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Full text of "Principles of physiological psychology"

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THE LIBRARY 

OF 

THE UNIVERSITY 
OF CALIFORNIA 

LOS ANGELES 



VOL. I. 

INTRODUCTION 

;>ART I. THE BODILY SUBSTRATE OF THE MENTAL 

LIFE 



SOME OPINIONS OF THE PRESS. 

" By this excellent translation Professor Titchener has con/erred a rare sen-ice on 
English students of philosophy." Bookman. 

" A profound and suggestive work." Pall Mall Gazette. 

" Professor Titchener has done his part of the work with notable success. Wundt's 
style is laboured and complex even for a German, and it was no easy task to render his 
lengthy and parenthetical sentences into good English." Glasgow Herald. 

" A more fully equipped translator than Professor Titchener could not be found, 
and he has laid all psychological students and investigators under a deep debt of grati- 
tude." Scotsman. 

" We congratulate Professor Titchener on his success in the difficult task of translat- 
ing the first part of Professor Wundt's book. It is practically essential for every 
serious student of modern psychology to be acquainted with Wundt's work, and this task 
will be greatly facilitated by the present translation." British Medical Journal. 

." It is the most complete and by far the most philosophic account of the nervous 
system of man that can be anywhere obtained. The reader who cares to compare 
this work with the three volumes of such a standard work as Quain's Anatomy, that 
deal with the central nervous system, the nerves and the organs of the senses, will at once 
see the difference between the work of the average first-class expert and that of a master. 
Dr. C. W. Saleeby in Occult Review. 



PRINCIPLES OF 
PHYSIOLOGICAL PSYCHOLOGY 



BY 

WILHELM WUNDT 

PROFESSOR OF PHILOSOPHY IN THE UNIVERSITY OF LEIPSIC 

Translated from the Fifth German Edition (1902) 

BY 
EDWARD BRADFORD TITCHENER 

SAGE PROFESSOR OF PSYCHOLOGY IN THE CORNELL UNIVERSITY 



VOL. I. 

WITH 105 FIGURES IN THE TEXT 




LONDON 

SWAN SONNENSCHEIN & CO. LIM. 

NEW YORK : THE MACMILLAN CO. 

1910 



FIRST ENGLISH EDTTION, December 1904. 
SECOND EDITION, December 1910. 



\MU 



Author's Preface to the First Edition 

THE work which I here present to the public is an attempt to mark out a 
new domain of science. I am well aware that the question may be raised, 
whether the time is yet ripe for such an undertaking. The new discipline 
rests upon anatomical and physiological foundations which, in certain 
respects, are themselves very far from solid : while the_experimental treat- 
ment of psychological problems must be pronounced, from every point of 
view, to be still in its first beginnings. At the same time the best means 
of discovering the blanks that our ignorance has left in the subject matter 
of a developing science is, as we all know, to take a general survey of its 
present status. A first attempt, such as this book represents, must show 
many imperfections ; but the more imperfect it is, the more effectively 
will it call for improvement. Moreover, it is especially true in this field 
of inquiry that the solution of many problems is intimately bound up with 
their relation to other groups of facts, facts that often appear remote and 
disconnected ; so that the wider view is necessary, if we are to find the 
right path. 

In many portions of the book I have made use of my own investiga- 
tions ; in the others, I have at least tried to acquire an independent judg- 
ment. Thus, the outline of the anatomy of the brain, contained in Part I, 
is based upon a knowledge of morphological relations which I have obtained 
by repeated dissection of human and animal brains. For part of the 
material employed in this work, and for frequent assistance in the diffi- 
culties which such a study offers, I am indebted to the former Director of 
the Heidelberg Anatomical Museum, Professor Fr. Arnold. The finer 
structure of the brain, as revealed by the microscope, is, of course, a subject 
for the specialist ; all that I have been able to do is to compare the state- 
ments of the various authors with one another and with the results of the 
gross anatomy of the brain. I must leave it to the expert to decide whether 
the account of the central conduction paths, as drawn from these sources 
in. chapter iv., is, at least in its main features, correct. I am fully conscious 
that, in detail, it requires to be supplemented and emended on many sides. 
Still, it receives a certain confirmation from the fact that the functional 
derangements induced experimentally by the extirpation and transsection 
of various parts of the brain are, as I seek to show in chapter v., readily 
explicable in terms of the anatomical plan. Most of the phenomena here 
described I have had frequent opportunity to observe in my own experi- 






vi Author's Preface to First Edition 

ments. In chapter vi. I have brought together the results of my Unter- 
snchitngcn znr Mechanik dcr N erven und Nervenceniren, 1 so far as these 
relate to the question which is one of psychological importance regard- 
ing the nature of the forces operative in the nervous elements. 2 

Parts II and III are concerned with the topics that first drew me, many 
years ago, to psychological studies. When m 1858 I began to work upon 
my Beitrdge zur Theorie der Sinncswahrnehmung, 3 German physiology 
was dominated, almost exclusively, by nativistic conceptions. My prin- 
cipal purpose in writing that work was to demonstrate the insufficiency of 
currrnt hypotheses regarding the origin of our spatial ideas of touch and 
sight, and to discover a physiological basis for a psychological theory. 
The views there set forth have since found general acceptance among 
physiologists as well as among psychologists ; though in the form which 
they have usually taken in the physiologies they could, perhaps, hardly 
hold their own against a rigorous criticism. I hope that, in the present 
work, I have succeeded in showing the inadequacy of modern physiological 
empiricism, as \\vll as the relative justification for nativism and the necessity 
with which both conceptions alike point to a more profound psychological 
theory. The hypothesis of specific sensory energies, which is really a 
survival from the older nativism, has, in my opinion, become untenable, 
despite the convenient explanation it affords of a large body of facts. My 
critical treatment of this subject will, no doubt, call forth many objections. 
But if the facts are viewed as a whole, the cogency of the argument will 
hardly be disputed. 

The investigations of Part IV, 4 especially the experiments on the appear- 
ance and course in consciousness of the sensory ideas aroused by external 
impressions, have occupied me for fourteen years, though, it is true, with 
many interruptions, due to other work and to the necessity of procuring 
appropriate apparatus. The first results were presented, as early as 1861, 
to the Natural Science Conference at Speyer. Since that time a number 
of notable papers on the same subject have been published by other in- 
vestigators. No one, however, has hitherto turned these results to account 
for a theory of consciousness and of attention. I have here sought to give 
this important chapter of physiological psychology at any rate a tentative 
systematic setting. 

Finally, I would ask the reader, when he comes upon polemical passages 
directed against Herbart, to remember that my criticisms are, at the same 
time, a proof of the importance which I attach to the psychological works 

1 Erlangen, 1871. A second volume followed later : Stuttgart, 1876. Translator. 

2 Chapters iv. and v. of the previous editions represent chapters v. and vi. of the 
present edition. What was formerly chapter vi. now becomes chapter iii. 

3 Leipsic and Heidelberg, 1862. Translator. 
* Now Part V. 



Author's Preface to First Edition vii 

of this philosopher. It is to Herbart, next after Kant, that I am chiefly 
indebted for the development of my own philosophical principles. So 
with regard to Darwin ; while I have, in one of the concluding chapters, 
opposed Darwin's theory of expressive movements, I need hardly say that 
the present work is deeply imbued with those far-reaching conceptions 
which, by his labours, have become an inalienable possession of natural 
science. 

W. WUNDT. 
HEIDELBERG, March, 1874, 



Author's Preface to the Fifth Edition 

WHEN this book first came before the world, nearly eight and twenty years 
ago, the status of the science for which it hoped to prepare a place was 
very different from that of the physiological psychology of to-day. At 
that time only one successful attempt had been made in Fechner's Elemente 
der Psychophysik to throw the light of an exact procedure upon philo- 
sophical problems that might, in the last resort, be regarded as psycho- 
logical. Fechner apart, the adventurer of an ' experimental psychology ' l 
was still reduced, in most instances, to borrow what he could from other 
disciplines, especially from the physiology of sense and nervous system. 
To-day all this is changed ; there is pouring in from all sides from the 
psychological laboratories proper, from neighbouring disciplines, from 
every science thai comes into contacl with psychological problems -an 
amount of expository material that, even now, is hardly calculable. At 
that time the investigator who sought to employ accuracy of method in 
any question of psychology was challenged at every point, by philosophy 
as by natural science, to prove that his endeavours were legitimate. To- 
day these doubts are hardly to be feared. But, to offset our advantage, 
thnv have appeared within psychology itself strongly divergent tendencies, 
some ol which cover profound differences of principle regarding the prob- 
lems and aims of the science, and the paths that it should pursue. 

From edition to edition of the present work, as I have attempted to 
adapt each successive revision to its altered circumstances, these changes 
of the times have been to me a source of ever-increasing difficulty. Honce, 
when the need arose for this fifth edition, I was strongly inclined to close 
my account with the book, and to leave it in the form which it had finally 
taken, unsatisfactory as I now felt that form to be. But, tempting as 
the idea was for many reasons, there was one paramount objection. The 
former edition contained many passages which I could not allow to pass 
as an adequate expression of my present convictions : for I would be the 
last to refuse, in the onward endeavour of our youthful science, to learn all 
that I can from new experiences, and in their light to better my theories. 
I resolved, accordingly, at least to put the book into such a shape that 
these discrepancies should, so far as possible, be done away with. How- 
ever, I soon found that this plan could not be carried out ; the result would 

This phrase appears to have been introduced by Wundt : see Beitidee 1862 
Vorrede. vi. Translator. 



Author's Preface to Fifth Edition ix 

be, after as before, a book that, in all probability, should satisfy the reader 
as little as it satisfied myself. And so, almost unawares, the new edition 
has become practically a new work. My principal purpose in this thorough 
recasting of the material has been not so much to give a complete survey 
of the entire literature of the subject, in its manifold branches, the 
numerous journals that are now published in the interests of experimental 
psychology render this an easy task for any one who will undertake it, 
as rather to present, in more adequate form and (where it seemed desirable) 
with greater detail of proof than had appeared in previous editions, those 
experiences and those interpretations of experience which had corne to 
me in the years of helpful association in research with all the younger 
investigators who have worked in the psychological laboratory at Leipsic. 
In its present form, therefore, the book is intended, first and foremost, to 
serve the purpose not of a compilation, but of an exposition of my own 
experiences and convictions ; though I have, of course, everywhere made 
grateful use of whatever I could take from the works of others. 

Although the text has been curtailed to the utmost, where curtailment 
v/as possible, particularly by the omission of a number of argumentative 
passages, directed against opinions and theories, current in philosophy 
or in the older psychology, which may now be regarded as obsolete, the 
change of programme has brought with it an increase in the bulk of the 
work. The two volumes of the previous editions have now become three. 
Volume ii. will contain the conclusion of the doctrine of mental elements, 
and the theory of ideas; volume iii., Parts dealing with emotion and 
voluntary action and with the interconnexion of mental processes, together 
with a closing chapter of philosophical import. 1 Dr. W. Wirth has under- 
taken the preparation of an index of names and subjects, to be included 
in this last volume. 2 I have also to thank Dr. Wirth for assistance in 
reading the proof-sheets. 

W. WUNDT. 

LEIPSIC, February, 1902. 

1 The present volume contains the Introduction and Part I, On the Bodily Sub- 
strate of the Mental Life. The remaining Parts are entitled as follows : Part II, Of 
the Elements of the Mental Life ; Part III, Of the Formation of Sensory Ideas ; Part IV, 
Of the Affective Processes and of Voluntary Actions ; Part V, Of the Course and the 
Connexions of Mental Processes ; Part VI, Final Considerations. Translator. 

a Now (1903) published separately. Translator. 



Translator's Preface 

WHEN I went to Leipsic in 1890, I carried with me a completed translation 
of the third (1887) .edition of the Grnndzilge der physiologischen Psychologic. 
I spent nearly a year upon its revision, and did not mention it to the author 
until the late summer of 1891. Professor Wundt took my presumption 
very kindly ; but the fourth edition was already on the horizon, and my 
manuscript was never offered to a publisher. 

I had not, however, given up the idea of a translation. As soon as 
other engagements allowed at the end of 1896 I set to work upon the 
edition of 1893. The work was finished, except for final revision, in 1899. 
But I found, on going over the first volume for the press, that certain 
chapters, especially those dealing with embryology and neurology, must 
be corrected and brought up to date. A year went by, with nothing to 
show for it but the writing of footnotes and additional paragraphs ; and 
when I was again ready, the fifth edition was in prospect for the immediate 
future. 

I fear that apart from my rather dearly bought experience, which 
should have profited me something I the_present translation is the worst 
of the Jhree. I might plead in excuse that one does not undertake the 
task of translating a large work for the third time and in mature life with 
the enthusiasm that one brings to it as a young student. I might also 
plead that the publishers, disappointed in the matter of the fourth edition, 
and naturally anxious, in any event, to bring out the translation as soon 
as possible after the appearance of the original, have put some little pressure 
upon me, though always of the friendliest kind, to get the work done out 
of hand. On the whole, however, I prefer to rest my case upon the diffi- 
culties of the book itself. Wundt 's style has often, of late years, been 
termed diffuse and obscure. I should not care to call it either of these 
things ; but I am sure that it is difficult. It has, perhaps, in a somewhat 
unusual degree, the typical characteristics of scientific German; the care- 
lessness of verbal repetitions, the long and involved sentences, the lapses 
into colloquialism, and what not. It has, besides, two special difficulties. 
The one- is intrinsic : Wundt, if I read him aright, has always had the habit 
of thinking two or three things at once, of carrying on certain secondary 
trains of thought while he developes his central idea ; and the habit has 
grown upon him. The consequence is that his use of connecting particles, 
of parentheses, of echo clauses, is now always complex, and at times extra- 



Translator's Preface xi 

ordinarily complex. The reader who opens the Physiologische Psychologic 
at haphazard, and runs through a paragraph or two, will think this state- 
ment exaggerated. If he will try not to understand, but to translate, 
and to translate not a page, but a chapter, its truth will be borne in upon 
him. I had hoped to use, for the present translation, certain parts of my 
former manuscript. But a new opening or closing sentence, even a new 
set of connectives, changes the whole colour of the German, and so demands 
a new phrasing, oftentimes a new vocabulary, from the translator. I soon 
found that my previous work was more of a hindrance than a help, and 
relegated it to the waste-paper basket. The second special difficulty in 
Wundt's style has also grown with the years ; it is his increasing tendency 
jto_clothe his ideas in conceptual garb, to write in a sort of shorthand of 
Abstractions. I^have never thought hirn, for this or for the other reason, 
obscure ; the meaning is always there, and can be found for the searching. 
But there are many and many passages where a half-way literal English 
rendering would be unintelligible ; where one is forced, in translating, to 
be concrete without losing generality ; and in cases like this the translator's 
lot is not a happy one. 

The present volume covers the first 338 pages of the German work, 
or the Introduction and Part I : On the Bodily Substrate of the Mental Life. 
The German pagination is printed, for convenience of cross-reference, in 
the page-headings of the translation 1 . For reasons stated in their place, 
j^have included a section from the fourth edition which the author has 
omitted. I have also added an index of names and subjects. 

E. B. TITCHENER. 
CORNELL HEIGHTS, ITHACA, N.Y. 



Contents of Vol. I 



INTRODUCTION 

i. THE PROBLEM OF PHYSIOLOGICAL PSYCHOLOGY ..... i 

2. SURVEY OF THE SUBJECT . . . . . . . . .11 

3. PREPSYCHOLOGICAL CONCEPTS ........ 16 

PART I. 

THE BODILY SUBSTRATE OF THE MENTAL LIFE 

CHAPTER 

THE ORGANIC EVOLUTION OF MENTAL FUNCTION 

i. THE CRITERIA OF MIND AND THE RANGE OF THE MENTAL LIFE . . 27 
2. THE DIFFERENTIATION OF MENTAL FUNCTIONS AND OF THEIR PHYSICAL 

SUBSTRATE .......... 33 

CHAPTER II 
STRUCTURAL ELEMENTS OF THE NERVOUS SYSTEM 

i. MORPHOLOGICAL ELEMENTS ........ 39 

a. The N*erve Cells ..... ... 40 

b. The Nerve Fibres .... .... 44 

c. Peripheral Nerve Terminations ....... 47 

d. The Neurone Theory . . . . . . . . .48 

2. CHEMICAL CONSTITUENTS . . . . . . . . 54 

CHAPTER III 
PHYSIOLOGICAL MECHANICS OF NERVE SUBSTANCE 

i. GENERAL PRINCIPLES AND PROBLEMS OF A MECHANICS OF INNERVATION . 57 

a. Methods of a Mechanics of Innervation ...... 57 

b. The Principle of the Conservation of Work ..... 60 

c. Application of the Principle of the Conservation of Work to the 

Vital Processes and the Nervous System .... 65 

2. THE COURSE OF THE PROCESSES OF STIMULATION IN THE NERVE FIBRE . 67 

a. Course of the Muscular Contraction following Stimulation of the Motor 

Nerve ........... 67 

b. Excitatory and Inhibitory Processes in Nerve Stimulation . . 70 

c. After-effects of Stimulation : Practice and Fatigue ... 75 
4. Stimulation of Nerve by the Galvanic Current . . , . .7? 



xi v Contents of Vol. I 

PAGE 

3. THEORY OF NERVOUS EXCITATION . .... 80 

{ 4. INFLUENCE OF THE CENTRAL PARTS UPON THE PROCESSES OF EXCITATION 85 

a. Course of the Reflex Excitation . .... 85 

ft. Enhancement of Reflex Excitability . .... 88 

c. Inhibitions of Reflexes by Interference of Stimuli . . . 91 

d. Chronic Effects of Excitation and Inhibition : ^Positive and Negative 

Tonus . . . . . -; . .93 

{ 5. THEORY OF CENTRAL INNERVATION. . 94 

a. General Theory of the Molecular Processes in the Nerve Cell . . 94 

b. Relation of Nervous to Psychical Processes . . . . . 101 

CHAPTER IV 

MORPHOLOGICAL DEVELOPMENT OF THE CENTRAL ORGANS 
i. GENERAL SURVEY ........... 

a. Object of the Following Exposition ... 

6. The Neural Tube and the Three Main Divisions of the Brain . " . 

c. The Brain Ventricles and the Differentiation of the Parts of the Brain 
2. THE MYEL IN THE HIGHER VERTEBRATES . .. 

