaces. They,

too, are 'association centres,' in the sense that they contain nodal points which serve to

centralise the functions, but centralise them by bringing into connexion all the different partial

functions, sensations, movements, reflexes, synergies of sensations [p. 315] and movements,

that work together in the functional whole, -- the connexions still admitting of continual

adaptation to external conditions.

There is an extensive region of the human brain that appears, so far as sensory and motor

symptoms are concerned, to be comparatively indifferent whether to external stimulation or to

internal change: the portion of the frontal lobes that lies anteriorly to the anterior margin of the

motor zone (Fig. 88, p. 205). Pathological observations show that injuries to this region,

sometimes involving the loss of considerable masses of the brain substance. have failed to

produce any derangement whatsoever of the motor and sensory functions.[114] As a rule,

however, the observers report, with equal definiteness, a permanent disturbance of the mental

attributes and faculties. In a famous American case, e.g., a pointed iron rod, one and a half

inches in diameter, was driven through the head by the explosion of a blast, entering at the

angle of the left lower jaw and emerging near the anterior extremity of the sagittal suture. The

patient, who lived twelve and a half years after the accident, gave no indication of disturbance

of sensation and voluntary movement, but suffered a complete change of character and

activities. "He combines the animal passions of a man with the intellectual activities of a child,"

so writes the attending physician.[115] In other cases, decay of memory, inability to concentrate

the attention, entire loss of will-power, etc., are quoted as characteristic symptoms.[116] These

results agree with the observation that the pathological degenerations of brain tissue which

accompany the decay of intelligence and will in cases of paralytic dementia usually have their

seat in the frontal lobes,[117] and with the general law that intellectual development keeps even

pace throughout the animal kingdom with the development of the prosencephalon.[118] It is

also said that highly developed human brains are often characterised by an especially

abundant formation of secondary gyres and fissures in the frontal lobes.[119] It can hardly be

maintained, however, that in these cases there is any marked difference between the frontal

and the other, e.g.. the parietal and occipital regions.[120][p. 316]

On the ground of these facts, it has been suggested by various investigators, among others by

MEYNERT, HITZIG, FERRIER and FLECHSIG, that the frontal brain stands in intimate relation

to the functions of the 'intelligence.' Now 'intelligence' is an exceedingly complex and indefinite

term. If, as we have seen, the act of vision and the different functions concerned in speech are

connected with 'centres' only in a very limited sense, it is, of course, impossible to conceive of a

localisation of the intellectual functions, or to connect them with a specific 'organ of intelligence.'

The utmost that we can say is that that this particular region of the cortex may contain certain

nodal points of conductions, whose abrogation produces disturbances of an intrinsically

elementary character, but manifesting themselves, in the complexity of functional co-operation,

as impairment of the 'intelligence' and derangement of the compound feelings. On the

anatomical side, the abundant connexions mediated by association fibres between the frontal

brain and other brain regions, furnish a distinct support to the view that the frontal lobes contain

nodal points of especial importance for the interrelation of the central functional areas. And the

objection that lesions of the frontal brain have occasionally been observed to pass without

permanent moral or intellectual injury is not decisive:[121] for local lesions in general are the

more easily compensated, by vicarious function of other parts, the more numerous and varied

are the connexions of the elements; and this condition is, on the whole, more adequately met

by the cortical areas occupied by the 'association centres' than it is by the direct sensory and

motor centres. Hence these negative results, in cases of local injury, are far outweighed by the

fact, as stated by the brain pathologists, that "lesions of any extent at all are never observed to

occur in this region without causing the most serious intellectual defects."[122]

Let us now attempt, so far as may be possible, to analyse the complex phenomena, grouped

together under the indefinite rubric of 'intelligence,' into their elementary processes. These

processes must be such as can be connected with a clear and simple psychological idea; and

this must, in its turn, be capable of correlation with a correspondingly simple physiological idea.

We find what we require in the elementary idea of the apperception of a mental contents, e.g. of

a sensation. What apperception means in detail we shall show later on:[123] here we

understand by it a psychological process in which, on the objective side, a certain contents

becomes clear in consciousness and, on the subjective, certain feelings arise which, as

referred to any given contents, we ordinarily term the state of 'attention.' Now the objective

component of this complex process, the 'clarification'[p. 317] of a contents, is surely suggestive

in the highest degree of determinate physiological concomitants. Just as, e.g., when a

sensation grows stronger or weaker, there is a parallel increase or decrease of the

physiological processes of excitation in particular nervous elements, so must we suppose that,

when sensations or other conscious contents grow, as we have put it, clearer or more obscure,

these changes are conditioned upon some sort of physiological substrate. And it is evident that

this substrate may very well consist of certain simple processes, consistent with the general

principles of nerve mechanics; whereas the idea of 'intelligence' is altogether so complicated

that the search for any kind of definite or limited physical substrate would, in its case, be

entirely hopeless.

It might, at first thought, be supposed that the elementary process of apperception which

appears in its simplest form when a sensation becomes clearer, consists, on the physiological

side, merely in an increase of the nervous excitation that runs parallel with the sensation; and

that the physiological change, when the sensation becomes more obscure, is a corresponding

decrease of the same concomitant excitation. But to say that a sensation 'grows clearer' or

'grows more obscure,' is, in reality, a very different thing from saying that it 'increases' or

'decreases in intensity.' To speak relatively, a weak sensation may be clearly, and an intensive

sensation obscurely apperceived. And a little introspection suffices to show that a sensation, in

growing stronger or weaker, alters its own intrinsic character; while, if it grows clearer or more

obscure, the change is primarily a change in its relation to other conscious contents. A

particular impression is always apprehended as 'clearer' in contrast to other impressions which,

as compared with it, appear obscure. These facts suggest that the substrate of the simple

apperception process may be sought in inhibitory processes which, by the very fact that they

arrest other concomitant excitations, secure an advantage for the particular excitations not

inhibited. If we postulate an inhibitory process of this kind, we are able to explain how it is that

apperception as such does not consist in an intensification of the sensation contents. And if we

assume, further, that the inhibitory influence, in this special case, is not exerted directly upon

certain excitations in progress within the sensory centres, but rather upon the conduction of the

elicitations to the higher centres in which the sensory contents are combined to form complex

resultants, we avoid doing violence to the obverse fact that the conscious contents obscured by

inhibition do not on that account lose in intensity. The arousal of the inhibition, since on the

psychological side it is ordinarily dependent upon particular conscious contents, past and

present, must be physiologically conceived as analogous to that of the reflex inhibitions

occurring in various forms in the lower nerve centres. There is, however, a difference. The

inhibitory effects are liberated, here [p. 318] as elsewhere, by certain excitations that are

conducted to the centre; but their liberation is at the same time influenced by that incalculable

manifold of conditions which, for the most part, we can merely group together under the

indefinite name of the current disposition of consciousness, as determined by past experience

and the circumstances of the time.

We thus regard apperception as the one elementary process indispensable to any sort of

'manifestation of intelligence' and, indeed, to the higher mental functions at large. The

considerations put forward above with respect to its physiological substrate are, of course,

hypothetical. We have fewer data in this case even than we had for our discussions of the

functions of the visual and speech centres, and we are accordingly thrown back upon

conjecture and tentative hypothesis. These must be based, almost exclusively, upon the results

of a psychological analysis of the functions. Except for the meagre analogy of reflex inhibition,

physiology furnishes us with nothing more than the general principles of nerve mechanics.