3. THE OBLONGATA . . . . . i . 

4. THE CEREBELLUM .......... 

5. THE MESENCEPHALON ......... 

6. THE DIENCEPHALON .......... 

7. THE PROSENCEPHALON . 

a. The Brain Cavities and the Surrounding Parts .... 

b. Fornix and Commissural System ....... 

c. The Development of the Outward Conformation of the Brain 

CHAPTER V 

COURSE OF THE PATHS OF NERVOUS CONDUCTION 
{ i. GENERAL CONDITIONS OF CONDUCTION . ... 150 

f 2. METHODS OF INVESTIGATING THE CONDUCTION PATHS . . . .152 

3. CONDUCTION IN THE NERVES AND IN THE MYEL . . . . .155 

a. Origin and Distribution of the Nerves . . . . . .155 

6. Physiology of the Conduction-Paths of the Myel . . . .159 

c. Anatomical Results . . . . . . . . .163 

4. PATHS OF CONDUCTION IN OBLONGATA AND CEREBELLUM . . . 167 
a. General characteristics of these Paths . . . . . .167 

f. Continuations of the Motor and Sensory Paths . . 168 

c. The Regions of Origin of the Cranial Nerves and the Nidi of Cinerea in 

the Oblongata . . . . . . . . .170 

d. Paths of Conduction in Pons and Cerebellum . . . 172 
i 5. CEREBRAL GANGLIA AND CONDUCTION PATHS OF THE HIGHER SENSORY 

NERVES I 8 



Contents of Vol. I xv 

PAGE 

a. The Cerebral Ganglia .... .... 178 

b. Conduction Paths of the Nerves of Taste and Smell .... 179 

c. Conduction Paths of the Acoustic Nerve . . . . .182 

d. Conduction Paths of the Optic Nerve . . . . . .185 

6. PATHS OF MOTOR AND SENSORY CONDUCTION TO THE CEREBRAL CORTEX 190 

a. General Methods for the Demonstration of the Cortical Centres . 190 

b. Motor and Sensory Cortical Centres in the Brain of the Bog . . 193 

c. Motor and Sensory Cortical Areas in the Monkey . . . .198 

d. Motor and Sensory Cortical Centres in Man ..... 204 

7. ASSOCIATION SYSTEMS OF THE CEREBRAL CORTEX . . . .213 

8. STRUCTURE OF THE CEREBRAL CORTEX . . . . . . .218 

9. GENERAL PRINCIPLES OF THE PROCESSES OF CENTRAL CONDUCTION . . 225 

a. The Principle of Manifold Representation . . . . .225 

b. The Principle of the Ascending Complication of Conduction Paths . 226 

c. The Principle of the Differentiation of Directions of Conduction . 227 

d. The Principle of the Central Colligation of Remote Functional Areas ; 

Theory of Decussations ........ 229 

CHAPTER VI 

PHYSIOLOGICAL FUNCTION OF THE CENTRAL PARTS 

i. METHODS OF FUNCTIONAL ANALYSIS ....... 241 

2. REFLEX FUNCTIONS .......... 242 

a. Spinal Reflexes ......... 242 

b. Metencephalic and Mesencephalic Reflexes ..... 244 

c. Purposiveness of the Reflexes. Extent of Reflex Phenomena . .250 
3. AUTOMATIC EXCITATIONS ......... 253 

a. Automatic Excitations in Myel and Oblongata . . . .253 
6. Automatic Excitations in the Brain Cortex ..... 256 
4. FUNCTIONS OF THE MESENCEPHALON AND DIENCEPHALON . . . 258 
a. Functions of the Mesencephalon and Diencephalon in the Lower 

Vertebrates 258 

6. Functions of the Mesencephalon and Diencephalon in Man . . 269 

c. Striatum and Lenticula , . . . . . 270 

5. FUNCTIONS OF THE CEREBELLUM . . . . .271 

6. FUNCTIONS OF THE CEREBRAL HEMISPHERES ...... 280 

a. Phenomena of Abrogation after Partial Destruction of the Prosen- 

cephalon ....... 280 

b. Phenomena of Abrogation after Total Loss of the Cerebral Hemispheres 283 
f. Results from Comparative Anatomy and Anthropology . . . 285 

The Hypotheses of Localisation and their Opponents ; the Old and the - -j 
JJe\y Phrenologies , , ' 2 ^7 



xvi Contents of Vol. T 

PAGE 

7. ILLUSTRATIONS or THE PSYCHOPHYSICAL ANALYSIS OF COMPLEX CEREBRAL 

FUNCTIONS .......... 298 

a. The Visual Centres 298 

6. The Speech Centres 3 2 

c. The Apperception Centre . . . . . . . 3 J 5 

{ 8. GENERAL PRINCIPLES OF THE CENTRAL FUNCTIONS ..... 320 

a. The Principle of Connexion of Elements . . . . . 320 

6. The Principle of Original Indifference of Functions . . *': . 322 

c. The Principle of Practice and Adaptation 4 324 

^> d. The Principle of Vicarious Function . . . . . . 325 

t. The Principle of Relative Localisation 328 

INDEX OF SUBJECTS ........... 333 

INDEX OF NAMES .......;.. 345 



INTRODUCTION 

I. The Problem of Physiological Psychology 

' \ L 

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. Psycho- 
logy, 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 bodily life in other creatures as indicate 
the presence ol 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 pro- 
cesses of bodily life from conscious processes than we can mark off an outer 
experience, mediated by sense perceptions, and oppose it, as something wholly 
j>eparate and apart, to what we call ' inner ' experience, the events of 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 pro- 
cesses 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 
;xternal objects that arise in consciousness either to direct affection of 
the organs of sense, or, in the case of memory images, to physiological ex- 
citations within the sensory centres, which we interpret as after-effects 
of foregone sense impressions. 

P. * B 



2 Introduction [2-3 

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 pi ex- 
cesses 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. 
' hologists, 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 
tor ;i science of mind than the direct apprehension of conscious proo 
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 t cti 
that properly belong to psychology, although it has often been hamp red 
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 



3-4] Problem of Physiological Psychology 3 

the character of its problems, to advance more quickly towards the applica- 
tion 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 con- 
sciousness, which, as we have seen, form part of vital processes at large, 
it is clear that physiology is, in the very 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 ex- 
perimental 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 psycho- 
logy, 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 only in method, and not in aim or purpose. Physiological psycho- 
logy, 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 psycho- 
physics. 1 If we free this term from any sort of metaphysical implication 

1 The word was coined by Fechner ; see his Elemente 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 inter- 
action 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. 



4 Introduction [4~5 

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 psycho- 
logy, 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 organisation, 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 psycho- 
physical problem that we have assigned to physiological psychology proves 
to be itself psychological. In execution, it will be predominantly physiolo- 
gical, 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 " physio- 
logical 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 im- 
portance 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 introduc- 
tion of the experimental method. From one point of view, indeed, the 
change wrought is still more radical : for while in natural science it is pos- 
sible, under favourable conditions, to make 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 makes a scientific in- 
trospection possible. For all accurate observation implies that the object 



5-6] Problem of Physiological Psychology 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 in- 
dependence 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 occurred 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 production 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 pro- 
cess remains approximately unchanged. The great importance of the ex- 
perimental 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 where 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 psycho- 
logical discipline, ethnic psychology. The results of ethnic psychology con- 
stitute, at the same time, our chief source of information regarding the 
general psychology of the complex mental processes. In this way, experi- 
mental psychology and ethnic psychology form the two principal depart- 
ments 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 in- 
strument of introspection. Finally, child psychology and experimental 
psychology in the narrower sense may be bracketed together as individual 



6 Introduction [6-7 

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 
;i:iimal psychology are of relatively slight importance, as compared with 
I he 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 de- 
velopmental 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. 1 The reasons that he gives for this opinion 
have often bee.i repeated in later times. 2 In the first place, Kant says, psycho- 
logy cannot become an exact science because mathematics is inapplicable to 
the phenomena 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, conform- 
ably 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 representa- 
tion 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 
representation 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. 3 

It Herbart, nevertheless, failed to accomplish the task which he set himself, 
th<- 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. 4 At 

1 KANT, Metaphysische Anfangsgrunde d. Naturwissenschajt. In SammtlicJie \\'erke, 
id. by ROSENKRANZ, v. 310. 

8 Cf. csp. E. ZELLER, Abh. d. Berliner Akad., 1881, Phil.-hist. Cl., Abh. in. Sitz- 
ungsber. of the same. 1882, 295 tf. ; and my remarks upon the question, Philos. St'udicn 
i. 250. 463 ff. 

3 HEREART. Psychologic als Wissenscha^t neu gegriindet auf Erfahrnne Metaphysik 
u. Malhemafik. In Gcs. Werke. ed. by HARTENSTEIN, vols. v., vi. 

* FECHNEK, El. d. Psychophysik. ii. 9 ff. An interesting light is thrown upon the 



?-8] Problem of Physiological Psychology J 

the present day, experimental psychology has ceased to regard this forrmilation 
of mental measurements as its exclusive or even as its principal problem. Its 
aim is now more general ; it attempts, by arbitrary modification of conscious- 
ness, to arrive at a causal analysis of mental processes. Fechner's determina- 
tions 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 sub- 
stitute for this subjective method, whose sole resource is an inaccurate inner 
perception, a true and reliable introspection, and to this end it brings conscious- 
ness 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 ; 
lor 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 case, 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. 1 
The ^mission, 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 know- 
ledge of the mental life gained, in independence of experience, simply and solely 

origination of the idea of ' mental measurement ' in Fechner's mind, and also upon the 
inspiration that he derived from Herhart, by the " Kurze Darstellung eines neuen 
Princips mathematischer Psychologic " in his Zendavesia, 1851, ii. 373 ff. For a 
detailed treatment of mental measurement, see Ch. ix. below. 

1 On the question of method in general, cf my Beitrdge zur Theorie der Sinnes- 
wahrnehmun%en, 1862, Einleitung : Ueber die Methoden in der Psychologic; 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 Wa>irnen*nun%, 
in the Philos. Studien, iv. 292 ff. ; and Volkerpsy-hologie, i. i, 1900, Eiuleitung. 



8 Introduction [3-9 

from metaphysical concepts. The result has proved that any such metaphysical 
treatment of psychology must7 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. Psycho- 
logy 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. 1 

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 psycho- 
logy. Sometimes the rational part is restricted to a fe.v 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 for the discovery by this whole introspective 
school of one single undisputed fact, it would be offered in vain. 2 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 experi- 
mentation, but the whole jof 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. 

Kxperimental 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 jnterpretation of conscious phenomena by 

1 Cf. with this the essay Philosophie it. Wissenschaft, in my Essays, i ff. ; and the 
article Uebcr d. Eint/ieilung d. Wissenschaften, in the Philos. Studien, v. \ ff. 

2 F. A. LANGE, Geschichte des Mater ialismus, ate Aufl., ii. 383 ; History of Materi- 
alism, iii., 1892, 171. 



9-lo] Problem of Physiological Psychology 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, con- 
scious processes (we are told) have their specific character, their peculiar con- 
stitution ; but it is impossible by psychological means to discover uinformities 
of connexion among these elements. Hence the only road to a scientific 
description or explanation of complex mental experiences lies through the know- 
ledge of the physiological connexions obtaining among the physiological processes 
with which the psychical elements are correlated. 1 On this conception, there is 
no such thing as psychical, but only physical causation, and every causal explana- 
tion of mental occurrence must consequently be couched in physiological terms. 
[t is accordingly termed the theory of ' psvchophysical materialism.' The 



theory as such is by no means a new thing in the history of philosophy. ^ 
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. 2 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 
nothing else than it has always been, a metaphysical assumption. More than 
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 propositions that 
has led certain psychologists, who would otherwise take the same theoretical 
position, to divide the problem of psychology, and to recognise the inter- 
connexions of mental processes as a legitimate object of inquiry, alongside of the 
investigation of their dependence upon determinate physiological processes within 
the brain. In the psychological portion of their works, these writers usually 
adopt the fEeory of the ' association of ideas,' elaborated in the English 
psychology of the eighteenth century. 3 They adopt it for the good and 
sufficient reason that the doctrine of association, from David Hartley (1705-1757) 

1 H. MUNSTERBERG, Ueber Aufgaben und Methoden der Psychologic, in Schriften der 
Gesellschaft f. psychol. Forschung, i. in ff. Practically the same position, though with 
minor changes of expression, is taken by the author in his Grundzuge der Psychologic, 
i., 1900. 382 ff. 

2 MUNSTERBERG, Grundzuge der Psychologic, Vorwort, viii. Cf. the same author's 
Psychology and Life, 1899. This view, o f the irrelevancy of psychology to the mental 
sciences, is further shared by certain modern philosophers : see the criticism of it in my 
Einleilung in die Philosophic, 1901, 4. 

3 Cf., e.g., T. ZIEHEN, Leitfaden der physiologischen Psychologic, 5te Aufl., 1900, 3 ff. ; 
Introduction to Physiological Psychology, 1895, 3 ff. 



10 



/;/ t reduction [ I o- 1 1 






down to Herbert Spencer (18.20-1904), 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, 
iy calculated to make the. experimental method comparatively barren, 
M> tar 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 
tine of association. Ideas are treated, after as before, as if they were 
i nmutable objects, that come and go, form connexions of sequence with one 
another, obey in these connexions the well-known laws of habit and practice, 
nd finally, when arranged in certain groups, yield the not very startling result 
i hit they can be brought under the same logical categories that have proved 
icrally serviceable for the classification of all sorts of concepts. 1 
Now physi >'ogy and psychology, as we said just now, are auxiliary disci- 
pline, and licit IHT can advance without assistance from the other. Physiology, 
i i its andysis of the physiological functions of the sense organs, must use the 
ivjults of subjective observation of sensations; and psychology, in its turn, 
ncc.ls t:> kno.v the physiological aspects of sensory function, in order rightly to 
appreciate the psychological. Such instances might easily be multiplied. 
M >re >ver, in vie.v of the gaps in our knowledge, physiological and psychological 
aliUe, it is ine. -liable tint the one science will be called upon, time and again, 
i > 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 clo.sing 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 an- to be I mind 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 
thejnental life lacks all causal connexion, and that the real and primary object 
ol 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 psycho- 
logical point of view, so that for psychology as such their results are generally 
valueless, llcncc the gravest danger that besets the path of our science to-dayX/ 
comes not from the speculative and empirical dogmas of the older schools, but/" 

On the doctrine ot association, see Part v., below. For a general criticism of 
psychological materialism, cf. the articles Ueber psychische Causalitat and Veber die 
Definition der Psychologic, in the Philos. Studien, x. 47 ff. xii. i ff. 



H-I2J Survey of the Subject li 

\7rather from this materialistic pseudo-science. Antipsychological tendencies can 
hardly find clearer expression than in the statement that the psychological . 

y' interpretation of the mental life has no relation \\hatever to the mental life itself, 
as manifested in history and in society. 

Besides this application of the term ' experimental psychology ' in the 
interests of pyschological materialism, we find it used in still another sense, 
which is widely different from that of our own definition. It has become custo- 
mary, 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 inaccept- 
able 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. 
i indeed, is proved by Vogt's observations on the analysis of the feelings in the 
hypnotic state. 1 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 stili uncertain. 2 

2. Survey of the Subject 

Physiological psychology is primarily psychology, and therefore has for 
its subject the manifold of conscious processes, whether as directly expe- 
rienced by ourselves, or as inferred on the analogy of our own experiences 
from objective observation.. Hence the order in which it takes up parti- 
cular 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 psycholo- 
gical exposition. While, then, the following Chapters of this work arc 
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 

1 O. VOGT, Die directe psychology sche Exp crime ntalmcthode in "hypnotise hfn L\ 
vuussiseinsziistdnden. In the Zeitschr. fur Hypnotismus, v., 1897, 7, 180 ff. 

2 For a general discussion of hypnotism, see Part v.. below. 



1 2 Introduction [12-14 

of the discussion, by the 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 reading of a 
book like the present implies. Thus_a_critical review of the results of brain 
anatomy and brain ^hysiology^ with reference to their value for psychology, 
- 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. 1 

Combining in this way the demands of theory and the precepts of prac- 
tical method, we shall in what follows (i) 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, 
:, infon ed al various goings '';. gathojogj .ml general biology. This ma>s 
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 
cmr task to follow brain anatomy into all the details which it has brought 
__ to lighLconcerning the connexions of fibres within the brain, into all those 
minute points whose interpretation is -nil altogether uncertain, and whose 
truth is often and again called in question. It will only be necessary lor 
us_tp__obtain a general view of the structure of the central organs and of 
j>uch 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 

In my Grmlriss der Psyclnljjiz Ute Aufl.. 1911 ; 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 supplement and as introduct.on to the present work. 



' 



14-15] Survey of tJic Subject 13 

of psychology proper, with the doctrine of the dements 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 subjec- 
tive 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., jeelings. It is important that the terms be 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 ' sensa- 
tion,' but as an ' idea ' ; and when we come to deal with the formations 
resulting from a combination of feelings we shall term them expressly ' com- 
plex 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 con- 
sidered 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 later 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, 



14 Introduction [i$-l6 

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 when 
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 than the image of direct 
perception. But this difference is by no means constant ; we may sense in 
breams, or in the state of hallucination, as intensively as we sense under the 
operation of actual sensory stimuli. 1 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 origi- 
nate 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 sensa- 
tions, 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 ele- 
ments and of their direct interrelations. A physiological psychology, on 
the other hand, is bound to regard this psychophysical aspect of the prob- 
lems of mind as one of its most important objects of investigation. 

(3) The course of our inquiry proceeds naturally from the mental ele- 

1 For a more extended d.scussion of these terminological questions see Ch. vii. 
| i, below. 






1 6- 1 7] Survey of tJic Subject 15 

inents 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 ideas that arise from the connexion of sensations. Since physio- 
logical 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 accord- 
ingly designate the contents of this section a study of the composition of 
sense ideas. Our conclusions will, however, apply equal!}- 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 doctrire 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 physi- 
cal 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 inter- 
connexion 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. _J5ensji- 
tions^ and feelings are, always and everywhere, complementary constituents 
of our mental experiences. Hence the conscious contents that are com- 
pounded of feelings can never occur except together with ideational contents, 
and in many cases the affective elements are as powerful to influence sensa- 
tions and ideas as these are to influence the feelings. This whole group of 
subjective experiences, in which feelings are the determining factors, may 
be brought under the title of Gemilthsbewegungen und Willenshandlungen. 
Of these, Gemiithsbewegungen 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 implica- 
tions 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 



I f> Introduction [ I /- 1 8 

of Gemiilhsbewegungen 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. 1 

(5) Having thus investigated^ense 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 conscious- 
ness and of the connexions of conscious contents. For all these conscious 
connexions contain, as their proximate constituents, ideas and emotions, 
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 be chiefly concerned with the arbi- 
trary modification of sense ideas and of their course in consciousness. \Vh<-n. 
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 fur 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 com- 
pleted, 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 know 
ledge, 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 2 

The human mind is so constituted, that it cannot gather experiences 

1 Gem&tksbewegvitgen, as first used above, means " complex affective, affective. 
'. Imonil and volitional processes." There is no exact English equivalent. See 
BVUAVIN'S Did. of Phil, and Psych., ii. 1902, 680. Willenshandlungen means, oi course, 
voluntary actions, internal and external. TRANSLATOR. 

2 In the first four editions OT the Physiologische Psychologic, the Introduction con- 



1893 : IO ] Prepsycholngical Concepts \j 

without at the same time supplying an admixture of its own speculation. 
The first result of this nai've 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 has 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 
roncepts of just the same kind, concepts that existed before the advent 
of any scientific psychology- The fact that the naive consciousness always 
ind everywhere points to internal experience as a special source of know- 
ledge, 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 refer- 
ence 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. JVtind, 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, un- 
tested, 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, they stood not merely 

sists of two sections, entitled respectively Aufgabe der physiologischen Psychologie, and 
Psychologische Vorbcgriffe. In the present, fifth edition, the second of these sections 
is replaced by an Uebersicht des Gcgenstandes. I here reprint the section on Psycho- 
logische Vorbeqriffe as it appeared in 1893. It was, in all probability, omitted mainly 
for reasons of space. Cf. Preface to the 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 hesita- 
tion, 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. 



!g Introduction [i93 : IO - n 

for^eparate modes of existence, lor ' 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 concep- 
tual ideas have passed so long from hand to hand in the service of the under- 
standing, 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 
Vof_subsTanres : but even of the suhstantialising of concepts. But we are not 

r \ called upon, on that account, to dispense with the use whether of the con- 
cepts themselves or of the words that designate them. JVe_speak of virtue, 

v/ 1 .honour, reason ; but our thought does not translate any one of these concepts 
i ntn ., substance. They have ceased to be metaphysical substances, and 
have become logical subjects. In the same way, then, we shall consider 
min.l. 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 meta- 
physics which invariably attach to concepts in their making by the naive 
consciousness. 

We must take up a precisely similar attitude to other ready-made con- 
cepts that denote special departments or special relations of the internal 
experience. Thus our language makes a distinction between,* mind ' .and 
' spirit.1 The two concepts carrv- the same meaning, but 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 
existem - asi onsiden d out ol all COHIK xion with an external existence; where- 
> , as ^jnind,' especially where it is explicitly opposed to spirit, presupposes 
/\ this connexion with a corporeal existence, giyerijr^external experience. 1 

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,' ' feel- 
ing,' ' 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 can 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 

1 The German terms for ' body ' and ' matter ' are Leib and Kor[>er ; for ' mind ' 

and ' spirit,' Seele and Geist. See BALDWIN'S Diet, of Phil, and Psych., ii. 680, 

TRANSLATOR. 



1893: 11-12] Prepsychological Concepts 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 hypostatisation 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. Neverthe- 
less, thjre is one legacy which has come down to modern science from the 
mythopoeic age. All the concepts that we mentioned just now have re- 
tained 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 classconcepts, 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 con- 
/ cejye 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 thtse 
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 thete is, to be sure, no special faculty 
of ideation or feeling or volition ; but the individual idea, the individual 
affective process, and the individual voluntary act are looked upon as inde- 






2O 



Introduction [ 1 893 : 12-14 



pendent processes, connecting with one another and separating from one 
another as circumstances determine. Now introspection declares that all 
these professedly independent processes are through and through inter- 
connected and interdependent. It is evident, therefore, that their separa- 
tion 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 an 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 orocesses 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 as 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 psychol< gy. 
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 sub- 
stance as its substrate ; on the other, they can see no way to escape a separa- 
tion 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 philo- 
sophy 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, 1 or whether, with Aristotle, it transfers to spirit 
the notion that it has abstracted from mind, and so substitutes a coincident 

1 Tintaeus, 35. JOWETT'S Plato, iii. 453-4. 



1893 14-15] Prepsychological Concepts 21 

form of definition for unity of substance. 1 .Modern spiritualistic philosophy has, 
in general, followed the path laid down by PLATO, though it affirms more de- 
cidedly 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 voca- 

_bulary. If a difference is made, it is made in one of two ways, Either spirit is 
taken as the general concept, within which the individual mind is contained ; 2 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, 3 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 word 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 

^isjhe same subject without reference to such connexion. We shall, accordingly, 
speak of spirit and of spiritual phenomena only when we can afford to neglect 
th;>se 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 be- 
tween 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, 4 the mediating thought (Siou/oia) and the emotion 

1 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 coOs TroiijrtKos, 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. i sub fin. WALLACE'S 
trans., 65 ; HAMMOND'S trans., 44 f. 

2 So WOLFF, Psycholoqia rationalis, 643 ff. 

3 Geist and GemMh. TRANSLATOR. 

* Timaeus, 77. JOWETP'S Plato, iii. 449-50. 



2 2 Introduction 

appear to stand in similar relation only to the faculty of reason. Hence these 
attempts at classification give us the impression that Plato worked out his 
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 of 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 three 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 
as the most general. Desire, in particular, is subordinated to sensation. 1 

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 miml 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, em- 
ploys" both of the Platonic divisions, side by side, but makes the faculty of 
k-eling subordinate to that of desire. The consequent dichotomy runs through 
liis whole system. He first of all separates ccgnition and desire, and then 
subdivides each of these into a lower and a higher part. The further progress 
of the classification is shown in the following table. 

I. FACULTY OF KNOWLEDGE II. FACULTY OF DESIRE 

1. Lower Faculty of Knowledge. i. Lower Faculty of Desire. Pleasant- 

Sense, Imagination, Poetic ness and unpleasantness, Sensu- 

faculty, Memory (remembering ous desire and sensuous aversion. 

and forgetting). Emotions. 

2. Higher Faculty of Knowledge. 2. Higher Faculty of Desire. Voli- 

Attention and reflection. Un- tion (affirmation and negation). 

. derstanding. 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 

* De anitna, ii. 2, 3. WALLACE'S trans., 65-77 '> HAMMOND'S trans., 48-56. 



1893: 1 6- 1 7] PrepsycJiological Concepts 23 

two principal faculties, or fesling 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 cf 
knowledge, reason for the faculty of desire, and judgment for feeling ; x while 
understanding, judgment and reason are eke.vhere bracketed together as under- 
standing in the wider sense. 2 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, under- 
standing the active side of knowledge. 3 Hence in his great Critique he opposes 
s3-i3i'oility to understanding. When connected with sensibility, understanding 
mediates empirical concepts ; alone, it gives us pure notions. 4 

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. 
The first is, in all essentials, a_legacy from the Wolffian psychology : the other 
two are peculiar to KANT. Previous philosophy had, in general, defined reason 
(Aoyos) 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 (1/01)5, 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 cf things. Sometimes, again 
it was considered as a special mode of manifestation of understanding. Illustra- 
tions 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, 
t'.ie capacity of ratiocinatio, of inferring individual truths from their grounds 
of ' reasoning ' First of all, ratio makes its appearance among the mental 
[.i?ult:Cj, in this latter significance, as faculty of inference ; later on, it appears 

1 Kritik d. Urlheilskraft, ROSENKRANZ' ed. ( iv. 14 ff. BERNARD'S trans., 1892, 

i ; if. 

zie, vii. 2, 100 and 104. 



1 l\nti/t d. I'l'iut'ii Vcmunfl, ii. 31, 55. MILLER'S trans., i8g6, r5, 40, 




24 Introduction [^93 : 17-18 

also as a faculty of insight into the grounds of things. And wherever the em- 
phasis fell upon this second meaning, reason shone forth as the very organ and 
instrument ot 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 inter- 
connexion of universal truths. 1 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 en- 
couraged 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 de- 
duction of the categories. He accordingly assumed that, since judgment stands 
judway^ between concept and inference (conclusion), the faculty of judgment 
>tands 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 
in : procei I, in the sami way, 1 > su] : '." . te this rul< to a lusher condition ; 
.1 nd so on, until in the last resort it arrives at the idea of an unconditioned. This 
idea, then, in its various forms as mindj world and God, remained the peculiar 
jjrqperty 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 application of the concepts 
and principles which understanding propounds. As faculty of transcendent 
ideas, it ranks high above understanding. JJnderstanding 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, %\hich 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 impera- 
tive form in which it demands obedience, it proves the truth of the idea of 
unconditioned freedom of the will. 2 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. 3 The three 
fundamental faculties of mind are thus referred to the three modes of manifesta- 
tion of the faculty of knowledge distinguished by formal logic. And we see at 
once how largely this reference is the product of an artificial schema tisation 
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 

1 WOLFF. Psychologia empirica, 483. 

* Kritik d. prakt. Vernunft, viii. 106. 

Kritik d. Urtheilskraft, iv. 15. BERNARD'S trans., 16. 



1893 : 18-19] Prepsychohgical Concepts 2$ 

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 judg- 
lent 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. 

HERB ART'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 ol 
his argumentation lies in the two following objections, (i) _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. 1 (2) The mental faculties are class-concepts, obtained by a provisional 
abstraction from the inner experience, and then raised to the rank of funda- 
mental forces of the mind and used for the explanation of our internal processes. 2 
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, con- 
tractility, 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. 3 

Objection has been taken to this distinction, on the ground that it pre- 
supposes a concept of faculty which is found only in the most unscientific form 
of the psychological faculty-theory. 4 Nevertheless, it must be conceded that 
the discrimination of the terms is not without significance. With the develop- 
ment 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 

1 HERBART, Werke, vii. 611. 2 Werke, v. 214. 3 Werke, vii. 601. 

* J. B. MEYER, Kant's Psychologic, 116. 



2 6 Introduction [1893: 19-20 

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 pos- 
sessed, 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 occur- 
rence) related phenomena wrested 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 cf (lie 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 in- 
carnate in a mythological being. It would, therefore, be unjust to psychology, 
were we to accuse Iv.-r 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 
it 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 use of it to the full. 

> HERBART not only realised the untenabliity of the faculty-theory ; he arrived 
x/at the positive conviction that mental processes must be considered asunitivy 
proo- -'-. Uut he sought to satisfy the requirement of unity by raising one of 
the products of current psychological abstraction above all the rest. He re- 
garded the idea as the real and only contents of the mind. Xay, he went so 
tar 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 
sec later, rest upon no better foundation than hypothesis, and bring their author, 
at every point, into conflict with an exact analysis of experience. 1 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. Never- 
theless, HERBAKT had taken the right path in his endeavour 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. 2 

1 Cf. ch. xix. [of the present edition], 

' Cf. with this the essay on feeling and idea, in my Essays, 199 ft* 



10-201 



Part I 
The Bodily Substrate of the Mental Life 

CHAPTER I 
The Organic Evolution of Mental Function 

i. 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, the_y_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 con- 
fronted with unusual difficulties. The distinguishing characteristics of 
mind are of a subjective sort ; we J<now them only from the contents of 
mirjDwn 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 when 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 spiritual- 
Jsm recognises no expression of mental life beyond the voluntary movements 
/ gf 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 workings 
our way downwards, from man to the animals, that we shall find the point 
at which mental life begins. 



^ Evolution of Mental Function [26-21 

Now, there are a very Jarg number J>f bodily movements^, having their_ 
source in our nervous system,. thajLdo.nat possess the character of conscious, 

"actTons. 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. ^Vhen, however, we rule out all the movements that 
may possibly go on without the participation of consciousness, there remains 

HBut 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. 

Hut the discovery of this criterion does not by any means remove the 
practical difficulties of our diagnosis of. mind. Jt_is not possible to dis- 
tinguish certainly in every case between a purely mechanical reflex or 
r\vn, in the lowest organisms, a movement due to external physical causes. 
>;irh as the imbibition of tumescent bodies, the change of volume from fluctua- 
tions 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 con- 
ditions, and brought into connexion with sense-impressions previously 
secured. Judgment on the ground of these criteria may, in the individual 

~case7remain doubtful ; since all vital processes, even those that are entirely 



i 1 

21-22] Criteria and Range of Mind 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 JMany 
of the ciliated infusoria pursue others, which they kill and devour. 2 These 
are all phenomena that point towards continuity of mental processes, though 

all probability to a continuity that extends only over a very sl.ort space 
of time. They point also, at all events in the case of the Ciliata, to a 
variation in the choice of means, forjthe satisfaction of the organic impulses, 
that would be unintelligible as a merely mechanical result of externr.l 
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 atjyyhich we notice the external voluntary 

tion, 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. 



me 

- " 



; 



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 cKemical origin, should be interpreted 
as reflexe;, is evidently a result not of observation, but rather of a_foregone theoretical 
conviction that voluntary actions must have developed out of reflexes. On this theory 
see "Part iv., below. 

2 FAMINZYN, The Menial Life of the Simplest Organisms, 1890 (Russian). Quoted 
by BECHTEREW, Bewusstsein und Hirnlocalisation, 1898, . 



30 Evolution of Mental Function [22-23 

^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 proto- 
_glasm occurring throughout the animal and vegetable kingdoms. Indeed, 
it is probable that all elementary organisms, whether they enjoy an inde- 
pendent existence or form part of a compound organism, possess the property 
of^ contractility at least during a certain period of their development. Con- 
sider, e.g., the lymph corpuscles, which are found in the blood and lymph of 
animals, and in pus, and wlncl^occur_asjriigratpry elements in the tissues. 

They are not onlv'entirelv similar in bodily configuration to certain of the! / 

lowest protozoa, bul they also undergo changes ol lonu which, in outward 
appearance, are indistinguishable from ih< movements of these unicellular 
organisnis (Fig. i). Only, the voluntary character of these movements is 
beyond the reach ol demonstration. It is true that similar structures par- 
ticularly the colourless blood-corpuscles 
of invertebrates have bcenseen t take 
up solid substances, and that this action 
may be interpreted as an inception of 
food. 1 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 
then- any definite imlic;it ion 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 move- 
ment. 2 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 and by the gases of respiration, that they 
at once suggest an origin in sensations. But, here again, we cannot be sure 
that the changes are not simply physical effects, as is undoubtedly the case 
with the movements evoked by hygrometric changes in the environment. 2 

1 M. SCHULTZE, Das Protoplasma der Rhizopoden, 1863. ENGELMANN, Beilrage 
tur Physiologic de* Proto plasmas, ii., 1869. VERWORN, Die Bewegung der lebendigcn 
Substam, 1892, 51 ff. ; Allgemeine Physiologic, 1901, 363 ff. (General Phvsiologv 1800 
146 ff.. 527). 

* DARWIN. Insectivorous Plants, 1875, esp. ch. x. PFEFFER, Pflanzen-bhvsioloeie 
2te Ann., r.TTSg;. 36.4 ff. 

> T.W. ENGELMANN, in PFI Cr.F.R'sArchiv. f. d. ges. Physiologie, xxvi. 537 ; xxix.^ic 
xxx. 95. PFEFFER, Uniersuchungen aits d. botan. Institut zu Tubingen, i. 363, 483 ' 
ii. 582. For further details, see Ch. vii. 3, below. On the physical causes of proto- 




FIG. i. Lymph-corpuscles. a k 

Changes of form in the living cell ; I the 
dead cell. 



. 

23-24] Criteria and Range of Mind 31 

We must, however, always rememb'cr, in passing judgment upon this 
whole group of observations, that the demonstration of physical conditions, 
to which the phenomena of protoplasmic contraction and oi 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 
ran 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 pro- 
perties 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. !n 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 Ihe 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 
'hack 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 
v of life have their ultimate basis in the general properties of matter, psycho- 
logy finds it no less obligatory to assume, in this same matter, the universal 
.bstrate 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 
x i not be confused, as hylozoism confuses it^with real life and actual conscious* 

plasmic movement, cf. BUTSCHLI, Untersuchungen iiber mikroskopische Schuuwic und das 
Protoplasma,; 1892, 172, 




32 Evolution of Mental Function [24-25 

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 oniy the common ground upon which they rest and whereby they become 
PCS-. |e ; ' secoi ; is wn - ' c us H posits .1 one-sided dependence. 
,-. ; - . . realitj \ r ( ' I i '': \< :. <! simultan< nisi] pr< sented l>ut 
incommensurable processes. We employ the concept of material substance 
to denote the ground of all objective phenomena. Hence it is the office oi 
this concept to make intelligible all the various forms of physical occurrence, 
including the physical manifestations of life. Now among these manifesta- 
tions 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 
twoextremes current in DESCARTKS'<UIV. 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 ; * another looks upon life 
and mind as convertible terms, and accordingly endows plants as well as animals 
A-ith consci - - ! i i onner 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 ex- 
cluded the other, as, indeed, it did, in the metaphysical dualism of DESCARTES. 
But this is misleading. The close interconnection of the phenomena of the 
PJiysical life and the processes of eonsciousness makes the relation ' physical 
JiHJpsychical/ on the face of it, much more probable. We should, as a matter 
of fact, admit at once that, c.g.._a sensation is a psychical quality, \\ithout 
meaning to deny that it is accompanied by a physical process in the sense-' 
and the sehse-centre. And such a coexistence of the two kinds of vital pr<" 
is, in many cases, 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- 

1 A. BETHE, Diirfen wir den Ameisen und Bienen psychische Qualitdten zuschreiben ? 
In PFLCGER'S Arch.f.d. ges. Physiol., Ixx. 1898, 15 ff. Cf . the critical remarks of \Y.\ss 
MANN, Die paychischcn Fahigkciten der Ameisen, 1899, and Biol.Centralblatt, xviii. 189*, 
578. 

* FECHNER, Nanna oder iiber das Seelenlcben d(r Pflamen, 1 84g_; 2nd ed., 1899. 



25-6] Differentiation of Mental and Bodily Functions 



33 



prelation is, at best, grave enough. ( Manyjiovements^that may in all prob- 
ability be regarded as purgly automatic, are, as we said above, purposive in 
character ; and majTyMDf^ihem, again, are self-regulating. It is, therefore, very 
difficult to draw the line of division in the concrete case. 1 

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 he 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^D 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 con- 
ceive 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. 





FIG. 2. An amoeba in two differ- FIG. 3. Actinosphaerium. a Incepted food- 
ent phases of movement, n Nu- particle, making its way into the soft body 
cleus. i Ingested food-particle mass, b Cortical layer, c Central parenchyma. 

d Ingested food-particles, e Cilia of the cortical 

layer. 

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 1he 

1 Cf. with this the later discussions of impulsive movement (Part iv.) and of con 
sciousness (Part. v. ). 

2 FECHNER, Zendavesta oder iiber die Dinge des Himmels uwfl des Jenseits, i., 851, 
2nd 



p. 



N/ A 

~A oi 



Evolution of Mental Functions [26-7 

body. The only sense that is plainly functioning is the sense of touch, 
n impression m^^upor^an^pj)rtionofjhe_contractile protoplasm first 
: all n-lrasrs a movement at the place of direct impact, which may then 
extend to pin |>iively co-ordinated motion of the entire body. 

The beginnings of a differentiation of mental function can,_however, 
;. .,,...: ev e n ;: . th< protozoa, wherever the cortical Layer surrounding 
t he , onti 11 tile ' odj subsl LI i - has devj lp] ed leeciaj organs oi movement. 
cilia and flagella (Fig. 3). Oftentimes this development goes hand in hand 
with a differentiation of the nutritive functions, An oral aperture and 
digestive cavity are found, and in many instances a system of open canals 
appears, whose fluid content? are kept in motion by a contractile vesicle. 
The cilia with which these infusoria are furnished render them incomparably 
more motile than the^rganisms lying_at the very lowest point of the organic 
seale. the monera and rhizopods, which consist merely of a viscous body- 
masa. 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- serisation ; 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 alike manifest the primitive contractility of 
protoplasm. I:, course oj tjme^jioweyerjjthey become m<><liti<-d 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 develop- 
ment 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. 1 

1 The relations of the various cavities, three or four in number, are in reality much 



27-8] s Differentiation of Mental and Uoiiily 1< unctions / 33 

f V- / 

At this stage^ sensation and mo_vnient 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 





FIG. 4. Yolk in the FIG. 5. Division of the cell- 
final stage of fissipar- mass produced by yolk- 
ous division. cleavage into a peripheral 

(c) and a central (d) layer. 