Nevertheless, it is worth while to attempt a schematic representation of our theory; we can at

least show the general possibility of a physiological interpretation of the complex phenomena in

question. Such a representation is given in Fig. 105. We assume that the central area of

apperception AC stands in connexion with a twofold system of conduction paths: the one

centripetal, ss', hh', that brings up sensory excitations from the primary sensory centres; and

the other centrifugal, la, gf, etc., that carries, conversely, to subordinate centres the inhibitory

impulses proceeding from AC. We then

have, according as these impulses are

transmitted to sensory or motor centres,

either the apperception of sensations or

the excitation of voluntary movements. In

the former case, it is other sensations, in

the latter other motor impulses, aroused

by internal or external stimuli, that are

forced into the background. It is plain that

the transmission here assumed presents

a certain analogy to the reflex process,

and particularly to the reflex inhibitions. At

the same time, the way in which

apperception depends upon the sensory

excitations coming in at the moment

marks a wide divergence from the

schema of the reflex mechanism. In the

reflex, we find the motor excitation or

inhibition following of necessity from the

action of the sensory stimuli; in

apperception and voluntary movement,

we can speak simply of a [p. 319]

regulative influence of the excitations in

progress, -- the implication being that a

large number of intermediate terms,

which our methods cannot reach, exert a

determining influence upon the final

result. The physiological character of

these intermediaries is wholly unknown to

us. We can only infer, from psychological

experience, that definite dispositions take

shape in every brain, as the consequence

of generic and individual development,

and determine the excitatory processes

that run parallel with the act of

apperception. If, then, we refer the

apperceptive acts to a particular physiological substrate, we can do so only on the condition

that we endow the central area in question with connexions to the other central parts, in virtue

of which the excitations released in it are dependent upon these dispositions. Hence the

centrifugal paths that issue from the apperception organ AC and serve to conduct the inhibitory

excitations must, in general, take two directions: a centrifugal sensory and a centrifugal motor.

In both directions they are connected, both directly and indirectly, by way of intermediate

centres that represent nodal points of conduction for certain complex functions, with the direct

sensory centres (SC, HC) and the motor centres (MC). The part of intermediary is played,

within the centrifugal sensory path, by certain intermediate sensory centres (O and A); within

the motor path, by analogous motor centres of complex character (B and L). The term 'centre'

is here used, of course, only in the relative sense defined above, in the discussion of the visual

and speech centres. We found, e. g., that the speech centres were not to be regarded as

independent sources of the functions ordinarily ascribed to them, but simply as indispensable

intermediaries in the mechanism of speech associations and apperceptions; and the same

conclusion holds here. The physiological significance of these centres may be roughly

illustrated in this way. We will suppose that various sensations, belonging to the domain of

speech, have arisen in the sense centres proper, SC and HC. The corresponding excitations

are at once combined, in the intermediate sensory centres O and A, into an unitary excitation

process; whereupon the apperceptive inhibition can operate to render this, or the primary

excitations in progress in the centres SG and HC, clearer or more obscure. The processes in O

and A will thus have the significance of resultants, which correspond to the functional

unification of the two associatively connected elements, phonetic utterance and script form.

These resultants must not, we need hardly repeat, be regarded as traces, stamped indelibly

upon certain cells, but rather as transitory processes of extremely complex character,

embracing a large variety of elements, -- processes akin to the stimulation processes in the

peripheral sense organs, and to other processes in the central nerve substance, all of which

leave a disposition to their renewal behind them. A like function must be assigned to the

intermediate motor centres B and P in which an act of apperception releases (by the paths g f r

s, g fr s) a determinate motor excitation, corresponding to the sensory excitations brought up

from SC and HC (by ss', hh'), or from O and A (by ek, ek); or else an unmediated activity on the

part of the two elements, phonetic utterance and script form, releases (by the paths e f, ef) the

corresponding motor impulses, without interference from the apperception centre, i.e. by way of

a direct reflex excitation. These impulses are then, in all cases, carried (by the paths f r s, fr s)

to the general motor centres MC, whence they are transmitted along the further nerve

conduction to the muscles.

In the hypothetical schema of Fig. 105, the paths that lead towards AC between subordinate

centres are represented by [p. 320] uninterrupted, the centrifugal paths that lead away from AC

by interrupted lines, and the direction of conduction is further indicated by arrows. Besides the

direct motor centres MC, and besides the auditory and visual centres SC, HC, chosen as the

chief representatives of the sensory centres, the schema includes, as examples of more

complex central areas, the four 'speech centres' mapped out in Fig. 102. Suppose, now, that a

series of impressions is carried by the optic nerve S to the visual centre SC: we have, taking

only the principal cases, the following possibilities. (1) The impressions are not conducted

farther. Then the sensations remain in the state of mere perception or indistinct apprehension.

(2) An individual impression a is apperceptively enhanced by inhibition of the impressions b c d:

the inhibition is released by way of ss' and hh', and conducted to the centre SC by the path la.

We then have perception of b c d and apperception of a. (3) Besides the apperception of the

impression a, there is a conduction by way of O to the centre A. Here a resultant is released,

which produces in the auditory centre HC (by way of ee a) the verbal idea a corresponding to

the visual image a. At the same time, by means of inhibitions released in the centres A and SC

along the paths ke and ga, the resulting word idea and the phonetic utterance are apperceived.

(4) The processes described under (3) combine with (a) a conduction of the resultants from A

by way of L to MC (along ef and frs): involuntarily pronunciation of the word designating an

apperceived idea; (b) a conduction from AC by way of L to MC (along gf and frs): intentional

pronunciation of the word in question; (c) a conduction from HC by way of A to O, and from O

again by way of SC to certain other elements, not shown in the Figure: involuntary association

of the word idea to the script form. (5) If the original impression a is the script form of a word,

we have the following possibilities: (a) direct apperception, again, by means of an inhibition la;

(b) conduction from SC to O, and apperceptive inhibitions along the paths la and ke:

apperception of a word whose meaning is familiar; (c) conduction from SC to O, and from O by

way of A to HC, with the fourfold apperceptive inhibition la, ke, ke and ga: apperception of a

visual and of the corresponding auditory word idea (the ordinary process in reading); and so on.

For the rest, this Figure, too, necessarily leaves out of account all those moments that, in the

nature of the case, cannot be given a place in any pure conduction schema: so, more

particularly, the intimacy and direction of the associations, specially indicated in Fig. 104 for the

functions of speech; the influences of practice and of vicarious function, which are constantly at

work to change the face of the phenomena; and finally the influences, wholly rebellious, like the

last, to any attempt at schematic representation, which are exerted, psychologically by the

configuration of consciousness, physiologically by the general state of the nervous dispositions,

upon the associations and apperceptions in progress at any given time.

§ 8. General Principles of the Central Functions

(a) -- The Principle of Connexion of Elements

THE principle of the connexion of elements may be understood in an anatomical, a

physiological, and a psychological sense. It may therefore be formulated in three different

ways, each one of them individual, in [p. 321] contents and meaning, but each one again, in all

probability, closely related to the others.