FIG. 6. Neuromuscular cells 
of Hydra, after KLEINENBERG 
(epithelial muscle- cells of 
HERTWIG). m Muscular pro- 



layer of cells, the mesoderm, forms between the two. The initial stages 
of development are thus identical over the whole series of forms from coel- 
enterates to vertebrates, the differentiation of organs beginning always 
with the distinction of three germinal layers. The outermost layer is the 

Y-^ . ^ " - 7 ii j , 

y source of the nervous system and sense-organs, as well as of the muscular 

f-y\ _ | _ I !! !!! 1^ O ~* f - - . 

^system; the innermost furnishes the organs of nutrition; and the inter- 
mediate layer, the vascular system. In the vertebrates, the skeleton is 
also derived from the ectoderm. 1 

This discrimination of organs is accompanied by a differentiation of the 
elementary contituents 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 pro- 
cesses 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 
af it (Fig. 6). 2 In the next stage, the properties of sensation and contrac- \/ 
tility pass to special and spatially separated cells, while connective elements 

more complicated. It would be more nearly true to say that, where the change indi- 
cated in the text takes place, the body-cavity gradually replaces the cleavage-cavity. 
>Cf. MINOT, Embryology, 1897, ch. ix. TRANSLATOR. 

1 The author gives no references here. The mesoderm is now divided, by~the best 
writers, into mesothelium, 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. 

2 KLEINENBERG, Hydra, eine andtomisch-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 (KLEINEN- 
BERG'S ' neuromuscular ' cells) are now to be regarded as muscle-cells. Later Note, 
by AUTHOR.] 



Evolution of Mental Functions 



[28-9 



develope, to mediate the functional interconnexion of the different struc- 
tures. 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 n<>\\ 
In-come external organs, devoted to the reception of physical stimuli. At 
the same time, they undergo a differentiation, which fits them for excita- 
tion by various forms of movement-process in the outside world. Simi- 
larly, the contractile cells become organs for receiving and converting into 
rxtrrnal 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 inter- 
connexion of the two groups of organs. Hence the simplest scheme of a 
nervous system is given with a centrally situated nerve-cell, connected on 
the one hand witli 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 longi- 
tudinal 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 func- 
tions, auxiliary to the strictly psycho- 
physical processes, which run their 
course without the participation of 
consciousness. Others enter into in- 
timate 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 menfal 




FIG. 7. Ganglion of the ventral nerve- 
cord of the earthworm (Lumbricus), 
after RETZIUS. G Ganglion, st Ventral 
nerve-cord, n', n' Nerves. 



^9~3] Differentiation of Mental and Bodily functions 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 ot the mass of the nervous ele- 
ments, 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 ccelen- 
terates and the lower worms and molluscs, up to the brain-like ganglionic 
masses of the arthropods 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 ma- 
terials into the two layers of the germ- 
primule, there appears in the ectoderm 





FIG. 8. Embryonic area of the rabbit, 
with the embryonic primule. a Primitive 
groove, containing primitive streak. b 
Embryonic primule. c Internal crescentic 
portion (area pellucida), and d external 
discoidal portion (area opaca) of the 
embryonic area. 



FIG. 9. Transverse section through one 
half of the neural tube, after His. To the 
right, on the inner side of the tube, lie 
unmodified germinating cells ; to the left, 
on the outside, are nerve-cells in process 
of development, m Ventral (motor), $ 
dorsal (sensory) nerve-root. 



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 



38 Evolution of Mental Functions [3 " 1 

sheaths. 1 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. g). 2 

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 physio- 
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. 



1 The myelic furrow is now known to be entirely distinct from the primitive groove. 
See O. HERTWIG, Embryology. 79 ff., 125, 416 6. TRANSLATOR. 
* His, Archiv fur Anatomic u. Physiologic, Anat. Abth. 1890, 95. 



31-2] Elements of Nervous System 39 



CHAPTER II 
Structural Elements of the Nervous System 

I. 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 tor the nervous elements. 
They serve, further, to carry the blood-vessels. And the perineurium 2 

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. 



46 Structural Elements of Nervous System \ 

imparts to the peripheral nerves, which have no solid wall of bone 
protect them, the necessary power of resistance to mechanical injury. 



to 



(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 




FIG. 10. Bipolar nerve- 
cell from the ventral 
cornu of the myel of the 
ox, unstained. After 

JUL. ARNOLD. 



FIG. ii. 
staining. 



Multipolar ganglion cell, with aniline 
After BETHE. 



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* 



32-4] MorpJwlogical Elements : Cells 4 1 

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-evils, and they remain throughout in connexion with the (vll> 
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 v/ , 
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. 1 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 
incloses 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. u). 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). Finalty, 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 

1 DEITERS, Untersuchungen tiber Gehirn u. Ruckenmark des Menschen u. der Sduge- 
thiere, 1865, 53 f. His, Arch. f. Anatomic, Supplementband, 1890, 95 ff. VON LEN- 
HOSSEK, Der feinere Ban des Nervensy stems, 2te Aufl. 1895, 36 ff. HELD, Arch. f. 
Anatomic, 1897, 2O 4 ; Suppl. 273. BETHE, Arch. /. mikroskop. Anatomic, 1900, lv., 
513. GOLGI, Verhandl. d. anatom. Gesellschaft auf d. 14 Vers. 2u Pavia, 1900; Anat. 
Anzeiger, xviii., Ergdmungsheft. 



Structural Elements of Ncn>ous System 



[34-5 



into the dendrites, so that they too are, in all probability, to be looked upon 
as nervous structures (Fig. 13). 

^Jervfc cells are classified, according to the number of processes they 
send out) a* ""jpftlari ^'j^far -"'! 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, 
multi] < lar. As a rule, t very 
sii( h . ell gives "t! a single 
neurite, and an indeterminate 
number of dendrites. The di- 
vergent 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 paral- 
leled by a corresponding differ- 
ence 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. 1 This scheme can, however, hardly 
be 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 nst, 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 

1 RAMON Y CAJAL, Les nouvelles idles sur la structure du systeme nerve ux chez I'homme 
et chez les vertebres, 1894. 




FIG. 12. Multipolar ganglion cell, showing clearly 
the twofold mode of origin of the fibrils (DEITERS' 
type). After NISSL. Neurite to the right. 



36] 



Morphological Elements : Cells 



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 v_ \ f 

of the contraction of protoplasm and of muscular 





FIG. 13. Pericellular reticula of nerve- 
cells, stained by the silver method. Alter 
GOLGI. 



FIG. 14. Pyramidal cell from the 
cerebral cortex, stained by the 
silver method. After RAMON y 
CAJAL. pp Denclrites. n Neurite. 
cc Collaterals. 



tissue, or whether they are not rather simply the direct physical and 
Chemical effects of the stimuli applied. 1 

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 

1 RABL-RUCKHARD, Neurol. Centralblatt, 1890, 199. DUVAL, Soc. de Biologic, 1895. 
Cf. KQLLiKER.FerA. d. Wurzburger phys-med. Gesellschaft, 1895. 



44 Structural Elements of Nervous System [3^-7 

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 colls of (ioi.Gi's type ; and the cells without marked distinction 
of the processes are called intermediary or intercalary cells. 1 




FIG. 15. Cells of PURKINJE, from the cortex of the cerebellum, with richly branching 
dendrites. After KOLLIKER. n Neurite. k Collaterals. 



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 trending 
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. 

(6) 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 

1 These intermediate cells (intermediare oder Sr.haltzellen) 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. 



37-8] 



Morphological Elements : Fibres 



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 con- 
tinuation 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 bul- 
bous masses ; later in its course, it becomes 
surrounded by a structureless membrane, 




FIG. 16. Nerve-fibres, a Cerebrospinal nerve-fibre 
with primitive sheath, myelinic (medullary) sheath, 
and broad neurite. b A similar fibre, whose neurite 
is coagulated by collodion, c Sympathetic nerve- 
fibre without myelinic sheath ; contents very 
finely striated ; primitive sheath nucleated (' fibre 
of REMAK '). 



;f 



FIG. 17. Schematic dia- 
gram of the structure of 
a meduhated nerve-fibre. 
a Neurite. 5 Primitive 
sheath of SCHWANN. 
rr Nodes of RANVIER. 
hi Corneal sheath (axi- 
lemma) of KUHNE. 



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, wb 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 



46 Structural Elements of Nervous System [38 

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). 1 While the myelinic sheath is thus subdivided, the neurite 
itself runs uninterruptedly from its point of origin to the conclusion 
of its course. It is made up. as was first observed by MAX SCHULTZE, of 
numerous prjrnitive fibrils, which in many places, and especially where 
it issues from the nerve-cell, give it a finely striated appearance. 2 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 

__ j _ * J *J 

function ; and that the remaining investments are merely protecting 
structures. 8 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. 4 

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 
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. Tliis view finds support 
ir^ observations made upon the peripheral terminations of the nerve- 
Jibres. 

1 EWALD andKCHNE, 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 PFLCGER'S Arch. /. d. ges. Physiol., xxii., 1880, i ff. ; KOLLIKER, 
Gewebelehre, 6te Aufl., ii., 13. 

2 MAX SCHULTZE, in STRICKER'S Gewebelehre, 1871, 108. POWER'S trs., i., 1870, 150. 

* PIERSOL, Normal Histology, 63 f. TRANSLATOR. 

* See below, Ch. v. 2. 



39-4] Morphological Elements : Peripheral Nerve Huttings 47 



(c) Peripheral Nerve Terminations 

The termination of a nerve in the peripheral organs may take one of 
two forms. Either the ends of the nenritic threads divide up into a 
fascicle or network of finest den'dritic 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 develop- 
ment, by the penetration 
of a nerve fibril into an 
epithelial cell. The two 
forms of nerve-termina- 
tion occur side by side, 
in these their character- 
istic 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 termina- 





FIG. 18. Typical forms of sensory nerve-termination. 
A Free ending of a cutaneous nerve-fibre. 5 Sensory 
fibre, n Terminal reticu'.um between the cells of the 
epidermis. B Ending of an olfactory nerve-fibre. 
r Olfactory nerve-fibre, g Olfactory cell, e Epithelial 
cells. 



tions 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 deli- 
cate fibrils, whose dendri- 
tic 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 
y the cells of the deeper lying connective tissue, and thus transform' these V' 

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 ners r e-fibre enters, in the olfactory mucous 
membrane, into a nerve-cell. This cell, which lies between epithelial cells, 



Structural Elements of Nervous System 



[40-1 




FIG. 19. Motor nerve-terminations in the 
cross-striated muscle-fibres of the rat. 
After SZYMONOWICZ. 



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. 1 

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. 2 In the 
muscles of reptiles, birds and mam- 
mals, 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). 3 

(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 

1 For a detailed account of the central and peripheral terminations of the sensory 
nerves, see ('hs. v. vii., below. 

2 PIERSOL, Nornwl Histology, 90. TRANSLATOR. 

3 KCHNE, in STRICKER'S Gewebelehre, ii. ( 1871, 682. POWER'S trs., i., 1870,210. 
SZYMONOWICZ, Lehrbuch d. Histologie, 1901, 306. MACCALLUM'S trs., 1902, 312 ft. 



41-2] Morphological Elements ; Neurone TJieory 49 

re 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, 
neurone. In the light of the neurone theory, the whole of the central 
lervous 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. 1 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 schema of the motor neurone train, 
jjiyeji in Fig. 20, consists of two neurones, the one of which (N { ), as motor 
cell (Zf) in the ventral cornu of the myel, is attached directly to a peripheral 
muscle-fibre (M), while the second (iV//) belongs to a higher nervous centre. 
The neurite proceeding from the cell Z n gives off a certain number of 
collaterals, and finally resolves into fibrils that come into contact with the 
dendrites of the cell Z t . 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, N T , that has its 
centre in a bipolar spinal-ganglion cell Z/, and a central neurone, N n , that 
belongs to a nerve-cell, Z n , lying somewhere in the higher regions of myel 
or brain. The neurone N r 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 N n . 
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., VI1L). 

Whether the individual cell territories are, always and everywhere, 
related to one another in the manner indicated by these diagrams is, we must 

I \VALDEYER Uebereinige neuere Forschungen imGebietder Anatomie des Centralnerv- 
ensy stems, in the Deutsche med. Wochenschrift, 1891, nos. 44-48. For the history of the 
theory, see M. von LENHOSSEK, Der feinere Bau des Nervensy stems, 2te Aufl., 1895, 
103 ff. ; M. VERWORN, Das Nfuron in Anatomie und Physiologic, 1900. 

P. E 



Structural Elements of Nervous System 



[42-: 



admit, still an open question. So far, the neurone theory must be regarded 

s rnj h is an hypothesis thai l>7in^ together, in a very happy way, a large 
numbci . oi the data oi current histology. Whether the definition oi the 
neuron, in gcnei il, md 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_atjhe present^ 






FIG. 20. Schema of a motor neurone train. 
After RAMON Y CAJAL. 



FIG. 21. Schema of a sensory 
neurone train. 



the theory does not want for opponents. Fortunately, the settlement 
of these controversies among the morphologists is no^of_decigive 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, lattr 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 ide^i of neurones, and the view that this idea 



43-4] Morphological Elements : Neurone Theory 5 1 

suggests of a connexion between the central elements which is relatively 
variable, and in certain circumstances perhaps de terminable by the exercise 
>f 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 proye 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 facts 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 ven- 
tral cornua of the myel. GERLACH discovered the fibrillar structure of the inter- 
cellular substance, and His the embryological connexion of nerve-fibres with 
nerve-cells. GOLGI, KOLLIKER, NANSEN, W. His, G. RETZIUS, RAMON Y CAJAL 
and many others have made the nerve-cell a subject of special investigation. 1 
It is but natural that the results obtained should not be always in agreement. 
GOLGI and NANSEN supposed that thedendrites 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 pro- 
pounded 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 of 
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 cen- 
tripetally 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- 

1 Cf. the bibliographies inM. VON LENHOSSEK, op. cit. 36 ff. ; KOLLIKER, Gewebelehre, 
6te Aufl. ii. 5 ff. 



52 Structural Elements of Ncivous System [44- 5 



nccted. fg^rjpffil, in this SRnsp , are all conduction-pa ths that co 
citations to determinate nerve-cells; centrifugal; ill conduction-paths th.at 
carry cxcitati": ":;'. them. In general, therefore, the peripheral sensory 
nerves will In-long to a centripetal, and the_ motor nerves to a centrifugal system, s 
Hut 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 game riffle. there has always been a certain amount of dissen t , based 
, spe i; Ih upon the oft n peated obsi rvation ol the continuity <>l ihe fibrils 
wnfifirTne nerve-cells. 1 It^has even been maintained that the fibrils pursue 
.in 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, 2 and now revived on the ground of further work upon the 
same morphological phenomena. 3 

The structural schema of RAMON Y CAJAL, and the neurone theory that is 
based uj < n it, si md in the forefront i recent neurolqgii .M investigation. Anato- 
mists 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 accumula- 
tions of granules (Figg. 10, 14),* and the observations made in many quarters 
on the fibrillar structure of the nerve-cells. 5 Neither of these, it is true, has 
passed unchallenged ; 'x>th the granular masses and the fibrils have been ex- 
plained as precipitates from the cell-substance, due to microchemical treat- 
ment or to post-mortem coagulation. 8 Nevertheless, the hypothesis that these 
structures exist in the living tissue is confirmed by the fact that they have been 
observed in fresh preparations, untreated by staining reagents (Fig. io). 7 

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 cer- 
tain parts of the cell-body, while the nucleus becomes farther and farther dis- 

1 NISSL. Kritische Fragen der Nervenzellenanatomie, in the Biol. Centralblatl, 1896, 
1898. HELD, Arch.f. Anatomic, 1897, 204; Suppl., 273. BETHE, Biol.Centralblatt, 1898, 
no. 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. 

2 MAX SCHULTZE inSxRiCKER's Gewebelehre.iBfi, 108 ff. POWER'S trs., i. 172. 

3 APATHY, Biol. Central blatt, 1889 and 1898 (vols. ix. and xviii.). Mittheilungen am 
der Zoo/. Station zu Neapel, xii. 1897 ; also in Amer. Journ. of Insanity, lv., 1898, 51 ff. 

4 NISSL, Allg. Zeitschr. f. Psychiatric, \., 1894. 

5 FLEMING, Arch. f. Mikrosk. Anatomic, xlvi., 1895, 373. LENHOSSEK, ibid., 345. 
MUNCKEBERG and BETHE, ibid., liv., 1899, 135. 

a HELD, op, cit. Sometimes, as was discovered by BOTSCHLI (Untersuchungen iiber 
mikroskopische Schdume 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. 

' J. AKNOLP, Arch. f. mikwsh. Atiatnmir, Hi., 1898, 542. 



45-6] 



Morphological Elements : Neurone Theory 



S3 



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 
V_jjf 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 proceeding 
from it, and which apparently makes the cell their nutritive as well as their 
functional centre. 1 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 de- 
generation. 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 (6). These in turn are slowly 
resorbed (c) until they altogether dis- 
appear : so that, finally, nothing is left 
of the nerve but its connective tissue 






FIG. 22. Degenerated nerve-cells. A 
Cell in the state of inflammatory turges- 
cence. B Atrophied cell. After FRIED- 
MANN. 



FIG. 23. Secondary degeneration in a 
nerve-fibre, whose connexion with the 
centre has been severed, a, b, c Different 
stages of the degenerative process. 



investments. 2 It is, however, probable that the appearance of these degenera- 
tive 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 cellshrinkage sets in com- 
paratively quickly, after extirpatiorToT the region of nervous diffusion. It has 
also been observed in man, as a secondary atrophy of the nerve-centres. 3 

1 NISSL, Allg. Zeitschr. f. Psychiatric, xlviii., 1892. MARINESCO, Arch. f. Physiol., 
1899, 89. VON WENDT, Skandin. Arch. f. Physiol., xi., 1901, 372. M. FRIEDMANN, 
Neurolog. Centralbldtt, 1891, I. 

2 MONCKEBERG and BETHE, Arch. f. mikrosk. Anatomic, liv., 1899, 135. 

3 GUDDEN, Arch. f. Psychiatric, ii., 693. 



54 Structural* Elements of Nervous System [4 6 ~7 

Such arc the phenomena that occur as after-effects of enhancement or aboli- 
tion 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 question- 
able nature. Ajrincbnid movements of the dendrites were first described by 
RABL-RiicKHARD. They may possibly be explained as phenomena of imbibi- 
tion and coagulation. At any rate, the psychophysical theories of sleep and 
waking, dissociation of consciousness, and what not, that certain authors have 
erected upon them, are rjur^Vy_in}aginary ^psychological constructions, based on 
an extremely scant and more than doubtful foundation of physiological obser- 
vation. 1 

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 surrroundin^ the myclin, which 
is said to consist of a corneal substance allied to epithelial tissue, and termed 
neurokeiatin. 2 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 as- 
signed the empirical formula C no H 2tl N i P0 22 . This formula is, naturally, 
intended merely to give an approximate idea of the extreme complexity 
of the chemical molecule of this compound. 3 From prQtagon_are derived 
/ lecithin and cerebrin, decomposition-products which probably occur along- 
side of it in the nerve-substance, and together with it form the myelinof 
Jhe 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- 

1 DUVAL, Hypothtse sur la physiol. des centres nerveux, in the Comptes rendus de la 
socielt de biologie, 1895. SOUKHANOFF, La thiorie des neurones, etc., in the Arch, de 
neurologic, 1897. QUERTON, Le sommeil hibernal et les modifications des neurones, 
Institut Solvay, Bruxelles, 1898. 

2 W. KOHNE and CHITTENDEN, Zeitschr. f. Biologie, N.F. viii., 1890, 291. 

3 LIEBREICH,. 4nn. derChemieu. Pharmacie, cxxxiv., 1865, 29. According to KESSEI 
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 thegeneral relations of the chemism of nerve-sub- 
stance to the physiological processes. 



47~8] Chemical Constituents 55 

stitutjon^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. 1 Lecithin Tias two characteristic properties. The large pro- 
portion of carbon and hydrogen which it contains gives it a high heat of 
combustion ; and its complex nature renders it easily decomposable. Cere- 
brin, if boiled with acids, yields a sugar and other, unknown, decom- 
position 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. 2 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 com- 
bustion. nervous tissue contains substances which are classed with the 
proteins, but oi 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. 3 Some observers have, in fact, 
maintained that this free acid increases, as a result of activity, just as it 
does in muscle. 4 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_^eripheral 
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 
e blood are ultimately transformed into compounds of still greater com- 




1 The constitution of ordinary lecithin, according toDiAKONOW, is C^ 
distearyl-glycerin-phosphoric acid + trimethyl-oxethyl-ammonium-hydroxide. Accord- 
ing 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. 

2 W. MULLER (Ann. d. Chem. u. Pharm.cv., 1858, 361) has worked out for cerebrin 
the empirical formula, C 37 H 33 NO 3 . On the cerebrin series, cf. PARCUS, Journ. f. prakt. 
Chemie, 1881, 310 ; NEUMEISTER, Physiol. Chemie, 2te Aufl., 472. 

3 GSCHEIDLEN, in PFLtJGER's Arch. f. d. ges. Physiol., viii., 1874, 71. 

4 MOLESCHOTT and BATTESTINI, Arch, de biologie ital., viii., 1887, 90. 



56 Structural Elements of A T enwts System 

plexity, representing (as their high heats of combustion show) a very con- 
siderable 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 pro- 
teids into more complex. Apart from the undisputed observation of 
synthetic processes within the body, 1 we have further evidence in the fact 
that substances containing phosphorus, which closely resemble the albu- 
minates in their composition and chemical conduct, appear under con- 
ditions that definitely suggest their formation within the organic cell. 
A compound of this kind, jnuclein, appears in particularjo form the princi- 
pal constituent of the cell -nuclei. 2 Hence we ma.y 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. 



1 E. BAUMANN, Die synthetischen Processe im Thierkorper. Inaugural lecture. 
Berlin, 1878. 

2 MIESCHER, in HOPPE-SEVLER'S Physiologisch-chemische Untersuchungen, 4, 452 ; 
Lu BAVIN, ibid., 463. 



49-50] 



CHAPTER III 

Physiological Mechanics of Nerve-Substance 
i. 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 in- 
quires into the changes which these properties evince as a result of physio- 
logical 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 mor- 
phological investigation, mentioned above (p. 53), the study of the func- 
tional changes of the central elements is, as yet, hardly more than a pro- 
gramme. And our knowledge of the internal processes in the peripheral 
nerves is also severely limited. We know that their functioning is attended . 
X by electrical and chemical changes, the, meaning of which is still obscure : / 
\wejoiow: 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 syste- 
matic completeness. If the molecular mechanics of the nervous system 



* 



58 Physiological Mechanics of Nerve Substance [50-1 

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 func- 
tions, is termed in physiology a.^stiruMlus. 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. Jf we do this r 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 out- 
side 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 sur- 
rounding 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 in- 
fluences exerted upon the organism by the external world in which it lives. 
_As regards nerve-substance, therefore, all stimuli whatsoever are to be 



classed is 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 

\^ - - ' ' . - j 

are almost entirely beyond the range of experimental control. We ac- 
cordingly 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. \ve then begin with that peripheral effect of nervous excitation 
which i- 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 measure- 
ment, that is released in the central organ by the stimulation of a centrally 
directed nerve-fibre, the reflex contraction. In neither of these cases, 



5 1-2] Methods of a Mechanics of Innerrntion 59 

however, does the muscle-contraction afford a direct measure of the pro- 
cesses 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 cer- 
tain 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 
\y 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, _th_e jmtscular 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 corres- 
ponding change in nervous excitability. Orf the contrary, since the_con- 
\jractile substance has its own intrinsic irritability, which it maintains in 
face of stimulation whether directly applied or transmitted to it by the 

y 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 

-^yf -j ii~ ^*s __, r__5?_ - -_ - - J- \ 

/\ quite different effects in muscle : or conversely, may set up different pro- - 
cesses in the nervous substance, while the contractile substance shows 
the same reaction. \Ye 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 con- 
stancy in the properties of the muscle experimented upon, or at least make"" 
such constancy highly probable. For the rest, the properties of the con- \> 
tractile su~bstance itself, and the related phenomena of the course of the 
muscuVir contraction, may here be left out of consideration, as their interest \ 
js__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 

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 

1 A good summary of the most important facts regarding the mechanical properties 
of muscle will be found in TIGERSTEDT'S Lehrbuch der Physiologic, ii., 1898, 128 ff. The 
reader should compare with this the recent papers of ROLLETT (PFLUGER'S Arch. f. d. ges. 
Physiol., Ixiv. and Ixxi.), SCHENCK (ibid. Ixii., Ixiii., Ixiv., Ixv., Ixvii., Ixxii.) and KAISER 
(Zeitschr. f. Biologic, xxxiii., xxxv., xxxvi., xxxviii.). 





60 Physiological Mechanics of Nerve Substance [5-~3 

in explaining the interrelations of various forms of movement-process. 
This is the law of the conservation of work. 

(6) 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 num- 
ber 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 oppo- 
site transformation of mechanical work into its equivalent amount of mole- 
cular 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, em- 
ployed 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 sepa- 
rates the molecules. 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 



53-4] Principle of Conservation of Work 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 interpre- 
tation 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, f n 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 dis- 
gregation 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 can 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 thejljsgregation is abolished. But a lifted weight remains lifted 



62 Physiological Mechanics of Nerve Substance [54~5 

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 mole- 
cules effected, and the moment at which the work required for these opera- 
tions 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 me- 
chanical 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 transforma- 
tions 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 re-appear 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 conserva- 
tion 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 sum 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 ' mole- 
cular work. 



55-6] Principle of Conservation of Work 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, illus- 
trations 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 con- 
taining 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 applica- 
tion 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 of 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 de- 
composition and recomposition of chemical compounds. In every sub- 
stance we can distinguish between the state of physical and the state of 
chemical equilibrium. For every molecule, in the physical sense, con- 
sists 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, accord- 
ing to the character of 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, 



64 Physiological Mechanics of Nerve Substance [5 6-7 

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 station- 
ary 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 can- 
not maintain it, but are always compelled at once, by the forces of chemi- 
cal 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. 1 
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 com- 
pounds. 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 instable 
in the aggregate state of solidity. In general, the compounds of homogene- 
ous 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 ' sub- 
stances. 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 con- 
versely. There is, as ST. CLAIRE DEVILLE proved, no compound so stable 
that it cannot be dissociated by the application of heat in sufficient quan- 
tity. Here, as in the change of a liquid into a gas, a certain amount of the 
1 CLAUSIUS, Abhandlungen zur mechanischen Wdrmethenric, ii., 1867, 214. 



57~8] Conservation of Work in Nervous System 65 

internal work of the heat disappears, transformed into work of dissocia- 
tion. When the dissociation is complete, the atoms are in a new state 
of equilibrium. In the dissociation of water, e.g., the more stable con- 
nection H 2 gives place to the less stable forms H.H and 0.0, in which 
the vibratory condition of the atoms differs from that in the stable com- 
pound H 2 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 com- 
pound 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 com- 
position 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. 

(c) Application of the Principle of the Conservation of Work to the Vital 
Processes and the 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. Car- 
bonic 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 retransformed 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 thVsame 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 ex- 
ternal work of the contractile substance. The central station, from which 
all these processes of the animal body are directed, is the nervous system. 

p. F 



56 Physiological Mechanics of Nerve Snbstana 

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 
, ,,; i;; ,, - i. ;mi ] 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 con- 
stant alternation of decomposition and recomposition of chemical com- 
pounds ; 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 re- 
uniting, enter into their old connexions, or into connexions of the same 
order, but combine afresh to form simpler and more stable compounds. 
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 cer- 
tain work, which, however, is counteracted by the work of other atoms, and 
consequently is not perceptible outside the molecule. This internal 



59~6o] Stimulation- Processes in Xerve-Fibre 67 

molecular work is far more considerable in an instable chemical com- 
pound, 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 com- 
pound ; 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 ' posi- 
tive ' 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 un 
changed, by the continual compensation of the existing quantities of posi- 
tive and negative external molecular work. Or, to put 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 contracticn 
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 

FIG. 24. 
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- 



68 Stimulation- Processes in Nerve- Fibre [60- 1 

tomatic of the stimulatory process. The muscle carried no weight beyond 
the light writing lever, and gave a twitch in response to stimulation. Nu- 
merous observations have shown that ..ike loading of a muscle increases its 
irritability. L'ndci the present circumstances tin- 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. 1 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 
arc 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 o'oS 
o'l sec. Provided that the nerve is stimulated directly above the muscle, 
about O'oi 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, 
J _^_ the other as near to 

i ' -*Z^~ ^ l -S^>< ' *"^ -^^^ 

it as possible, and 
FIG. 25. 

when the experi- 
ment 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 contrac- 
tion 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 IJELMHOLTZ discovered, the 
curve of contraction given by the more remote stimulus begins later has 

1 Muscle-curves of this kind are termed, as proposed by A. PICK, ' 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 ob- 
tained 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. 



6 1 -2] Muscular Contraction from Motor Nerve-Stimulation 69 

a longer latent period than the other. Secondly, as PFLUGER first showed, 
Jjie 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 con- 
traction 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 stimu- 
lation is relatively slow, for the frog-nerve at ordinary summer tempera- 
ture it averages 26, for the nerves of warm-blooded animals at normal 
body temperature 32 m. in the i 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 strik- 
ingly in the uncut nerves of the living animal. 1 

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. 

1 Cf. my Unlersuchungen zur Mechanik der N erven 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 PFLUGER (Untersuchungen iiber die 
Physiologie des Elektrotonus, 140), has been referred by many physiologists, following 
HEIDENHAIN (Studien des physiol. Instituts zu Breslau, i., i), 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 (PFLUGER'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. 



70 Stimulation-Processes in Nen>e- Fibre [6*-$ 

Here, as 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 constant, the observed changes 
must necessarily depend upon the processes taking place in the nerve. 

(6) Excitatory and Inhibitory Processes in Nerve-Stimulation 

We must suppose, if we apply the principle of conservation to the pro- 
cesses in the nerve, that every process 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 stimula- 
tion-processes of so little intensity that they are unable of themselves, 
without the intervention of the test-stimulus, to release any muscular con- 
traction 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 
' ' ' h and ':. ontra tion normally released by the test-stimulus ;uv 
suppressed, or at most only -a minimal (what is called an 'isometric') 
twitch remains possible. If now, during the progress of the first stimulation, 
tin- 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. Fig. 26 gives an 




FIG. 26. 



illustration of this procedure. The stimulation -process here under investiga- 
tion 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 



64-5] Excitatory and i nliib itory Processes *]\ 

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 performed, 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, /, g, gave the contractions 
b', c', d', e', /', g'. The course of these curves shows clearly that the stimu- 
lated 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 considera- 
bly 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 6. 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 =o) ; 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 



Stimulation-Processes in Nerve-fibre 



[6 5 -6 




FIG. 27. 



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 
overloaoT the muscle, in the manner indicated above, and so to reduce the 
contractions C and R to zero or to a minimal height. Adopting this method, 
we find that, as a matter of fact, in experiment B the excitatory effects had 

the upper hand. 

i c n ~ 

A 



Now the dif- 
ference 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. 1 

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 demon- 
strable. 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 stimula- 
tion-process when the effect of weak test-stimuli is wholly suppressed, the 
effect of strong stimuli may be increased. 2 

It follows from the above results that, if we desire to gain a quantitative 
expression of the relation which the inhibitory effects bear 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, 

1 Experiments on the superposition of two contractions were first made by HELM- 
HOLTZ (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 con- 
tractions. The greater heightening of the curve of summation has, however, been 
confirmed by KRONECKER and STANLEY HALL (Archiv. /. Physiologic, 1879, Supple- 
mentband, 19 f.). 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. 

a Mechanik der Nerven, i., 109 ff. 




66-7] Excitatory and Inhibitory Processes 73 

equally sensitive to inhibition and to excitation. Experiments made under 
these conditions show that the stimulation-process developing as the result 
of a. momentary stimulus, e.g. of an electric shock or mechanical concussion, 
J'uns its course as follows. At the moment of stimulation, and lor a brief 
period afterward, the nerve does not react to the weak test-stimulus at all ; 
the process takes precisely the same form as it would it the stimulus had 
not acted. 1 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^). We obtain the sams 
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, 
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). __T_his. 

enhancement of irritability A "_ ^ : ^. \.~ 

increases up to a point cor- ^ ^-""^ 7T~-^ X '-^' 

responding roughly with the ^^ -- ;N ' ^" 

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 

^Jcourse of the stimulation-process is in general divisible into three stages :, 
the stage of inexcitability. the stage of increasing, and the final stage of 

/ V decreasing excitability. 

Oftentimes, however, this third stage is interrupted by a brief interval, 
during which irritability shows a sudden marked decrease, quigklyJbllowed 
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 in- 
crease 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 stimula- 
tion 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. 
1 Mechanik der Nercen, i., 63, 100. 




74 Stimulation- Processes in Nerve-Fibre [6/-# 

The curves of A were obtained from a fresh nerve, those of B from a nerve 
that had already been subjected to repeated stimulation. 1 In this pheno- 
menon, 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 

RC 



Fig. 29. 

conditions residing in the muscle itself. Thus, the stage of inexcitability. 
which appears .,: thi beginning <>t stimulation, may very possibly i>e 
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 ex- 
plained 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 
a :cruing 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 _o_f excitation thus resembles a suddon discharge, in 

1 Mechanik der Nerven, i., 86, 190, 200. 



68-9] 



Practice and Fatigue 



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 that of 
the inhibitory effects ; in the latter case, the intensity of the inhibition is 
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 *IG. 30. 

curve of inhibition is char- 
acterised 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 pro- 
cess, are no longer observable. 

(c) After-effects of Stimulation : Practice and Fatigue 

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. 4t_the samejime, the contractions 
become longer ; while the longer after-effect shows further that the course 





76 Stimulation-Processes in Nerve-Fibre 

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 em- 
ployed, 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. 1 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. 2 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 em- 
ployed 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 tKe" 
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 ; though these, in their gradual 

1 Wt'NDT, Arch. f. Analimie u. Physiologic. 1859, 537; 1861, 781. Unlersuckungen 
zur Mechanik der N 'erven, i., 177 ff. 

2 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 cu BOIS-REYMOND'S Arch. /. Physiologic, 
1888, 538. 



/o-i] Practice and Fatigue 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 
jthej>henomenon 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 be- 
ginning, then more slowly. Then, from a certain point onwards, the height of 
lift remains the same, while the duration of the contraction is quite consider- 
ably 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. 1 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 be 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 con- 
traction of the muscle during the application of stimuli to the nerve. With 
a sufficiently overloaded muscle, e.g., no amount of repeated stimulation 
will bring out 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. 2 

1 WUNDT, Arch. f. Anatomic u. Physiologic, 1859, 549, 

9 BOWDJTCH, Journal of Physiology, vi,, 1887, 133, Arch, f. Physiologie, 1890, 505, 



78 Stimulation- Processes in Nerve-Fibre [? l ~ 2 

We must conclude from these results that the elementary phenomena 
of practice and fatigue are of radically different origin. The prime condi- 
tion 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. JHere, 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 conse- 
quently 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 move- 
ment. They are secondary phenomena, 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 ex- 
haustion. 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 pheno- 
menon 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. 



72-3] Stimulation of Nerve by Constant Current 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-pheno- 
mena at large ; and the phenomena which accompany this mode of stimula- 
tion 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, inhibi- 
tory 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 de- 
creases 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 i sec., very much slower than the rate of the excita- 
tory 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 elec- 
trodes : at make, the excitatory forces predominate in the neighbourhood 



8c Mechanics of Innervation [73-4 

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. 1 

The phenomena of nervous stimulation are attended by other phenomena 
besides that of muscular work. Only the thermal and electrical changes, how- 
ever, 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. 2 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 stimula- 
tion-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 i sec. ; the anodal wave of in- 
hibition travels, as BERNSTEIN found, at the rate of only 8 to 9 m. in the i sec. 3 

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 dis- 
engagement of heat or in some form of external work, such as muscular con- 

1 PFLCGER, Unlersuchungen iiber die Physiologic des Elektrotonus, 1859. WUNDT, 
Untersuchiingen zur Mechanik der Nerven, i., 223 ff. 

2 A. PICK, Mechanische Arbeit und Wdrmcentwickelung bei der Muskelthatiekeit 
1882. 

3 BERNSTEIN, Monalsher. d, Berliner A kademie, 1880, 186. 



74~5] Theory of Nervous Excitation 8 1 

traction. The negative molecular work will counteract these positive effects : 
heat will become latent ; the progress of a muscular contraction will be in- 
hibited. Equilibrium of the two opposed kinds of molrr.ular work brings 
about the stationary condition of the nerve, during which there is no change 
of temperature and no accomplishment of external work. Hence, when- 
ever 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 dis- 
cussions, 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 con traction, 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 be 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 connexions. 
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 trans- 
formed 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. o 



82 Mechanics of Innerration 

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 rela- 
tions 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-estab- 
lished in large measure, and in nerves of high functional capacity probably 
in full measure, during jhe actual progress of the contraction. This ten- 
dency to the maintenance of equilibrium between positive and negative mole- 
cular 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 essen- 
tially 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 portion 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. Simi- 
larly, 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 irrita- 
bility, both during and after the contraction. Hence a supervening stimulus 
will invariably find 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 



76-7] Theory of Nervous Excitation 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 ofjthe_ sums of positive and negative molecular work which 
obtains in the latter case. While the current is closed, there is preponder- 
ance 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 oil. At 
both places, that is, the work 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 disap- 
peared may be set free again. The phenomena of stimulation lead us to 
infer that the first of these processes takes place regularly in the neighbour- 
hood 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 con- 
sequence 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 o'oooG to 0^0007 sec. after the application of the 
stimulus, 1 and therefore falls completely within the period of nervous in- 

1 According to BERNSTEIN'S investigations, PFLUGER'S Arch. f. d. ges. Physiol., i., 
190; Untersuchungen ilber den Erre$ungsvo;-<*ang imNerven- und Muskelsysteme, 1871, 30. 