Anatomically regarded, the nervous system is an unitary complex of numerous elements; and

every one of these morphological elements stands in more or less close connexion with others.

This fact of interrelation is expressed in the very structure of the essential elements, the nerve

cells. Not only are the connexions mediated, in general, by the cell processes, but the character

of these processes, as dendrites and neurite, oftentimes indicates the direction in which the

proximate connexions are made. It is the merit of the neurone theory to have shown how this

principle of the connexion of elements is exhibited in the morphological relations of the central

nervous system. And the merit would remain, even if the theory, in its present form, should

ultimately prove untenable.

Physiologically, the principle of the connexion of elements implies that every physiological

activity, which is open to our observation and analysis, is composed of a large number of

elementary functions, the nature of which we may, under favourable circumstances, be able to

infer, but which we can never completely isolate from the given complex activity. In particular,

e.g., the physiological process underlying however simple a sensation or muscular contraction

is a complex process, involving the activities of many elementary parts. This may be seen at

once from its physiological analysis in a given case, whether the analysis be applied directly to

the group of actual functions, or whether it be performed inferentially by a study of the

connexions of the elements concerned in them. Our discussions of the act of vision and of the

functions of speech hear witness, in both directions, to this physiological significance of the

principle of elementary connexions.

Lastly, there is a psychological, as well as an anatomical and physiological, formulation of the

principle. It means, psychologically, that the simplest psychical contents discoverable by

analysis of the facts of consciousness always presuppose, as their physiological substrate,

complex nerve processes, the result of the co-operation of many elementary parts. This

complexity of the physical conditions of elementary psychical facts manifests itself in two ways:

first, in psychological observation itself, in so far as the psychical elements, simple sensations

or simple feelings, are always products of psychological abstraction, and never actually occur

except in connexions (a simple colour sensation, e.g., is given as a coloured object in space,

and so on: and secondly, in the physiological fact that no psychical process can be imagined,

however simple it may be, which does not require for its origination a large number of

functionally connected elementary parts. Thus, in the arousal of a sensation of light or tone we

have not only the action of stimulus upon the peripheral structures, but also and invariably the

processes of nervous conduction, the excitations of central elements in the mesencephalic [p.

322] region, and finally certain processes in the cortical centres. If the sensory excitation is of

central origin, as is sometimes the case, -- e.g. in memory images, -- then, conversely, it is first

co-ordinate centres and then peripheral regions that are involved in it. Hence every conscious

contents, though it be as in these instances quite simple, conceived of in isolation from its

connexions, and therefore, psychologically, insusceptible of further analysis, is always,

physiologically considered, a complicated formation made up of various nerve processes

spread over a large number of elementary parts.

(b) -- The Principle of Original Indifference of Functions

The principle of the connexion of elements, in its anatomical and physiological signification,

cannot but suggest the hypothesis that, wherever the physiological functions of the central

elements have acquired a specific colouring, -- recognisable psychologically, say, in the

peculiar quality of a sensation, or physiologically in the release of a muscular contraction or the

origination of a secretory or other chemical process, -- this specific character of their activity is

conditioned not upon the elements themselves, but upon their connexions. The connexions to

which we must thus ascribe a determining influence upon the development of specific functions

are, however, not so much the connexions of the nervous elements with one another, as rather

their connexions, first, with the organs and tissue elements that directly subserve the functions

themselves, and secondly, with the external stimuli by which the organs and elements are

adequately excited. As regards sensation, in particular, which in virtue of its psychological

significance is the most important of all specific functions, the determining factors cannot be

found in the specific energy of any set of nerve fibres or nerve cells, but only in the physical

action of the stimuli upon the sense organs and its immediate consequences, viz., certain

changes in the sensory elements that serve to transmit the stimulation to the sensory nerves. It

is a matter of indifference, for the present argument, whether these elements are themselves

nervous in character, as they are in the olfactory mucous membrane and perhaps in the retina

of the eye, or whether they simply represent epithelial appendages to the nervous system, as

they do in the organs of touch and audition. But if, now, the specific activity of the nervous

elements that belong to a particular sense department is the result of development, if it has

been acquired under pressure of the external conditions of life, then the hypothesis of an

original indifference of function follows of itself. And this principle, once formulated, immediately

suggests the further hypothesis that the functional indifference will have persisted in all cases in

which special conditions have not been at work to produce specific differences. There are, as a

matter of fact, two phenomena which make it extremely probable that such functional

indifference has persisted in a high degree [p. 323] among the central elements. The first is

this: that a fairly long continuance of the function of the peripheral organs is required, if the

sensations of the corresponding sense department are to appear in consciousness. Those who

are born blind or deaf, and even those who have lost the sense of sight or hearing in early

childhood, lack the sensation qualities of light and sound. And these sensation qualities are

evidently wanting at a time when the atrophic degeneration of the central sensory elements,

which results where the functions have been abrogated for a considerable period, cannot yet

have set in.[124] The second phenomenon is this: that the functional disturbances occasioned

by central lesions may be compensated without disappearance of the lesions themselves, i.e.

under conditions that force us to assume a vicarious functioning on the part of other elements.

But this clearly presupposes that, under stress of the conditions of life: the elements may take

on novel functions of a special kind. In such cases, we are able to trace the rise of specific

functions during the lifetime of the individual. We are, of course, bound to suppose that the

conditions of the principal functional differentiations have been operative during the evolution of

the race. Still, the facts just cited prove that we inherit nothing more definite than certain

dispositions, which are given with the connexions of the nervous elements; and that the

development of specific functions demands the actual discharge of these functions, and is

therefore altogether dependent upon the direct action of the vital stimuli during the course of

the individual life. This dependence of the elementary nervous processes upon external

impressions must now be localised, primarily, in the nervous elements that come into closest

contact with the sensory stimuli, i.e. in those situated at the periphery; it may be looked for far

less probably in the more central regions, where, as we have said, substitution and exchange

of functions play a leading part. Since, therefore, the immediate contents of consciousness

always find expression in the elementary qualities originating in direct connexion with the

peripheral functions, everything favours the view that the activities of the higher central

elements consist solely in the effects which they produce by the combination and, under certain

circumstances, by the inhibition of the excitations conveyed to them.

It follows, then, that the principle of the indifference of elementary functions admits, like its

predecessor, of an anatomical, a physiological, and a psychological derivation. Anatomically, it

is supported by the essential identity of structure that we find throughout the elements of the

nervous system. The neurones differ both in form and in extent. But striking as these variations

may sometimes be, the structural differences that they exhibit are, at most, such as indicate

merely a difference in direction of conduction; and even these are apparently confined to the

more highly dif-[p. 324]ferentiated nerve cells (pp. 42 f.). Physiologically, the principle of

indifference of function is attested by the uniform character of the forces that reside in the

nervous elements. The two mutually supplementary forms of energy that we designated, from

their mechanical effects, excitation and inhibition, or positive and negative molecular work (pp.