Mechanics of Innervation [77~S 

excitability. 1 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 physiolo- 
gists not infrequently substitute for them the words ' assimilation ' and ' dis- 
similation,' antithetical terms borrowed from the vocabulary of metabolic pheno- 
mena, 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 
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 ' assimila- 
tion ' and ' dissimilation ' are, apart from the effects from which these names 
arc derived, altogether unknown to us ; and the effects themselves are simp'v 
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, 
Jhe_decomposition of the complex molecules, and the combustion-processes re- 
sulting 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 ho\v in detail the interchange of energy is effected 
in the two case?, the reply is that as regards dissimilation very little, and as 
regards assimilation practically nothing is known. The various processes in- 
volved 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 comp'ication is vouched for by the fact, now fairlv 
well established for all such processes, that in dissimilation there is a constaiu 
interplay of decomposition and recomposition ; existing chemical connexions 
are dissolved, and new compounds formed, in continual interdependence. A* 
a rule, energy is liberated during these dissimilations, in the form of heat o. 
of mechanical work. But ; again, we cannot say which phase of the dissimilation 
process is responsible for this result ; nor do we know if a similar interchange 

1 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 inexcitabihty. 



78-9] Course of Reflex Excitation 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, but 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 per- 
mit of our answering these questions. The theory of assimilation and dis- 
similation 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 dissimila- 
tion 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. 1 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 compli- 
cated. When we consider, therefore, that the words ' assimilation ' and ' dis- 
similation"' 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 syndromus, to their simplest components. We 
thus discover that every process of fatigue and recuperation contains two ele- 
mentary terms, variously interrelated in the particular case, -excitation and 
_ inhibition. Or, on the other hand, we may attempt to refer the effects in ques- 
tion 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 straightforward 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 sensa- 
tions, in the theory of auditory sensations, applied to all other conceivable 
physiological and psychophysical phenomena. It almost seems, indeed, as if 
this pair 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 nine- 
teenth 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 sub- 
stance by stimulating a peripheral nerve, and endeavouring to find out how 
the course of stimulation is altered, if it is compelled to pass through centra' 

- i BIEDERMANN, Etektrophysiologie, 1895. 7i_ff. 



86 influence of Centre on Excitation Processes [79~ 8 

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. 1 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 



FIG. 31. 

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. Ltjs 
true that intensification of stimulus increases not only the height but also 
the duration j)f the_contractions. while decreasing thejatent period. But 

1 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 (0-032 to 
at most 0-004 m g-) f strychnine. I have convinced myself, by experiments specially 
directed upon Ihe point, that minimal quantities of this poison do not affect the tem- 
poral cours? cf the reflex contractions. See Untersuchungen zur Mechai ik der N erven 
und Neruencr.il en, ii., 1876, 9. 



X 



8o-l] Course of Reflex Excitation 87 

the weakestjreflex contractions are always noticeable by their long^ duration^ \S 
and the strongest by the length of their latent period, even when we com- ' 
pare the former with the strongest and the latter with the weakest direct 
contractions. 1 ^is clear, now, that the time which the stimulation requires 
to j>ass 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-curves 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 th 
stimulation is transferred from a sensory root to a motor root bd >ngiii 
,to_the same nerve-trunk : we may term this the case of same-sided reflex 
^xcitation. 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 b2en 
predicted, relatively shortest for same-sided reflex excitation, where under 
normal circumstances it amounts to 0*008 to 0*015 sec - It K 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 o'oo^ 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. 2 
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 longitu- 
dinal conduction. An explanation is, without any doubt, to be found 
in the fact that longitudinal conduction (as we shall sec presently, in 
Chapter v., when we come to discuss the morphology of the myel) is sub- 

1 Exceptions to this rule may occur, though very rarely, in cases of maximal reflex 
excitation and minimal motor stimulation : op. cit., 21. 

2 Op. cit., 14, 30, 37. 






' 4 
88 Influence of Centre on Excitation Processes [82 -j 

served for the most part by the fibres of the white substance, wiriiJe 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 con- 
duction, 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. J ust, then, as there are mechan- 
isms in the spinal ganglia which diminish the irritability of the incoming 
nerves, so must there be 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 tlie advent of useless sensory excitations. 1 

(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 develope 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, 2 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. 3 Within certain limits, the reflex makes its 
appearance after the same number ot single stimulations, whether these be 
given in quick or slow succession. On the other hand, the duration of a 

1 Op. dt., 45 f. 

2 Cf. Note i, p. 86, above. 

3 KRONECKER and STIRLING, Berichte d. k. sdchs. 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 char- 
acter (Arch. f. Physiologic, 1878, 23). I cannot follow them in this statement. It may 
evidently be explained by the fact that KRONECKER and STIRLING did not avail them- 
selves 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 under- 

vx stood 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. 



83-4] Enhancement of Reflex Excitability 8c) 

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 ba of 
longer duration, the tetanus disappears earlier than the stimulation itself. 1 
Another phenomenon, that shows very clearly _the differences in excitability 
_betweerijhe^eripheral 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, 2 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 i sec., the muscle traced the upper 

curve when the motor nerve was 
stimulated, the lower, when the 
stimulation was applied to thr- myel, 
IG ' 32 ' winch had been severed below the 

oblongata. 3 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, leleased 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 o'i66 sec., while movement 
initiated by voluntary innervation required 0*296 sec. 4 

We can easily see the reason for the greater effect produced upon the 

myel by frequent repetition of the stimulus. Every stimulation leaves 

__behind i t anjwfwnce^rsflex excitability. Here, again, however, the central 

substance merely exhibits, on a laiger scale, phenomena with which we 

are already familiar in the case of the peripheral nerve. On the other hand, 

there aie 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 

about the changes in question with unfailing certainty. Strychnine prob- 

1 BEAUNIS, Reck, exper. swr les conditions de I'aclivite cerebrale et sur la physiologie 
des nerfs, 1884, 106. 

2 HEI.MHOLTZ, Monatsberichte d. Berliner Akademie, 1864, 307. 

3 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). 

* HELMHOLTZ and BAXT, Monatsber. d. Berliner Akad., 1867, 228 ; 1870, 184. Ex- 
periments by VON KRIES (Arch. f. Physiol., 1886, Supplementband, i ff.) gave like results, 



90 



Influence of Centre cm Excitation Processes 



[84-5 



ably owes its power in this regard to the circumstance that its effect is 
limited almost exclusively to the central substance of the myel ; wherer.s 
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. 1 

The action of these poisons is, in general, as follows, (i) 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 the 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 
extraoidinary slowness. An illustration of these changes is given in Fig. 




FIG. 33. 



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 can 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 

i Unlersuchungen zur Mechanik der Nerven, ii., 64. 



85-6] inhibition of Reflex l>y Interference 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. 1 

(c) Inhibitions of Reflexes by Interference of Stimuli 

We have spoken so far only of the influences which enhance the ex- 
citability of the_central_elements. Here, however, just as in the case of the 
nerve-fibre, thejexcitatory 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 
_aiter~removal of trie_ brain. SETSCHEXOW, 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. 2 
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 ; 3 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 4 
becomes coextensive with the sphere of sensory excitation ; and the assump- 
tion of specific inhibitory centres falls to the ground. At the same time, 
while any possible sort of sensory stimulation, whether it affect other sen- 
sory nerves or sensory central parts, may arrest the progress of a reflex 

1 Op. cit., 56 f. ROSENTHAL (Monatsber. d. Bcr'iner Akad., 1873, 104; 1875, 419) 
speaks of a decrease of the latent period in strychnine tetanus, and BIEDERMANN (F.lek- 
trophysiologie, 1895, 501) accepts his statement. I do not understand this result ; 
though with a high degree of strychnine poisoning, and with stimuli of moderate in- 
tensity, the increase of the latent period is not so pronounced as to be obvious at once, 
without the aid of some chronometric instrument. 

2 SETSCKENOW, Physiol. Studien iiber die Hemmungsmechanismen fur die Reflex- 
thatigkeit des Ruckenmarks, 1863. SETSCHENOW and PASCHUTIN, Neue Versuche am 
Him und Ruckenmark des Frosches, 1865. 

3 HERZEN, Sur les centres moderateurs de V action reflexe, 1864, 32. SETSCHENOW, 
Ueber die elektrische und chemische Reizung der sensibeln Ruckenmarksnerven, 1868, 40. 

4 GOLTZ, Beitrdge 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 unin- 
jured 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, I33)and BOFTICHER 
(Ueber Reflexhemmiinz, in 'Sammlung physiol. Abhandl., ii. Reihe, Heft 3) show that the 
same result may be obtained by blinding the animals. 



c)2 Influence of Centre on Excitation Processes [86-7 

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 excita- 
tions in the same central territory, quite generally, an interference of 
stimulations. Then the result of such interference is dependent upon four 
things. It depends (i) 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 
Jiand, the original stimulation occurred some time before the application 
ofjthe 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 de- 
pends (4) upon the state of the central organ. The more completely the nor- 
mal 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 
poisons, 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 in- 
hibition. 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 be observed at all. 1 

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. 2 Such an hypothe- 

1 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 PFLCGER'S 
Arch. f. d. ges. Physiol., xxvi., 137 ff. 

2 E. CYON has turned this idea to account for a theory of the central inhibitions: 
Bulletin de I'Acad. de St. Pttersbourg, vii., Deer., 1870. The facts which he adduces 



87-8] Chronic Effects of Excitation and Inhibition 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, how- 
ever, that the phenomena of inhibition are in this case much more pro- 
nounced 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 be produced, side by side, by the simultaneous stimula- 
tion 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 

To nits 

If we inquire into the nature of these special circumstances, we find 
as the most important, the connexions in which the various nervous ele- 
ments 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. 1 This permanent tension 

Jn_the state of rest is known as the tp-nus of a muscle. Its disappearance 
when the nerve is cut indicates that it has its ground in a chronic excita- 
tion 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 In- 
abrogated, not only by section of the motor nerves themselves, but also 
by section of the sensory roots of the spinal nerves. 2 We must therefore 

in support of it, so far as they are taken from the phenomena of vascular innervatipn, 
have been called in question byHEiDENHAiN. PFLVQ-ER'sArch. f. d. ges. Physiol., iv., 

1 BRONDGEEST, Over den Tonus der willekeurigen Spieren, Utrecht, 1860. 

2 CYON, Berichte d. k. sacks, Ges. d. Wiss., math.-phys. Cl., 1865, 86. Cf. on the other 
side, G, HEIDENHAIN, in PFLCJER'S Arch. f. d. ges. Physiol., iv., 1871, 435. 



94 TJieory of Central Innervation [88-9 

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 per- 
sists 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 con- 
tinuations, a surplus of inhibitory forces. Here, e.g., belongs the observa- 
tion that increase of tonus in a determinate muscle-group is regularly 
followed by^ decrease of tension in the antagonists : so that increased ex- 
citation of the flexor muscles of a limb brings with it a decrease of excitation 
in the extensors, and conversely. 1 We may term this phenomenon that 
of negative tonicily. It then becomes evident that the two opposing 
forms of tonus may be brought into relation with the fundamental phe- 
nomena 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 l.irivlv (Irprndont 
upon the influences to which the central elements are subjected, in virtue 
of their connexion with other like elements and with the stimulation pro- 
cesses which these latter convey to them. 



5. Theory of Central Innervation 
(a) General Theory of the Molecular Processes in the Nene-Cell 

The phenomena of central innervation have referred us to the 
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- 

1 H. E. HERING and SHERRINGTON, in PFLUCER'S Arch. /. d. ges. Phvsiol., Ixviii., 
1897, 222 ff. 



89-90] Molecular Processes in Nerve-Cell 95 

derance in the increase of the negative molecular work ; so that a mome.ntary 
stimulus-shock is, as a rule, unable to release any excitation whatever. 
If, however, the stimuli are repeated, then, as one follows another, the 
jarm>unt__of negative molecular work is gradually diminished, in proportion 
to the positive, until at last this latter attains such dimensions that excita- 
tion_arises. We may therefore suppose that the typical process of stimu- 
lation 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 com- 
pounds, 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. 1 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 jthe nerve is the formation of definitive 
r products of combustion, with the performance of positive work ; character- 
istic 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, dis- 
solutions 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 
infertile" formation of nervous substance are, in some cases, more complex, 

1 Cf. p. 53, above. 



g6 Theory of Central In nervation L9 O ~ I 

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. 1 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-celb, 
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 obtain- 
ing 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 
ail. There is, further, a constant movement of material, in the direction 
from centre to periphery ; so that the fibre is in continual receipt of sub- 
stances in which potential work is stored up. Here, it is plain, we have 
the explanation of the nutritive influence which the central substance every- 
where 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 con- 
nected 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. 2 

The differences in the response of the nerve-cells to the stimuli con- 
ducted 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. 

1 Cf. pp. 55 f., above. 

2 Cf. the discussions in Part V. of the abnormal enhancement of excitability in the 
cerebral cortex, which presumably underlies certain forms of derangement of con- 
sciousness (dream, hypnosis). 



91-2] Molecular Processes in Nave- Cell gj 

The central region, we may suppose, is devoted pre-eminently to the forma- 
tion of the complex compounds of which the nervous substance is com- 
posed ; 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 pro- 
duction 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 rein- 
forced by new stimulus-impulses, it is able to produce an excitation. For 
the rest, here as in the nerve, rHs 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 accumu- 
lated 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 somewnat 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 much more considerable, in the former case, if the valve is to be 
opened ; but, when this is done, the steam rushes out with a correspond- 
ingly greater force. It should be added that the peripheral region of the 
nerve-cell probably evinces a different conduct in different cases, approach- 
ing sometimes more, sometimes less nearly to the character of the peri- 
pheral 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. H 



g8 Theory of Central Innervation [9 2 ~3 

complete, while at the same time difficulties are placed in the p:ith of the 
stimulation. 

We are now in a position to interpret the peculiar enhancement of 
renex^xcitability produced by the repetition of stimulus or by the action 
of jxusons. 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 ex- 
citation 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 ex- 
hausted. 

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 pro- 
cesses (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 dis- 
tributed over one and the same mass, according to the direction from which 
the stimulation proceeds. We must, however, remember that these pro- 
cesses 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 con- 
tinuous and gradual transition from the one region to the other. \\ V 
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 pre- 
ponderance of inhibitory, in that of the cathode a preponderance of ex- 
citatory 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 



93-4] 



Molecular Processes in Nerve- Cell 



99 




FIG. 34. 



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, 5 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 proceed- 
ing 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 ar- 
rested 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 d^ndrites. 
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. 1 

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 peri- 
pheral 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 con- 
tinuous 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, 

1 See above, Ch. ii. pp. 42 if. I may be permitted to remark that this theory of 
the directions of central conduction was formulated, on the ground of purely physio- 
logical 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. 



ioo Theory of Central In nervation [94~5 

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 obtain- 
ing in the skeletal muscles; In all regions of innervation that do not stand 
under direct voluntary control, the mechanisms which subserve the pro- 
duction and accumulation of excitatory and inhibitory effects are to be 
found, in part, in the muscles themselves ; so that, under these circum- 
stances, the muscle-substance is endowed, within limits, with the attri- 
butes elsewhere reserved for the peripheral and central nerve-substance. 1 
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 -move- 
ments, 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. 2 We are thus led to the following conclusions. 
First, when an excitatory process is frequently conducted through a ganglion 

1 T. W. ENGELMANN, PFLUGER'S Arch. f. d. ges. Physiol., lvi. ( 1894, 149 ff. 

I take this very useful term Bahnung from S. EXNER, who first proposed it (Ent- 
wurf einer physiologischen Erkldrung der psychischen Erscheinungen, i.,i894, 76), without 
meaning thereby to commit myself in any way to the views and hypotheses put for- 
ward by this author. [As the word ' facilitation ' does not fit the present passage, 
we seem to have no better English term than ' canalisation.' TRANSLATOR.] 



95-6] Nervous and Psychical Processes 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 con- 
nexions. Translated into terms of the hypothesis developed above, this 
would mean that the frequent repetition of conduction, in a certain direc- 
tion, 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, 
jhis^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 inter- 
nal forces. It is to this fact that we must turn for explanation of the prin- 
ciple 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 indispens- 
able 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 prin- 
ciples 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 



IO2 Theory of Central Innervation 

their transmission to other substances, whose properties show them to 
be related to the nervous elements, more especially, therefore, to the sub- 
stances that are of peculiar import for the psychical aspect of vital phe- 
nomena, 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, ;it 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 ot 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, how- 
ever, 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 be 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 thr one to the other. As a matter of fact, we can no more 
derive the mechanics of nerve-substance from the connexions and relations 



97-8] Nervous and Pyschical Processes 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 t<> 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. 



[98-9 



CHAPTER IV 
Morphological Development of the Central Organs 

i. 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, th.2 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-su 1 ..ctnce that has been worked out from 
individual, isolated structures. We must be content to do this in the rough, 
and as a matter of general direction. At the same time, the more com- 
plicated 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 a.* 
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 



gg-lOO] Development of Central Organs 105 

private use, has wrought havoc from the days of GALL and phrenology 
down to the present time. Great advances have been mide in our knowl -:\ -*e 
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 liie 
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 con- 
ditioned 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 mule : 
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 (. , -j 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 



106 Development of Central Organs [ic -1 

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 develop- 
ment. 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 verte- 
brates 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 



IOI-2] 



Neural Tube and l>rai>i Divisions 



107 



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 (lie three anterior sense- 
organs. The nervous primule of the olfac- 
tory 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 products 
of the growth of the fore brain, we must, in 
the light of indisputable physiological facts, 
jook 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 



FIG. 35. Embryonic primule of 
the ovum of the dog, after 
BISCHOFF. a. Neural tube, with 
the three brain vesicles at its 
anterior end. a' Extension of 
the neural tube in the lumbar 
region (sinus rhomboidalis 01 
intumescentia lumbalis). b Pri- 
mule of the vertebral column. 
c Primule of the body-wall. 
d Place of separation of the 
ectodermal and mesodermal 
layers of the germinative vesi- 
cle. / Entoderm. 



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 mammals, 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 epencephalon 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. 



io8 



Development of Central Organs 



[102-3 



Uittelhim 



The three brain-vesicles, then, represent expansions of the anterior end 
of the neural tube. \Yith the tube itself, they form a closed system, whose 

parts intercommunicate by 
way of the continuous cen- 
tral cavity. But the de- 
velopment of the two se- 
condary vesicles from the 
first and third primary 
vesicles brings other changes 
with it. The roof of the 

fore and hind brains divides 
Mtd.'sfm. 



/ Hintcrhim 

Kachhim 



Vorderhirn 




Mittclhirn 



Zttrisckttikir* 



Vorderhirn 



FIG. 36. Brain of a human embryo (seven weeks), 
magn. 3 cliam. A Lateral, B dorsal view. Vorder- 
hirn Fore brain. Mittelhirn Mid brain. Hinterhirn 
Hind brain. Zwischenhirn 'Tween brain. Nach- 
hirn After brain. Lob. olf. Olfactory lobe. Opticus 
Optic nerve. Med. spin. Myel. After MIHAL- 
KOVICS. 



up longitudinally ; so that 
two slit-like apertures ap- 
pear 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 prosen- 
cephalon into its two hemi- 
spheres, and leaves the 
diencephalon open above. 
The mesencephalon, which 
does not share the advance 
in organisation characteris- 
tic 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 
casss, 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 



103-4] 



The Brain Ventricles, etc. 