60, 80), appear throughout as the simple substrate of nervous function. It is true that, in

working out the fundamental ideas of a physiological mechanics of the nervous system, we

allowed ourselves to be guided, in the first instance, by the phenomena of muscle, i.e. of the

mechanical structures appended to that system. At the same time, these peripheral

phenomena came into consideration only in their symptomatic significance. We expressed the

facts in a certain way, at the instance of external conditions. But, mode of expression apart, we

have every reason to assume the essential identity of the nervous processes. Lastly, the

principle derives its principal support, on the psychological side, from the fact that the specific

differences in the sensory contents of consciousness, if they are of an elementary nature, may

always be resolved into qualities of sensation and feeling that depend upon the functions of

peripheral elements. In so far, therefore, as the central nervous system is concerned in the

higher psychical processes, it must be occupied, not with the origination of new specific

qualities, but solely with the indefinitely complex interrelation of these sensory elements of our

mental life.

'Practice,' in the ordinary acceptation of the term, consists in the perfecting of a function by its

repeated performance. Hence the principle of practice, as applied to the functions of the

nervous system, signifies that every central element, whether considered by itself or regarded

as co-operating in some especial way, determined by the conditions of life, with other like

elements, becomes better and better fitted to discharge or to share in the discharge of a

particular function, the more frequently it has been called to its service by pressure of external

conditions. We are already familiar with the elementary phenomenon of practice, in the

increase of excitability by stimulation (p. 75). This elementary phenomenon is common to all

elements of the nervous system: it may be demonstrated even in the isolated nerve, though it is

observed at its best, and its after-effects are: more persistent, in the connected neurones and

neurone chains. We thus have every reason to look upon it as responsible for the marked

changes that are continually taking place, as the result of function, in the nervous apparatus

and their appended organs: changes that, especially if we extend them beyond individual to

generic development, represent the organs themselves as, in large measure, the products of

their functions.[p. 325] The obverse of the practice processes is seen, on the other hand, in the

decrease and ultimate abrogation of functions which, as the result of functional inactivity, is at

the same time connected with degeneration and waste of their morphological substrate (p. 53).

The effect of practice need not be limited to the quantitative enhancement of a given function. It

may lead up to new combinations of elementary processes, by which the qualitative value of a

complex function is altered, and the function itself, in accordance with the general character of

practice, moulded into more complete correspondence with existing conditions. Under these

circumstances, the process of practice is termed 'adaptation.' Adaptation, that is, can never be

anything else than a result of elementary practice processes. At the same time, it is a more

complex process, since it consists, by its very nature, in a number of concomitant practice

courses, which culminate, definitely and purposively, in a single combined result. This complex

character of adaptation makes it possible, further, that increase of practice in a given direction

may coincide with decrease of practice in another, or that certain elements may be gaining in

practice while other elements, alongside of them, are losing. Change in the conditions of life

may thus render certain nerve paths more practicable, and others impassable. And the same

shift of function that appears here, in the closure and improvement of conduction paths, may

occur over whole regions of nerve cells. Hence the adaptations of chief significance for the

nervous functions are those in which newly practised elements take the place of others, whose

activity has been suspended under stress, internal or external, of adverse conditions. In view of

their great importance for the central processes we may group the resulting phenomena under

a special principle of vicarious function.

(d) -- The Principle of Vicarious Function

Whenever it occurs, whether in the central nervous system or in other physiological

departments, the phenomenon of vicarious function is simply a special case of practice and

adaptation. In the present context, it may be termed a limiting case, in the sense that it extends

to functions which the elements involved have never before been directly called upon to

discharge, though they must, of course, have carried within them the latent possibility of their

new offices. Habituation to the requirements of vicarious function may, as the preceding

argument has shown, be brought about in two ways. On the one hand, elements and

complexes of elements which, up to a certain time, performed only some part of a composite

function, may afterwards, owing to the functional disability of the elements correlated with the

remaining phases of the function, take upon them the duties of the whole. On the other hand, a

certain group of elements may be compelled, [p. 326] by the incapacity of other elements with

which they are in some way spatially co-ordinated, to play a part that is altogether strange to

them. We may denominate the first of these cases a substitution by extension of the area of

function, and the second a substitution by aquisition of novel functions. The first form is

conditioned upon the original functional interdependence of departments of the nervous system

that may be widely remote from one another; the second is conditioned, conversely, upon the

spatial connexion of elements between which no original functional relation can be

demonstrated. This spatial connexion may itself consist either in the immediate proximity of

neurones lying upon the same side of the brain, or in the union of distant areas by association

fibres. In the latter case, substitution occurs most often between the symmetrically situated

cortical areas of the two cerebral hemispheres, and is mediated by the association systems that

run from the one half of the brain to the other. This statement applies, e.g., in all probability, to

the functional areas of the speech centres, whose development is usually unilateral (see above,

p. 238).

The first of these two forms of substitution, that by extension of function, appears in general as

a gradual compensation of the disturbances due to the partial impairment of a functional area of

some magnitude, by way of enhancement of activity in other areas which, from the first, took

their share in the same total function. These compensations may proceed from the higher

centres, which, in favourable circumstances, almost entirely annul the disturbances produced

by lesions in the lower; or may, contrariwise, be the work of the lower centres, which to a

certain extent, though never completely, make good the loss sustained by the cessation of

activity in the higher. Instances of compensation of the former kind, resulting from vicarious

function on the part of superior centres, are not uncommon; we have them, e.g., in the gradual

disappearance of the disturbances in cases of injury to the cerebellum, or of lesion of the

diencephalic and mesencephalic region. An example of the second kind is the partial recovery

of functions, normally conditioned upon the co-operation of certain cortical centres, by an

enhanced compensating activity in the diencephalic and mesencephalic centres, such as may

be observed more particularly in decerebrised animals (pp. 259 ff.). For both forms of

substitution, the principle of connexion of elements is, besides that of practice and adaptation,

of primary importance. None of these compensations would be possible, if the central function

were not always divided into a number of partial functions, in each of which there is co-

operation of all the factors necessary to the function as a whole; so that the higher central area

contains the fundamental constituents required for the activity of the lower, and the lower in its

turn those required for the function of the higher. Thus, to all appearances, we have repeated in

the visual cortex, only on a [p. 327] higher plane and in more complicated form, the same

relations of sensory and motor conditions that characterise the mesencephalic portion of the

visual centres (cf. Fig. 78, p. 185). This is the reason that disturbances arising in the latter may

be compensated from the visual cortex, and that, conversely, even cortical defects may, to a

certain extent, be made good by the mesencephalic centres. At the same time, these

compensations are, on the whole, the less likely to occur, in the case of simple functions, the

nearer the lesions approach to the periphery; and are the less complete, in the case of complex

functions, the higher the functional centre that is the seat of the disturbance. Interruption of the

sensory nervous conductions in the myel and in the peripheral nerves is altogether beyond the

reach of compensation by vicarious function. And, on the other hand, the disturbances of sense

perception and of its associative connexions that are caused by destruction of cortical areas

can never be more than imperfectly compensated by habituation of the lower centres.

The case stands very differently with those other forms of vicarious function which have their

source in the spatial connexion, whether direct or mediated by association fibres, of the

nervous elements. Here, the newly habituated parts apparently acquire wholly new functions.

Under these circumstances, it is natural to suppose that the areas concerned are such as,

before their assumption of the vicarious function, discharged no duties whatsoever. Indeed,

there are many authors who take this assumption for granted, -- connecting it, for the most part,

with hypotheses regarding the complex functions of the elements themselves, -- and who

accordingly consider the cerebral cortex, in particular, as a functional department that contains

within it a large number of reserve elements, intrinsically functionless, and provided simply as

substitutes for elements that may have become defective. Now the fringe of teleological ideas

that surrounds this notion of 'functional reserves' would alone make us hesitate to accept it.

But, that apart, the hypothesis runs counter to the fact that functionless elements always evince

a gradual degeneration and decay (Fig. 22, p. 53). Suppose, e.g., that the 'speech centres' of

the right hemisphere were entirely functionless: it would then be impossible to understand why

they do not become altogether atrophied in the course of a long individual life, still more in that

of many generations. This difficulty disappears if, as the principle of the connexion of elements

requires, we hold to the opinion that every complex function presupposes an intricate co-

operation of central elements and their connecting conductions. But then we cannot fail to see

that there is another hypothesis, indefinitely more probable than that which we have just been

discussing: the hypothesis, namely, that, here too, the elements destined to vicarious duty have

always had a certain, only a comparatively unimportant share [p. 328] in the normal function;

and that the substitution consequently consists, again, simply in an enhancement of activities,

along lines already familiar to the elements in their normal state. Looked at from this point of

view, the appearance of aphasia after destruction of the speech centres of the left side would

not imply that the speech functions had their exclusive seat in this left hemisphere, or, as has

sometimes been suggested by those who support the hypothesis of word localisation, that only

certain subordinate word forms, e.g. the interjections, are localised on the right. It is much more

reasonable to suppose that the co-operation of neurone territories, where it is so extraordinarily

complicated as it must be in a function of the complexity of speech, involves wide-spread areas

in both halves of the brain; though, as a matter of fact, the areas of the left hemisphere are

normally the more practised, and their destruction accordingly produces specially obvious

defects. The larger practice of the motor organs on the right as compared with those on the left

hand side of the body furnishes an admirable parallel to this functional habituation. Genetically,

it is, in all probability, related to the unequal development of the speech centres; and factually,

it repeats that inequality point by point. For the organs on the right hand side of the body are

the more practised, but they are not the sole vehicles of function; and just because they are

not, is substitution by new habituations possible.

(e) -- The Principle of Relative Localisation

There can be no doubt that, in a certain sense, the central functions, like those of the peripheral

organs, are spatially distinct. But there can also be no doubt that the central organ, as its

names implies, represents in contradistinction to the peripheral organs a centralisation and

thus, at the same time, an unification of functions; so that any absolute localisation of function,

which should confine each separate activity within fixed limits, is a priori impossible, as it is also

unsupported by the facts of observation. In the peripheral organs, where the demands of

external function have produced diversity of structure, the principle of division of labour is

strictly observed, and the localisation of function follows in the train of its observance. In the

centres of the nervous system, the principle is broken through in two different ways. On the one

hand, every central function divides, and divides the more definitely the higher its place in the

ascending series of activities, into a number of subordinate and auxiliary functions, which of

themselves embrace wide and, in part, widely remote areas of the central nervous system. On

the other, the processes of practice, adaptation and vicarious function show that the spatial

centralisation of a function is not fixed, but dependent upon its exercise, and upon the

conditions under which this exercise is placed, so that any rigid spatial limitation is out of the

ques-[p. 329]tion. The name of 'visual centre,' e.g., is by no means to be restricted to the region

of the occipital brain known as the 'visual cortex.' The. nodal points of the optic conduction in

the lower nerve centres have good right to share in the title, seeing that they are not only

constantly involved in the normal functions of vision, but are also able, to a certain extent, to do

substitute duty for the functions of the higher region. Since, therefore, the principle of vicarious

function, like the principle of practice and adaptation, implies a dependence upon external and

internal conditions which allow the elementary functions a certain freedom of exchange, the

localisation of the central functions at large cannot be looked upon as anything more than

relative, i.e. dependent in this way upon the functional conditions of the time, and varying with

their variation. The principle of localisation thus stands in the closest relation to the principles of

the connexion of elements and of the original indifference of function. For without the connexion

of elements that is required by every, even the simplest form of central activity, and without an

original and, in the case of many central elements, a permanent functional indifference, there

could be no shift of the limits of a function with change in its conditions. In fine, then, the

principle of relative localisation gathers up and includes all the preceding principles, as its

necessary presuppositions; while an absolute localisation of the central functions, such as is

oftentimes assumed, comes into direct conflict with every one of them.

The five principles laid down in the text have gained ground but very slowly in the development

of the theory of the central functions; and even at the present day they have failed, in many

instances, to overcome the opposing views. Their progress was hindered, from the outset, by

the authority of scientific tradition; in some measure, more particularly in the domain of

anatomical and physiological research, it is so hindered still. Rightly to appreciate this

resistance, we must remember that each one of them was obliged, before it could gain

acceptance for itself, to oust a diametrically opposite opinion. These five dogmatic

preconceptions had practical possession of the field; and the advance of modern nerve

physiology has consisted in their gradual refutation, point for point, under compulsion by the

facts.

(1) In opposition to the principle of the connexion of elements many physiologists, even now,

prefer to posit an autonomy of the elements. Their assumption is, not that all contents of

consciousness, even the simplest, presuppose complex functions in which numerous

physiological elements take part, but contrariwise that these physiological elements, the nerve

cells, can mediate extremely complex psychical functions. Thus, a single nerve cell may,

according to circumstances, be the vehicle of a sensation or of a compound idea, a concept.

The hypothesis is very seriously intended: its supporters have been at the pains to estimate the

number of ideas that, on emergency, may be lodged in an individual consciousness, by

counting the number of cells in the cerebral cortex.[125] Some-[p. 330]times, it is true, an

attempt is made to mitigate its crudity by the remark that it is merely 'provisional' and 'tentative.'

But even in this modified form it lacks all justification. For a provisional hypothesis is of use only

so long as it groups the known facts together in a formula that can further the progress of

investigation. If, on the other hand, the hypothesis points us to a road that undoubtedly leads in

a direction diametrically opposed to the truth, then it has become nothing else than a pernicious

prejudice.

(2) In opposition to the principle of indifference of function it is generally held, at the present

time, that the law of specific energy, as it is termed, represents an especially valuable asset of

modern nerve and sense physiology. Nevertheless, the history of the law shows a gradual

regression. It slowly withdrew from the elements where advancing investigation had proved it to

be untenable, to other elements whose functional attributes were less thoroughly worked out;

and it has finally intrenched itself in those whose real differences of structure and function

warrant the assumption of distinct modes of activity. First of all, a specific function was

attributed to the nerve fibres. Then, as physiologists gradually became accustomed to consider

the nerves as relatively indifferent conductors of the nervous processes, the nerve cells were

selected as the vehicles of the specific functions. To-day, when the uniform character of these

central elements also is impressing itself more and more strongly upon the investigator, we

may confidently predict that the specific function will continue on its travels until it ultimately

comes to rest in the peripheral sensory elements. Here it may, with a certain conditional

propriety, be allowed to remain; whereas the central elements come into account only

indirectly, in so far, i.e., as the influences of practice and adaptation play a part among them.