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. 

(c) The Brain Ventricles and the Differentiation of the Paris 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 aheady 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 hemi- 





FIG. 37. Brain of Polypterus bichir, 
after J. MULLER. A Dorsal, B lateral, 
C ventral view. h Olfactory lobes. 
g Cerebrum. / Thalami. d Bigemina. 
be Cerebellum, a Oblongata. e Hypo- 
physis with the posthypophyses (lobi 
inferiores). ol Olfactory nerve. o 
Optic nerve. 



FIG. 38.^ Brain and myel of the frog, after 
GEGENBAUR. A Dorsal, B ventral view. 
a Olfactory lobes, b Cerebrum, c Bigemina. 
A shows, between b and c, a portion of the 
thalami. d Cerebellum, s Fossa rhomboi- 
dalis (oblongata). i Infundibulum ; the 
chiasma shows anteriorly. m Myel. m' 
Lumbar enlargement of the myel. / Ter- 
minal threads of the myel. 



spheres subdivides the first of these again into two s}7mmetrical 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 



IIO 



Development of Central Organs 



[104-5 



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 con- 
tain 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 from 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. 30). 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, 




FIG. 39. Horizontal longi- 
tudinal section through 
the brain of the frog, 
partly schematic, h Para- 
celes. z Diacele. m Meso- 
cele. s Canal connect- 
ing diacele and metacele 
(Sylvian aqueduct), r Me- 
tacele (fossa rhomboid- 
alis). c Myelocele. 





FIG. 40. Brain of a tortoise (A) and a bird (B), in sagittal section, after BOJANUS and 
STIEDA. 7 Hemisphere, ol Olfactory nerve, o Optic nerve, c Anterior commissure. 
Ill Bigemina . in B we see only the myelinic plate that connects the two bigemina, 
marked a in A. h Hypophysis. IV Cerebellum. V Behind the anterior commissure 
lies the diacele, which passes under the bigeminal plate into the Sylvian aqueduct 
(mesocele) ; this then continues posteriorly, upwards into the epicele, and downwards 
into the metacele. 

1 The ' ventriclees ' are counted from before backwards, with the mesocele (Sylvian 
aqueduct) omitted ; two lateral ventricles (paraceles), third ventricle (diacele), fourth 
ventricle (metacele or metepicele). 



ios-6] 



The Drain Ventricles, etc. 



IT I 




diacele, Sylvian aqueduct (mesocele) and metacclc 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 con- 
sequently 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, ap- 
pear in the paraceles of the avian and mammalian 
brain. 

Both in the myel and in the brain (enceph- 
alon), the nervous mass is formed by prolifera- 
tion of the cells constituting the walls of the 
original cavities. Many of these cells evince the 
character of the formative cells of connective 
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 distri- 
bution of alba and cinerea persists, practically unchanged, in the struc- 
tures developed from the three stem-vesicles. In the developments 
from the secondary vesicles, on the other hand, the nerve-cells re- 
tain 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 



u 



FIG. 41. Transverse sec- 
tion through t-he brain of 
a fish (Gadus lota) in the 
region of the bigemina. 
Magn. After STIEDA. d 
Roof of the bigemina. v 
Mesocele. ts Elevation of 
cinerea upon the floor of 
the mesocele ; torus 
semi-circularis Halleri. 
Sylvian aqueduct (meso- 
cele). li Posthypophyses. 
h Hypophysis. Further 
forwards, v and a unite, to 
open into the diacele ; 
new branches lead from 
this into the posthypo- 
physes. 



112 Development of Central Organs [106-7 

of alba ; in the pallium, a mass of alba invested with a covering of cinerea. 
The cinerea 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 
c nerea, 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, 

FIG. 42. Lateral view of ^ en U p on alba, then upon the nidal cinerea, 

the brain of a human 

embryo (3 months), after then again upon alba, and finally upon the 

KOLLIKER. h Hemi- ectocinerea. 

sphere, m Mesencephalon 

(quadrigemina). c Cere- The series of changes that we have been de- 

bellum mo Oblongata. scr ibing hitherto is accompanied, in all the verte- 
S Sylvian fossa. 

brates, 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. 108). And tuc 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 prin- 
cipally conditioned by the growth of the prosencephalon ; so that degree of 
curvature runs practically parallel with development of the hemispheres. 1 

1 His, Die Formentwickelung des menschlichen Vorderhirns. In Abh. d. k. sacks. 
Ges. d, Wiss., math.-phys. Cl>, xv., 1890, 675. 




107-8] 



Tlie Brain Ventricles, etc. 



i. 



In the early stages of the development of the vertebrate brain, the 
prosencephalon extends anteriorly beyond the remaining brain- divisions, 
.vithout 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 mesen- 
cephalic flexure ; its most posterior portion that which covers mesen- 
cephalon 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 mam- 
malian brains, the concavity of 
the curve of growth is almost 
entirely obliterated, and the walls 
of the fossa draw together, leav- 
ing between them a deep and 
narrow fissure. 

The growth of the pros- 
encephalon over the caudex is 
necessarily followed by a modi- 
fication 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, 
und then, when the curve of the 
hemispheric arch begins to close 
in upon itself, inferiorly and 
anteriorly as well. The central 




FIG. 43. Median view of human prosen- 
cephalon, showing stages of growth : partly 
schematic. After FR. SCHMIDT. Embryos i 
of the 6th week ; 2 of the 8th week ; 3 of the 
loth week ; 4 of the i6th week, a Aula 
(foramen of Monro). b-d Anterior limiting 
lamella of aula. c Crus cerebri. e Lobus 
inferior of the hemisphere, i Posterior 
limit of aula. k Precommissure. g Callo- 
sum. h Marginal arch ; h' external, h" 
internal division, ff' Longitudinal fissure 
of the hemisphere-vesicle, bounding the 
gyrus fornicatus. Olfactory lobes. 



portion of the resulting cavity 

is termed the cell a ; 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. 



Development of Central Organs [108-9 

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, //'. 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 myeldevelopes 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. 



IIO-l] 



Myel in Higher Vertebrates 



All those nervous elements, on the other hand, that have relations to the 
striated muscles collect in a ventral group, corresponding to the animal 
jrmscle-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 be- 
comes 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 con- 
stituting 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 cen- 
tral canal of the myel, around which 
the two collections of cinerea enter into 
cross- connexion (Fig. 45 A). The 
dorsal and ventral sulci (fissurae 
medianae 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 c:j- 




FIG. 44. Transverse section of the 
embryonic myel of the sheep, after 
BIDDER and KUPFFER. cm Myelocele. 
par ly closed, c Epithelium of myelo- 
cele. a Cinerea, occupying almost the 
whole cross-section of the myel. 
b Place of origin of the ventral roots 
/. e Myelic ganglion, with the dorsal 
roots issuing from it. m Primule of 
the ventral and lateral tracts, n Pri- 
mule of the dorsal tract, h Ventral 
commissure, g Envelope of myelic 
ganglion and myel. d Primule of 
vertebra. 



116 



Development of Central Organs 



[Ill 



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 

m 

fibres follow a hori- 
zontal or oblique 



Dorsal sulcus 

Gelatinosa of 
Rolando 



Reticula 

Dorsal commissure 
Myeloccle 

Grey ventral com- 
missure 

White ventral 
commissure 

Indication of the 
lateral cornu 

Ventral sulcus 




B 



F.W 



or 

course ; and the same 
directions are, natur- 
ally, 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, particu- 
larly in the lumbar 
region of the myel, 
are broader and 
shorter, the dorsal 
longer and narrower. 
The former contain a 
quantity of large mul- 
tipolar ganglion-cells ; 
the latter consist ex- 
clusively of smaller 
cells. A good portion 
of the dorsal cornua 
consists, further, of 
the nervous ground- 
reticulum and its in- 
terpenetrating fibrils. 
This gives them a 
peculiarly bright ap- 
pearance, more 
especially as they ap- 
proach the periphery 
of the myel : the region 
is known as the gela- 
tinosa 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), 



Gelatinosa 



Clarke's column 



White ventral 
commissure 




W 



FIG. 45. Transverse section of the human myel, 
x 9. After GEGENBAUR. A from the lumbar en- 
largement, B from the thoracic region of the myel. 



II 1-2] My el in Higher Vertebrates 



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 differ- 
ence 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 spinal ganglion of the dorsal roots (e Fig. 44). 
The dorsal columns are not connected, as the ventral are, by white 
myelinated fibres, but by a grey commissure, composed of fine fibres run- 
ning transversely in the mass of cinerea behind the myelocele (dorsal comm., 
Fig. 45 .4). 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. q, p. 37 above). 

So long as the development of the central organs is confined to the forma- 
tion 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 distri- 
buted 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 enlarge- 
ment. 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. 1 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 2 ; 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 

1 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 con- 
tinuation of the myelocele in the oblongata, which persists as an open depression in all 
vertebrate forms. 

2 Cf. below, Ch. vii. 3. 



n8 Development of Central Organs [112-3 

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 ot nerve-fibres take n 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, dis- 
closing the fossa rhomboidalis (Figg. 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 

A B 

Nidus of funiculus gracilis Nid. of fun. gracilis 

Nidus of funiculus xf^CviaTlll$Xv J X x<^3lBr8^^>v ^ Nid. of fun. cuneatus 

cuneatus 

Dorsal cornu 

Dorsal cornu 




Pyramid. * Dentatum of olive 

Hypoglossal nerve 
Pyramid Pyramidal nidus. 

FIG. 46. Transverse sections of the human oblongata, X2. After GEGENBAUR. A 
from the posterior, B from the anterior portion, just before the opening of the fossa 

rhomboidalis. 

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 (funiculi 
siliquae). The lateral columns (s Figg. 47, 48) grow smaller and smaller 



1 3-4] 



Oblongaia 



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 dia- 
meter. In the lower por- 
tion of the oblongata, they 
are divided by a shallow 
sulcus into inner and outer 
columns, the slender and 
the cuneate funiculi (/g and 
fc Fig. 48), which at the 
lower end of the fossa rhom- 
boidalis carry knob - like 
prominences, produced by 
the grey nidi of the in- 
terior (nid. grac. and nid. 
cun., Fig. 46). Farther up- 
wards, the two funiculi 
appear to continue their 
course in the columns which 
bound the fossa rhomboi- 
dalis to right and left. 
These have been termed 
the restes (restiform bodies : 
pi Fig. 48). They are the 
largest columns of the ob- 
longata, and. like the funi- 
culi just mentioned, contain 
grey nidi. They are charac- 
terised by the intricate, 
trellis - like interlacing of 
their constituent fibres. 
Anteriorly, the restes pass 



n 




FIG. 47. Ventral view of the human oblongata, 
with the pons and the adjoining parts of the brain- 
base. On the left, the transverse fibres have been 
cut away, to show the continuation of the myelic 
columns through the pons into the crus, and the 
floor of the thalamus is exposed, p Pyramid. 
o Olive, s Lateral column, nd Dentatum of 
olive, br Pons. / Crusta. hb Tegmentum. Crusta 
and tegmentum are separated by a deep-lying 
bundle of transverse pontal fibres, cut across in the 
Fig. cc Albicantia. t Tuber with infundibulum. 
h Hypophysis, th Thalami. pv Pulvinar. k Geni- 
cula. sp Precribrum. pp Postcribrum. / XI, 
first to eleventh cranial nerves. / Olfactory ; 
// optic ; III oculomotor ; IV troc'ilear ; V trige- 
minal ; VI abducent ocular ; VII facial motor ; 
VIII auditory ; IX glossopharyngeal ; X pneu mo- 
gastric ; XI accessory. 



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 



120 



Development of Central Organs 



L'H-5 



n 



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 de- 
scribing is the formation 
of an entirely new system 
of fibre-groups surrounding 
the oblongata in a trans- 
verse direction. Some of 
them enter into the ventral 
sulcus and the sulcus separ- 
ating the pyramids from 
the olives ; others strike 
across the fossa rhom- 
boidalis. 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 peri- 
pheral nerves. The simple 
rule of the myel is no longer 
obeyed ; the nerve - roots 
are more or less displaced 




FIG. 48. Dorsal view of the human oblongata, with 
the quadrigemina, thalami and peduncles of the 
cerebellum. On the right, the radiation of the 
cerebellar peduncles within the cerebellum is shown. 
fg Slender funiculus. fc Cuneate funiculus. 
s Lateral column. The divergence of the lateral 
columns discloses the fossa rhomboidalis, on the 
floor of which may be seen the rounded prominences, 
et, divided by a median longitudinal sulcus. g 
Zonal fibres (f. arcuatae). pi Inferior cerebellar 
peduncles (restes). pm Middle cerebellar pe- 
duncles (crura ad pontem). ps Superior cerebellar 
peduncles (crura ad cerebrum), n Anterior, t 
posterior pair of quadrigemina (nates and testes). 
la Postbracl ia. Ik Thalamus. k Fostgeniculum, k' 
pregeniculum. z Epiphysis (pineal body), vm Val- 
vnla (velum medull. ant.). 

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). * 

1 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. 



tl6] 



Cerebellum 

4. The Cerebellum 



121 




FIG. 49. Brain of the common fowl 
(Gallus bankiva), after C. G. CARDS. 
A Dorsal, B ventral aspect, a Olfac- 
tory bulbs, b Cerebrum, c Bigemina. 
d Cerebellum, d' Rudimentary pilea. 
e Oblongata. 2 Optic nerve. 



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 prim- 
ule (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. no) ; 
laterally, it gives out tracts of trans- 
verse fibres, which run towards the 
lower surface of the oblongata, and de- 
cussate 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. 48) ; 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 supe- 
rius, vm), which forms 
the roof of the fossa 
rhomboidalis, and 
effects a direct con- 
nexion of the cerebel- 
lum with the adjoining 
anterior brain-divi- 
sion, the mesen- 
cephalon or quadrigemina. Finally, the white tracts proceeding froirfeither 
side of the cerebellum form the middle cerebellar peduncles (crura ad pontem, 
pni). The structure that arises at the brain-basis by the junction of the 
middle peduncles and their decussations with the longitudinal myelinated 




FIG. 50. Dorsal view of the human cerebellum. On the 
left side, the dentatum (en) and the arbor (av) have been 
exposed by an oblique section. W Vermis. H Right 
pileum (hemisphere). 



122 Development of Central Organs [116-8 

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 longi- 
tudinal 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 cerebellar 
peduncles are clearly visible at the very earliest stage of cerebellar develop- 
ment. 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 verte- 
brates, the fishes, the cerebellar cortex divides into a number of layers, 
characterised by differences of coloration. In the cerebellum of the amphi- 
bia, 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, en). 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. 



II 8] 



MesencepJialon ( Quadrigemina) 
5. The Mesencephalon 



123 




The mesencephalon the division of the cau lex 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 cinerca ; only the secondary vesicles develope the ecto- 
cinereal 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 tri- 
geminus) are in 
connexion with 
it. Ganglionic 
nidi are found 
scattered 
through the bige- 
rnina or quadri- 
gemina, and in- 



tercalated in the 
course of the 
myelinated fibre- 
tracts that pass 
below the Syl- 
vian aqueduct. 
These are paired 
tracts of alba, 

united in the median line, and forming in the first place continua- 
tions 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 
cerebii). 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 ; / Figg. 
47, 51), and an anterior and median portion, the tegmentum (tegmentum 
pedunculi ; lib 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 ; si Fig. 51). A second tegmental nidus is named from its colour the 



FIG. 51. .Crus and paracele of the right hemisphere of man. 
/ Crusta. sn Intercalatum. hb Tegrnentum. si Lemniscus. 
v Quadrigeminal lamina, z Epipliysis. th Thalamus. cm 
Medicommissure. cc Albicans. st Striatum. ca Precornu, 
op postcornu, ci medicornu of paracele. tp Tapetum of callo- 
sum. // Opiic nerve. 



124 Development of Central Organs [118-20 

rubrum (nucleus tegmenti ; hb Fig. 56). Dorsal of the crura lie the quadri- 
gemina (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. in), completely solid. They are connected by a lamina of alba, which 
posteriorly forms the direct continuation of the valvula, and anteriorly 
passes into the j ost- commissure (cp Fig. 53) running along the boundary- 
line of quadrigemini and thalami. 



6. The Diencephalon 

The diencephalon, or region of the thalami, is smaller than the mesen- 
cephalon in all the lower vertebrates (/ 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 dien- 
cephalon 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. in). 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 



I2O-I] 



DienccpJialon ( Thalmni) 



125 



ff 



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 perfo- 
rata posterior ; 
pp Figg. 47, 52). 
In the mammal- 
ian brain, two 
myelinated pro- 
minences, the 
albicantia (cor- 
pora candicantia 
or mammillaria ; 
cc), issue ven- 
t rally from the 
floor of the dien- 
cephalon. Situ- 
ated immediately 
in front of the 
anterior line of 
the pons, they 
bound the tuber 
posteriorly, a s 
i nfundibulum 
and hypophysis 
bound it ante- 
riorly. Their 
genetic signifi- 
cation is still 
uncertain. 

The dien- 

cephalon, 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 brajn- 




FIG. 52. Base of the human brain. Mo Oblongata. Cb Ven- 
tral surface of cerebellum, ft Floccule. to Tonsil, br Pons. 
/zs Crus. cc Albicantia. h Hypophysis. sp Precribrum 
(olfactory area), pp Postcribrum (between the divergent 
crura). I Olfactory nerve with olfactory bulb (rhinencephalon) : 
removed on the right of the Fig. II Optic nerve. 777 Oculo- 



motor nerve. V Trigemmus. VI ADaucent ocuiar. F 3 Sub- 
frontal gyre. F 2 Medifrontal gyre, sr Olfactory fissure. 
FI Superfrontal gyre. T v T 2 , T 3 Supertemporal, meditemporal 
and subtemporal gyres. O Occipital gyre. H Hippocampal 
gyre. 



126 Development of Central Organs [121-2 

baso, 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 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. in), the vascular trunk that penetrates the cavity of the 
diencephalon sends out a large number of branches from it into the hemi- 
cerebral 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 intemal 
faces to the diacele. 



122-3] 



Prosencephalon 



127 



The grey matter of the prosencephalon occurs in all three possible 
formations. As entocinerea, it covers the walls of the diacele, and there- 
fore, 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 




FIG. 53. Median section of the human brain, r Fossa rhomboidalis. br Pons. 
cc Albicans. rd Descending, ra ascending root of the fornix. h Hypophysis. II Optic 
nerve, ca Precommissure. cb Copula (lamina rostralis). 1 mo Porta. bk Callosum. 
sp Septum (septum pellucidum). / Fornix. cm Medicommissure. th Thalamus. 
cp Postcommissure. z Epiphysis. v Quadrigeminum. m Valvula. W Vermis of 
cerebellum with arbor. F 3 Subfrontal gyre. Gf Callosal gyre. C Callosal fissure. 
R Central fissure (fissure of Rolando). Vc Precentral gyre. He Postcentral gyre. 
H Hippocampus. U Uncus. Pr Precuneus. O Occipital fissure. Cn Cuneus. 
O' Calcarine fissure, a, /3 Lines of the transverse sections shown in Fig. 56. 