[126]

The law of specific energy still holds its own in current scientific thought. More headway has

been made against the dogmas that stand out in contrast and opposition to the principles of

practice and adaptation and of vicarious function: the hypotheses (3) of the originality and (4) of

the immutability of functional attributes. Neither possesses any present power in nerve

physiology, save as it is properly a logical consequence of the strict interpretation of the law of

specific energy. The numerous facts that bear witness to the influences of practice, adaptation

and vicarious function will not be deprived of their rights. But this merely aggravates the division

in modern physiological theory, and shows that it cannot maintain itself for any length of time.

The same intrinsic unsoundness appears (5) in the hypothesis of an absolute localisation,

which still persists in opposition to the principle of a merely relative localisation. It is the

connexion of this hypothesis with those of specific energy and of the more or less complex

character of the function of the nervous elements that constitutes the logical foundation of the

phrenological edifice. And the constantly recurring tendency to a revival of phrenology in some

modified form, adapted to the conceptions of current anatomy, physiology and psychology, is

therefore an evidence not only of the extreme vitality of these dogmas, but also of their internal

connexion. Indeed, if we grant the specific energy of the elementary functions, we have at the

same time admitted their originality, their constancy, their complex character and their absolute

localisation. Conversely, all these assumptions fall to the ground, as soon as we recognise the

just claims of practice and adaptation. For then we must grant the possibility, within certain

limits, [p. 331] of vicarious function; and from this admission follow, in the last resort, a merely

relative localisation of functions and, as regards the elements, an original indifference, which by

the great majority of the central elements has in all probability been retained as a permanent

possession. The five principles discussed above thus form an interdependent whole, no less

than the five antagonistic dogmas of the older nerve physiology. That a view of the central

functions which accords with the general status of our physiological and psychological

knowledge should meet, even to-day, with serious opposition is, perhaps, sufficiently explained,

first, by this very interdependence of the traditional doctrines, in which each assumption serves

to support the rest; again, by the simple formulation of the older theories, as compared with the

larger demands made by the new upon the physiological and psychological analysis of the

phenomena; and, lastly, by the natural longevity of ingrained prejudice. To suppose that the

specific contents of a sensation is given of itself with the existence of a central element; or that

the act of vision is completed with the 'projection' of the retinal image upon a central sensory

surface; or that 'memory for words,' 'intelligence,' and what not, that figure in popular

psychology as simple and undivided concepts, are localised as simply in sharply circumscribed

regions of the brain: to suppose all this is, naturally, very much easier than it is to work out the

conclusions that follow from the five principles enunciated in the text. But apart from the

objection that these various suppositions run counter to the facts, they are once and for all

impossible, for the reason that they rest upon a wholly untenable psychology, upon erroneous

physiological ideas, and, in the last instance, upon an antiquated conception of the structure of

the nervous system. If, therefore, the anatomists are still to be found to-day among the most

zealous champions of the phrenological view, they have really atoned for this fault in advance,

by contributing the most valuable materials from which others may construct a tried and tested

theory of the central functions.

Notes

[1] See above, pp. 43, 99.

[2] H. E. HERING and SHERRINGTON, in PFLÜGER'S Arch. f. d. ges. Physiol., lxviii., 1897,

222.

[3] See above, p. 88.

[4] See above, Ch. iii., pp. 80, 94, ff.

[5] MOTT and SHERRINGTON, Proc. of the Royal Soc., lvii., 1895, 481: C. BASTIAN, ibid.,

lvii., 89.

[6] SHERRINGTON, in THOMPSON YATES' Labor. Reports, i., 45, 175.

[7] These and the other mimetic reflexes are of great psychological importance, and are

accordingly discussed under the hearting of expressive movements in Part IV., Ch. xvii.

[8] For the relative autonomy of the cardiac movements, see especially T. W. ENGELMANN. in

PFLÜGER'S Arch. f. d. ges. Physiol., lvi., 1894, 149; for the autonomous functions of the

movements that fall within the sphere of the spinal reflexes, see GOLTZ and EWALD. ibid..

lxiii., 1897, 362.

[9] NOTHNAGEL, in VIRCHOW'S Archiv., xliv., 4. BINSWANGER, Arch. f. Psychiatr., xix., 759.

[10] See above. Ch. i., pp. 29 ff., and below, Chs. xvii., xviii., on Will and Consciousness.

[11] GOLTZ and FREUSBERG, in PFLÜGER'S Arch. f. d. ges. Physiol., xiii. 1876. 460.

[12] MASIUS, Bulletin de l'académia de Belge, xxiv., xxv., 1867, 1868.

[13] HEIDENHAIN, Physiol. Studien, 1856, 9. WUNDT, Lehre von der Muskelbewegung, 1858,

51 f.

[14] KUSSMAUL and TENNER, in MOLESCHOTT'S Untersuchungen zur Naturlehre des

Moonstone, iii., 1857, 77.

[15] LUCHSINGER, in PFLÜGER'S Arch. f. d. ges. Physiol., xiv., 1877, 383.

[16] MOSSO, Ueber den Kreislauf des Blutes im menschlichen Gehirn, 1881, 74 ff.

[17] Cf. with this the discussion of the psychology of dreams. Part V., Ch. xx. The excitatory

influence of the dypnoeic blood, mentioned in the text, is confirmed by the fact that other forms

of automatic or reflex stimulation -- dyspnoeic spasms, epileptiform twitches, and the like -- are

especially apt to occur during sleep.

[18] WERNICKE, Lehrbuch d. Geirnkrankheiten, ii., 10. KRAEPELIN, Psychiatrie, 6te Aufl., i.,

54.

[19] WOLFF, Allg. Zeitschr. f. Psychiatrie, xxvi., 273. ZIEHEN, Sphygmographische

Untersuchungen bei Geisteskranken, 1887.

[20] For this psychological aspect of mental derangement, and for an account of sleep and of

similar states (hypnotism), see Part v., Ch. xx.

[21] Here belong more particularly the experiments of LONGET (Anatomie et physiologie du

système nerveux, 1842; German trans. by HEIN, i., 385); SCHIFF (Lehrbuch d. Physiol., i.,

342); VULPIAN (Physiol. du système nerveux, 658), and others. These experiments will always

retain a place of honour in the history of the experimental physiology of the central organs. But,

from the modern point of view, they must be pronounced antiquated; if only for the reason that

they attempt to ascertain the functions of the parts by a purely symptomatic method, from the

phenomena of abrogation and stimulation, without regard to morphological relations and the

course of the conduction paths.

[22] FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed., 1842.

[23] GOLTZ, Beitrage zur Lehre von den Functionen der Nervencentren des Frosches, 1869.

[24] CHRISTIANI, Zur Physiologie des Gehirns, 1885.

[25] GOLTZ, Der Hund ohne Grosshirn, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 1892, 570.

[26] J. STEINER, Die Functionen des Centralnervensystems, Abth. 1-4, 1885-1900.

[27] J. STEINER, Die Functionen des Centralnervensystems, ii., 211 ff.

[28] GOLTZ, Die Functionen der Nervencentren des Frosches, 65.

[29] SCHRADER, in PFLÜGER'S Arch. f. d. ges. Physiol., xli., 1887, 75.

[30] SCHRADER, in PFLÜGER'S Arch. f. d. ges. Physiol., xliv., 1888, 175.

[31] CHRISTIANI, Zur Physiologie des Gehirns, 25.

[32] GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 570 ff.

[36] FERRIER, Functions of the Brain, 1886, 412. NOTHNAGEL was unable to discover any

marked symptoms whatsoever, even after extensive lesions of the thalami. See VIRCHOW'S

Archiv. lviii., 429 and lxii., 203.

[33] BECHTEREW, in PFLÜGER'S Arch. f. d. ges. Physiol., xxxiii., 413.

[34] STEINER, Die Functionen des Centralnervensystems, iii., 79 ff.

[35] SCHIFF, Lehrbuch d. Physiol., ii., 343.

[37] STEINER, op. cit., ii., 106; iii., 126; iv., 54 ff.

[38] GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., li., 520 ff.

[39] NOTHNAGEL, Topische Diagnostik, 204 ff. VON MONAKOW, Gehirnpathologie, 586 ff.

[40] WERNICKE, Lehrbuch der Gehirnkrankheiten, i., 370.

[41] NORTHNAGEL, in VIRCHOW'S Archiv, lvii., 209.

[42] MAGENDE,[sic] Leçons sur les fonctions du système nerveux, i., 280. Cf. also SCHIFF

Lehrbuch d. Physiol., i., 340.

[43] NOTHNAGEL, Topische Diagnostik, 262 ff. VON MONAKOW, Gehirnpathologie, 584.

[44] FLECHSIG, Plan des menschl. Gehirns, 41.

[45] LUCIANI, Il cerveletto, nuovi studi, 1891, 49. The results of these experimental

investigations, the most detailed made upon the cerebellum, serve in general to confirm the

statements of NOTHNAGEL (VIRCHOW'S Archiv, lxviii., 33), FERRIER (Functions Of the

Brain, 174), and BECHTEREW (PFLÜGER'S Arch. f. d. ges Physiol., xxxix., 362), as well as

the older observations of SCHIFF (Lehrbuch d. Physiol., i., 353). The only point upon which

there is some divergence of opinion concerns the direction of the imperative movements set up

by unilateral transsections. The differences are probably due to the fact that the cerebellar

peduncles were cut at different places.

[46] LUCIANI, op. cit. 32. LADAME, Hirngeschwülste, 93. WERNICKE, Gehirnkrankheiten, iii.,

353. VON MONAKOW, Gehirnpathologie, 624.

[47] In a case in which the cerebellum and pons were entirely lacking, voluntary movements

were possible, but there was pronounced muscular weakness; the patient frequently fell, and

her intelligence was extremely defective (LONGET, Anatomie et physiologie du système

nerveux, i., 764). Observations by KIRCHHOFF, on certain cases of atrophy and sclerosis of

the cerebellum, confirm this account upon all essential points (Archiv f. Psychiatrie, xii., 647 ff).

In a case of HITZIG'S, where, it is true, atrophy was only partial, the intelligence was affected,

but there was no disturbance of movement. HITZIG himself supposes that the symptoms

indicate a large measure of vicarious functioning, especially by parts of the cerebrum (ibid., xv.,

266 ff.).

[48] PURKINJE, in KUST'S Magazin d. Heilkunde, xxiii., 1827, 297. HITZIG, Das Gehirn, 196 ff.

Der Schwindel, 1898 (off-printed from NOTHNAGEL'S Pathologie, ix.), 36 ff.

[49] FLOURENS, LUSSANA and RENZI also observed effusion of blood in the cerebellum as a

result of intensive alcoholic poisoning; see RENZI in SCHMIDT'S Jahrbuch, cxxiv., 158.

[50] See Part III., Ch. xiii.

[51] BECHTEREW, Die Leitungsbahnen im Gehirn und Rückenmark, 361.

[52] VON LEYDEN and GOLDSCHEIDER. Die Erkrankungen des Rückenmarks. In

NOTHNAGEL'S Handbuch d. Pathologie, x., 149.

[53] LUSSANA, Journal de la physiol., v., 418; vi. 169. LUSSANA and LEMOIGNE, Fisiologia

dei centri nervosi, 1871, ii., 219.

[54] FLOURENS, Recherches expérimentales, 2nd ed., 28.

[55] In view of the close connexion of the olives with the conduction paths of the cerebellum

(see pp. 170 ff.), it is readily intelligible that injury to these centres should produce motor

disturbances akin to those set up by injury to the cerebellum itself. Such disturbances have, as

a matter of fact, been observed by BECHTEREW (PFLÜGER'S Arch. f. d. ges. Physiol., xxx.,

257). BECHTEREW found, further, that similar disturbances of equilibrium uniformly result from

injury to the walls of the diacele (ibid., xxxi., 479).

[56] LUCIANI, Il cerveletto, Germ. trans., 282.

[57] FERRIER, in Brain, xvii., 1894, 1 ff.

[58] Edition of 1874, 220.

[59] KAHLER and PICK, Beiträge zur Pathol. u. pathol. Anat. d. Centralnervensystems, 1879.

58.

[60] HITZIG, Der Schwindel, 42 ff.

[61] Cf. the discussion of the voluntary actions. Part IV., Ch. xvii.

[62] GALL, Anatomie et physiologie du système nerveux, iii., 1818, 85. COMBE, On the

functions of the cerebellum, 1838.

[63] R. WAGNER, Göttinger Nachrichten, 1860, 32.

[64] LUSSANA. Journal de la physiologie, v., 140.

[65] On phrenology in general, see below, § 6.

[66] LUCIANI, Il cerveletto, Germ. trans., 198. Cf. also FERRIER, Functions of the Brain, 178.

[67] LADAME, Hirngeschwülste, 99.

[68] Cf e.g., NOTHNAGEL, Topische Diagnostik, 78 ff. VON MONAKOW, Gehirnpathologie,

635 ff.

[69] LADAE. Hirngeschwülste, 186 f. NOTHNAGEL, Topische Diagnostik der

Gehirnkrankheiten, 435 ff. VON MONAKOW, Gehirnpathologie, 376 ff.

[70] GOLTZ, in PFLÜGER'S Arch. f. d. ges. Physiol., xlii., 1886, 484.

[71] LOEB, in PFLÜGER'S Arch. f. d. ges. Physiol., xxxiv., 1884, 67.

[72] GOLTZ, ibid., lvi., 1899, 411.

[73] GOLTZ, ibid., xxxiv., 1884, 450; xlii., 1888, 439.

[74] LONGET, Anatomie et physiologie du système nerveux, Germ. trans. by HEIN, i., 539 ff.

[75] Cf. the discussions of the centres of vision, speech and apperception, § 7, below.

[76] See below, Parts IV., V.

[77] LEURET and GRATIOLET, Anatomie comparée du système nerveux, ii., 290. OWEN,

Anatomy of Vertebrates, iii.

[78] HUSCHKE, Schädel, Hirn und Seele, 60. H. WAGNER, Massbestimmungen der

Oberfläche des grossen Gehirns, 1864, 33.

[79] TIEDEMAN, Das Hirn des Negers mit dem des Europäers und Orang-Utangs verglichen.

1837. BROCA, Mémoires d'anthropologie, 1871, 191.

[80] GALL and SPURZHEIM, Anatomie et physiologie du système nerveux, ii., 251.

[81] R. WAGNER, to whom we owe these reproductions, and those of certain other brains of

eminent men (Dirichlet, C. Fr. Hermann, etc.), was himself somewhat hesitant about drawing

this conclusion (Göttinger gel. Anz., 1860, 65; Vorstudien zu einer wissenshaftl. Morphologie

und Physiologie des Gehirns, 1860, 33). C. VOGT, however, justly remarks that it follows,

without any question, from WAGNER'S own figures, if we select from them the illustrations that

really apply to individuals of acknowledged mental pre-eminence. See also BROCA, Mémoires

d'anthropologie, 155. For the rest, it need not be pointed out that, here as elsewhere, the

concurrent factors of race, height, age, sex, must be taken into consideration. A normal

Hottentot brain in the skull of an European would, as GRATIOLET observed, spell idiocy.

[82] BOUILAUD, Recherches cliniques propres à demontrer que la perte de la parole

correspode à la lésion des lobules antérieures du cerveau et à confirmer l'opinion de M. GALL

etc. In Arch. gén. de méd., viii., 1825.

[83] FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed. 1842.

[84] See Ch. vii. below.

[85] DESCARTES, Les passions de l'ame, I. See above, p. 124.

[86] See the list in HALLER, Elementa physiologioe, iv., 1762, 397.

[87] See the historical critique of the doctrine of irritability in the author's Lehre von der

Muskelbewegung, 1858, 155.

[88] Elem. physiol., iv., 395.

[89] Ibid., 397.

[90] "Hypothesin esse video et fateor," he cautiously adds. Ibid., 399.

[91] GALL and SPURZHEIM, Anatomie et physiologie du système nerveux, i., 1810. Cf. also

the same authors' Recherches sur le système nerveux, etc., 1809 (contains the memoir

presented to the French Institute and the report of the Commissioners). GALL'S two main

services to brain anatomy are his introduction of the method of dissection from below upwards,

and his demonstration of the universally fibrous character of the alba.

[92] GALL'S system is set forth in detail in vols. ii.-iv. of the Anatomie et physiologie.

[93] COMBE, System of Phrenology. 1825 (Germ. trans. by HIRSCHFELD, 1833, 101 f.).

[94] Anat. et physiol., iv., 341.

[95] Ibid., i., 271; ii., 372.

[96] P. J. MÖBIUS, Franz Joseph Gall, in SCHMIDT'S Jahrbuch d. Medicin, cclxii., 1899, 260.

[97] P. J. MÖBIUS, Ueber die Anlage zur Mathematik: mit 51 Bildnissen, 1900.

[98] A criticism of the teachings of phrenology from the standpoint of comparative anatomy is to

be found in LEURET, Anatomie comparée du système nerveux, i.; a criticism based upon the

writer's physiological experiments, in FLOURENS' Examen de la phrenologie, 1842.

[99] MAGENDIE, Leçons sur les fonctions du système nerveux. 1839.

[100] FLOURENS, Recherches expér. sur les fonctions du système nerveux, 2nd ed. 1842.

[101] Cf., e.g., BURDACH, Vom Bau und Leben des Gehirns, iii. ARNOLD, Physiologie, i., 836.

HUSCHKE, Schädel, Hirn und Seele, 174.

[102] BROCA. Sur la siège de la facult du language, 1861.

[103] Cf. especially his discussions in PFLÜGER'S Arch. f. d. ges. Physiol., xx., 1879, 10 ff.

[104] Cf. Part II., Ch. viii.

[105] See below, Part III., Ch. xiv., on visual ideas.

[106] WERNICKE, Der aphasische Symptomencomplex, 1874.

[107] LICHTHEIM, Brain, a Journal of Neurology, vii., 1885, 437. A still simpler plan, including

merely the motor and sensory centres, was suggested by WERNICKE (op. cit.); it forms the

basis of LICHTHEIM'S schema, and is represented in Fig. 103 by the unbroken lines. A plan

resembling LICHTHEIM'S, and factually accordant with it, but somewhat more complicated in

outward appearance, had been drawn some years earlier by KUSSMAUL: Die Störungen der

Sprache, 1877, 183.

[108] Cf. with this discussion the critical remarks in my Völkerpsychologie, i., 1, 1900, 495 ff.

[109] S. FREUD, Zur Auffassung der Aphasien, 1891. STÖRRING, Vorlesungen über

Psychopathologie, 1900, 127 ff.

[110] KUSSMAUL, Störungen der Sprache, 163 f.

[111] GRASHEY'S case, Archiv f. Psychiatrie, xvi., 694 ff. This case, which is one of great

interest, has been further investigated by R. SOMMER (Zeits. f. Psych. U. Physiol. d.

Sinnesorgane, ii., 143) and G. WOLFF (ibid., xv., 1 ff.). Cf. the detailed discussion in my

Völkerpsychologie, i., 1, 1900. 502 ff., and in STÖRRING, Vorlesungen über Psychopathologie,

132 ff. A survey of the very extensive modern literature on aphasia and amnesia is given by O.

VOGT, Zeits. f. Hypnotismus, vi., 1897, 215, 266 ff. Cf. also PICK, Arch. f. Psychiatrie, xxviii., 1

ff., and C. BASTIAN, Aphasia and Other Speech Defects, 1898.

[112] Cf. the fuller psychological discussion of the processes of association in Part V., Ch. xix.

below.

[113] This schema is taken from my Völkerpsychologie, i., 1. 1900, 519 ff. (section on the

psychical structure of word ideas). The reader is referred to this passage for further explanation

the Figure.

[114] Cf. the cases collected by CHARCOT and PITRES (Revue mensuelle. Nov., 1877),

FERRIER (Localisation of Cerebral Disease, 1878), and DE BOYER (Études cliniques, 40, 54);

also BIANCHI, in Brain, xviii., 1895, 497.

[115] The report is printed by FERRIER, op. cit.

[116] Cf. DE BOYER, 45, obs. iv.; 55, obs. xxvii. VON MONAKOW. Gehirnpathologie, 491 ff.

The latter author

reports a similar case from the Zurich clinic, and cites other analogous observations of

JASTROWITZ.

[117] MEYNERT, Vierteljahrsschrift f. Psychiatrie, 1867, 166.

[118] FLATAU and JACOBSOHN, Handbuch d. Anat. u. vergl. Anat. d. Centralnervensystems

d. Säugethiere, i., 536 ff. MARCHAND, Die Morphologie des Stirnlappens, in Arbeiten des

pathol. Instituts zu Marburg, ii., 1893.

[119] H. WAGNER, op. cit.

[120] The reader may refer, for purposes of comparison, to the figures of the brains of GAUSS

and of an individual of moderate intelligence, Figg. 100, 101, p. 285, above.

[121] ZIEHEN, Leitfaden der physiol. Psychologie. 5te Aufl., 195; Introduction to Physiological

Psychology, 1895, 231.

[122] VON MONAKOW, Gehirnpathologie, 492.

[123] Part IV., Ch. xvii.; Part V., Ch. xviii.

[124] See p. 53 above; and cf. Part II., ch. viii., below.

[125] MEYNERT, Vierteljahrsschrift f. Psychiatrie, i., 1867, 80. H. MUNK, Ueber die Functionen

der Grosshirnrinde, Einleitung, 9.

[126] For the limitations to be put upon the law of specific energy in its application to the

peripheral sensory elements, cf. below Part III., ch. viii., § 4.

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