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, 

1 This Figure, with several others, has been printed without change in all editions 
of the Physiologische Psychologic. The reader will observe that the line from the 
abbreviation cb is carried to what has the general position of the terma (lamina ter- 
minalis). The real copula (the weisse Bodencommissur of the German ; Henle's com- 
missura 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, irrele- 
vant 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, 



128 



Development of Central Organs 



[123-4 



from which myelinated fibres radiate out towards the periphery of the 
hemispheres. 

The lowermost stratum of the floor of the paraceles is, therefore, com- 
posed 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). 






FIG. 54. Differentiation of the brain-ganglia, after GEGENBAUR. A Brain oi tortoise, 
B of foetal calf, C of cat. On the left, the roof of the paracele has been removed ; 
on the right, the fornix has also been cut away ; and in C, on the left, the passage of the 
fornix into the hippocampus (cornu Ammonis) is exposed. 7 Cerebrum. // Thalami. 
/// Quadrigemina or bigemina (lobi optici). IV Cerebellum. V Oblongata. ol Olfac- 
tory bulbs, st Striata. / Fornix. h (in C) Hippocampus, g (ibid.) Geniculum. 
sr Fossa rhomboidalis. 

The club-like extremity, lying anteriorly of the thalamus, is called the 
caput ; the narrower portion, surrounding the tl.alamus exteriorly, the 
cauda. Striatum and thalamus together cover the entire floor of the para- 
cele. 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 band of alba, the tenia (stria 
terminalis, stria 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 



124-5] 



Pfosencephalon 



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 : Ik), lies to the outside 

of the caudatum. In vertical sec- 
tion 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 myel- 
inated 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). 1 
These ganglionic nidi of the hemi- 
spheres take up many of the myel- 
inated fibres that spring from the 
quadrigemina and thalami ; others 
pass under the striatum 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 




A' 



FIG. 55. Thalami and striata of man ; 
in part after ARNOLD. On the left, the 
postcornu and medicornu of the paracele, 
with the hippocampus (cornu Ammonis) 
and calcar, are exposed. v Quadrige- 
mina. z Epiphysis". th Thalamus. cm Me- 
dicommissure. sc Tenia. st Striatum. 
fx Anterior portion of fornix, bk anterior 
portion of callosum, both in section. 
fx' Posterior portion of fornix, turned 
back, ci Medicornu of paracele. am Hip- 
pocampus, cp Postcornu of paracele. 
vk Calcar. ca Precornu of paracele. 



most various directions, towards 
the cerebral cortex. The terminal 
division of the great longitudinal fibre- tract, that begins in the columns of 

1 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. 



130 



Development of Central Organs 



[125-6 



Ih 



K 



the myel, then passes over into the columns of the oblongata, and there- 
after 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 cor texward 
must curve out 
around the ves- 
sels at the peri- 
phery, in order 
to attain t o 
their goal. 

The terminal 
division of the 
prosencephalon 
consists of the 
two olfactory 
bulbs or olfac- 
tory gyres (rhin- 
encephalon). In 
most fishes, the 
rhinencephalon is 
so strongly deve- 
loped that it not 
seldom surpasses 
in extent all the- 
remaining por- 
tions of the pros- 




FIG. 56. Transverse section through the human cerebrum, 
posterior aspect ; in part after REICHERT. The dorsal portion 
of the pallium of the hemispheres is not shown. On the left 
side, the section follows the line o, on the right side, the line ft. 
of Fig. 53. On the left, therefore, it passes through the medi- 
commissure and the hypophysis ; on the right, it traverses the 
brain a little more posteriorly, cutting the posterior portion of 
the thalamus and the albicans. bk Callosum. fx Fornix. ca Pre- 
cornu of paracele. st Nidus of striatum (caudatum). th Nidi 
of thalamus. Three of these are distinguished : a lateral, a 
median (bounding the diacele), and an anterior nidus, cm Medi- 
commissure. K Operculum. / Insula. m Radiations of the 
corona. Ik Lenticula. On the left hand, the three parts of the 
lenticular nidus are visible, cl Claustrum. Between cl and 
the lenticular nidus lies the external capsule of the lenticula. 
mk Amygdala, ci Medicornu of paracele. am Cross-section 
of hippocampus. // Optic nerve, t Infundibulum and hypo- 
physis. / Crusta. sn Intercalatum (substantia nigra). hb Teg- 
mentum with rubrum. fh Cleft in medicornu of paracele, by 
which a vascular plexus gains access to it (hippocampal fissure). 



encephalon. 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 olfac- 
tory bulbs con tain cavities, the rhinoceles, which communicate with the pros- 
ocele (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. 



126-7] Prosencephalon 131 

The olfactory bulbs lie far back mubr 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, pp. 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 (jh 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 post- 
cornu (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 out- 
ward 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. 



132 Development of Central Organs [127-8 

(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 curva- 
ture 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 ths 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. 1 

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 marsu- 
pials, 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 thus restricted to 
a mere cross-commissure between the two hippocampi, the precommissure 

1 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 Alan's Place in 
Nature, 1863, 100, 113. The older writers on the ape-brain figure the postcornu. 
Cf. e.g. TIEDEMANN, Icones cerebri, 1825, 54. OWEN himself, in his later work, describes 
the rudiment of a postcornu in the dolphin (Anatomy of Vertebrates, iii. 120). 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. 



128-30] Fornix and Commismral System 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 trans- 
verse lamella of alba (Fig. 53 cu) 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-cavi- 
ties (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 (/ and /' Fig. 57 B). 1 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, enclosing a narrow cleft-like cavity. 
These are the septa (septa pellucida : sp Fig. 53). 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 forni- 
columns (porta or foramen of Monro : mo Fig. 53 ; cf. * Fig. 43, p. 113). Be- 
tween 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 

i In human anatomy, the portion of the callosum which connects the two hippo- 
campi is termed the psalterium. 



134 Development of Central Organs t I2 9- 

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 calrar 
(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 (;' 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 terminals) of the aula ; and we have followed the course 





FIG. 57. Anatomy of brain of rabbit. In A the root of the hemispheres has been 
turned back, so that the callosum is visible over its whole extent. In B the callosum 
has been removed, and the paraceles are displayed. Mo Oblongata. C Cerebellum. 
V Quadrigemina. z Epiphysis. In B the beginnings of the striata are visible to the 
side of z under the hippocampi, am Hippocampus, bk Callosum. Anteriorly to the 
line bk lies the portion of the callosum that passes into the hemicerebral alba ; its 
interlacing with the coronal fibres can be seen in the Fig. Posteriorly to bk the hippo- 
campal commissure or fornicommissure begins, ol Olfactory lobes, ca Precornu of 
paracele. / Anterior, /' posterior portion of fornix. ci Medicornu of paracele. si Striata. 

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 contimious from the first, and the fornix has, there- 
fore, 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 
orcingulum : G/ Fig. 53, p. 127). 



130, 1 3 1-2] 



Fornix and Comuiissural System 



135 



In certain mammalian brains, where the frontal part of the prosen- 
cephalon 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 cal- 
losum, it again 
emerges p o s- 

teriorly at the ^JPTdrMMK^:.?. .-/. ' </- S 

brain-base. Here 
it passes over in- 
to a gyre lying 
behind the Syl- 
vian and bound- 
ing the median 
fissure ; the hip- 
pocampal gyre, 
the outer wall 
of the hippo- 
campus (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, con- 
sists of unmixed 
white matter. 
The sole excep- 
tion is a narrow 
stria of cinerea, 
isolated from the 
rest of the cor- 
tex, which has 
persisted in its 




FIG. 58. Callosum and paracele of human brain. Brain bar 
dened in alcohol. On the left side, the roof of the hemisphere has 
been removed, so as to expose the median portion of the callo- 
sum, and the callosal radiations into the hemicerebral alba are 
shown. On the right, the paracele appears in horizontal section. 
bk Callosum. sm Stria media (nerve of Lancisi). si Stria 
lateralis (tenia tecta ; part of callosal gyre), m Interlacement 
of callosal radiation with coronal fibres, m' Posterior uncrossed 
portion of callosal radiation. At this point the callosal tracts 
are reflected downwards, round the outer wall of the postcornu, 
to form the tapetum (tp Fig. 51). fa Fibrae arcuatae, con- 
necting the cortical portions of neighbouring gyres, st Striata. 
sc Tenia (stria cornea), th Thalamus, for the most part covered by 
the following parts : fx Fornix ; am Hippocampus ; vk Calcar. 



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 myelina ted fibres upon which 
it rests are entirely separated from the remaining white matter of the callosal 



136 



Development of Central Organs 



132 



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 mydinated stria, and have received a special name, 
tenia tecta or lateral stria (si 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. 1 
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 surf ace 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 gyms hippocampi. This means, of course, that the fasciola, which 
lies just below the lateral stria, mast divide from the rest of the cortex ; 

the lateral stria forms a parti- 
tion between cinerea and cine- 
rea. 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- 




FIG. 59. Hippocampal gyre and adjacent parts 
of callosum and fornix in the human brain. 
bk Callosum. si Stria lateralis. fc Fasciola 
(fasciola cinerea). fd Continuation of fasciola 
(fascia dentata). fx Lower extremity of fornix. 
H Hippocampal gyre, sr Reticular alba. 



1 The nerves of Lancisi. or striae mediae, belong not to the callosal gyre, but to the 
callosum itself. See sm Fig. <;8. 



132-3] 



Outward Conformation of Brain 



137 




Njj^*''* 1 '^^ 



cioia, 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 boun- 
dary 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 boun- 
dary 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 contri- 
bute 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) ; exter- 
nally to that comes a second layer of alba, 
the continuation of the lateral stria or the 
reticular alba (H) distributed over the cor- 
tex 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 (/*). 



FIG. 60. Hippocampal gyre and 
hippocampus of human brain, 
in transverse section, ci Medi- 
cornu of paracele. r Grey cor- 
tex of uncus. H Uncus with 
reticular alba, fd External cine- 
real lamella of hippocampus 
(fascia dentata). si Internal 
white layer of hippocampus ; 
continuation of the lateral stria. 
ft Reflected border of this layer 
(fimbria). 



(c)The Development of the Outward Conformation of the Brain 

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 



138 



Development of Central Organs 



[133-4 



G f. 




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 
iheir 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 correspond- 
ing appearance or 
non-appearance of 
certain nidal struc- 
tures (such as the 
olives) on the oblon- 
gata, and by the 
development of a 
pons. 

The point where 
the cerebral hemi- 
sphere 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 transfor- 
mation. 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 
(s l and s 2 . 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 



FIG. 61. Lateral aspect of brain of dog. Mo Oblongata. 
C Cerebellum. S Fissure of Sylvius, ob Olfactory lobes. 
Gf Callosal gyre, coming to the surface behind the olfactory 
lobes. H Hippocampal gyre, o Optic nerve. /, //, /// 
First, second and third typical gyres of the carnivore brain. 



r 34-5] 



Outivard Conformation of Brain 



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 /, 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 por- 
tion 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 (0) ; and the area situ- 
ated ventrally of it, the tem- 
poral 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). 



FIG. 62. Lateral aspect of brain of human foetus 
(7 months). Mo Oblongata. C Cerebellum. 5 Syl- 
vian fissure. s, Anterior, s 2 posterior ramus. 
K Operculum. R Central fissure (fissure of Ro- 
lando). F Frontal lobe. P Parietal lobe. O Occi- 
pital lobe. T Temporal lobe. 



146 Development of Central Organs [ 1 3$~ 1 37 

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 (hippo- 
campal 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 mam- 
malian 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 cere- 
brum, 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 ridgings 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 main- 
tained with approximate constancy throughout the whole period of develop- 
ment. 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 con- 
volution 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 is 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 ease 
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. no). 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 



1 3 7-8] Outward Conformation of Brain 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 spread- 
ing leaves and branches, termed by the older anatomists the arbor vitae 
(ay-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 arrange- 
ment 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, 1 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 de- 
scribed 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 oi 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 cere- 
brum 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 

i Here belong more especially the flocculus (ft 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* 



142 Development of Central Organs [138-9 

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 re- 
placed, suddenly or gradually, by an approximately transverse ridg-ing. 
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 callosal gyre comes to the surface in the frontal 
area, and is separated 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, G/). 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 longitudin- 
ally 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 regu- 
larity 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 
kind 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. 1 

1 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. 



139-40] 



Outivard Conformation of Brain 



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 marked- 
ly 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 extraordi- 
narily broad ; and the prin- 
cipal 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 pre- 
cuneus (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 (0). 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 




FIG. 63. Median aspect of brain of dog. Left 
hemisphere. Gf Callosal gyre, b Its anterior and 
superficial portion, ol Olfactory gyre. H Hippo- 
campal gyre, bk Callosum. fx Fornix. ca Pre- 
commissure. 



144 



Development of Central Organs 



[140-1 



Tr 



Cn 



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 (0'). We have, then, in all, on the 

surface of the occi- 
pital brain, five well- 
marked transverse 
fissures, three of 
which belong to the 
offshoots of the cal- 
losal gyre and the 
adjoining parts. On 
the other hand, the 
fissures and gyres of 
the frontal and tem- 
poral areas i.e. of 
the region anterior 
to the presylvian 
and ventral to the 
subsylvian fissures 
tend in general to 
pursue a longitudi- 
nal 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 (F t F 3 , 7\ T 3 ), 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 (O t 3 ). In the area of transition 
fromtheoccipitalto the temporal region, the foldsfollowa course that lies mid- 
way between the transverse and the longitudinal ; so that the three parietal 
gyres (Pi P 3 ) 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 




f* 



FIG. 64. Median aspect of brain of monkey (Macacus). Left 
hemisphere. After GRATIOLET. Gf, ol, H, bk, fx, ca as in 
Fig. 63. Pr Precuneus. Cn Cureus. O Occipital fissure 
(parieto-occipital fissure). O' Calcarine fissure (lateral 
occipital fissure). 



141-2] 



Ontivard Conformation of Brain 



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 pos 
terior, 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 




FIG. 65. * Fissures and gyres of the human brain. Lateral aspect of left hemisphere. 
FS Sylvian fissure : s t anterior, s 2 posterior ramus. Op Operculum, covering the 
insula (/, Fig. 56). F 1( F 2 , F 3 Superfrontal, medifrontal and subfrontal gyres. VC Pre- 
central, HC postcentral gyre. R Central fissure (fissure of ROLANDO). T\, T 2 , T 3 
Supertemporal, meditemporal, subtemporal gyres. P 1? P 2 , P 3 The three parietal 
gyres. O Occipital fissure. O' Calcarine fissure. O t , O 2 , O 3 The three occipital gyres. 

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 pos- 
terior 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. 

1 This Figure would, perhaps, be mere 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, n 
deed, but little is certainly known of the fissuration of the occipital lobe. TRANSLATOR. 



146 Development of Central Organs [ r 4 2 -3 

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. 1 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 develop- 
ment of the more posteriorly situated occipital fissure, which has accordingly 
been termed the ape-cleft (Affenspalte). 2 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, andO', 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-, mtdi-, 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 pre- 
central gyre. On the other, only one of the temporal gyres (the stipertemporal) 
runs a complete course to the postcentral gyre, curving boldly upwards round the 
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 (Pj-Pg). 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 (P 3 ), 
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 (G/). 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 (F 2 , F 3 , 
Fig. 52). 3 

1 ECKER, Archiv f. Anthropclogie, iii., 1868, 203. PANSCH, ibid., 227. 

2 The morphological relations of the Affenspalte (fissura perpendicularis externa; 
Wilder's pomatic fissure) are, in reality, more complicated than the author h?re re- 
presents them. TRANSLATOR. 

3 BISCHOFF, Abhandl. d. bayer. Akad. d. Wiss., x., 1868. ECKER, Die Hirnwin- 
dungen des Menschen, 1869. PANSCH, Die Furchcn und Wulste am Grosshirn des Men- 



145-4] Outivard Conformation of Brain 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 dis- 
posed about a longitudinal axis. Suppose, then, that the nssuration is uniform, 
all the folds taking a single direction : this will mean that the difference of sur- 
face-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 grow- 
ing structure, i.e., may be regarded as a coherent elastic body, in which the defor- 
mation 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 nssuration 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 i : o'9, while in the course of the fifth 
and sixth it has fallen to i : 07. This is the time of formation of the first per- 
manent fissures, all of which, without exception, take the transverse direction. 

schen, 1879. FLATAU and JACOBSOHN, Handbuch der Anatomie und vertfeichenden 
de$ Centralnervensy stems der Sdugethiere, i., 1899. 



148 Development of Central Organs [i44~5 

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. 1 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 widety 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 extend- 
ing 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 grow- 
ing more strongly in the transverse than in the longitudinal direction. The 
temporal lote 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 pre- 
dominantly 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. 2 

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 ac- 
quired 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 brachycephalic, predominantly transverse in direc- 
tion. As a matter of fact, a connexion of this kind between skull-form and domi- 



1 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. 

2 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. 



145] Outward Conformation of Brain 149 

nant gyre-trend has been made out by L.MEYER 1 and RuoiNGER. 2 But the 
actual convolution of a given brain will, of course, always be the resultant of 
the two sets of conditions : the intrinsic tensions 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. 

1 Centralblatt f. d. medicin. Wiss., 1876, No. 43. 

2 RUDINGER, Ueber d. Unterschiede d, Grosshirnwindungen nach d. Geschlecht beim 
Folus u. Neugeborenen, 1877, 5 ff. 



[H5-6 



CHAPTER V 

Course of the Paths of Nervous Conduction 
i. General Conditions of Conduction 

DUR 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, interconnected 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 cincreal 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 cnira 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 in- 
tergyral (arcuate) fibres, indicating the connexion of the various cortical zones 
of the same hemisphere. Hence from whichever point of view we consider the 
~utward 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-pro- 
cesses, 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 sug 
gested by the neurone theory, and yet with justice raise the question of the 
preferred lines of conduction, of the -lines which, under normal circum- 
stances, 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 trans- 
mission in particular instances of interrupted conduction or of inhibition of 
function. 



146-7] General Conditions of Conduction 151 

We distinguish two main kinds of conduction-path, according to the 
direction in which the processes of stimulation are transmitted : the centri- 
petal 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 sensa- 
tions. Frequently, however, this final effect is not produced ; the excita- 
tion 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 ' sen- 
sory ' 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 con- 
sideration 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 auto- 
matic movements. In the reflex, i.e., centripetal is followed by centri- 
fugal 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 con- 
ditions, 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 re- 
striction is entirely out of the question, as appears both from the general 
* Cf. the general discussion of reflex excitations in Chap, iii., pp. 85 ff. above. 



152 Course of Paths of Nervous Conduct ion ['47-S 

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 tins 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 stimu- 
lated 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 estab- 
lished 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 prob- 
lems 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 parts. 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 stimu- 
lus which the physiologist is therefore practically compelled to employ. 
The same thing is true of the disturbances consequent upon a division of 



148-9] Methods of investigating Conduction- Paths 153 

nervous 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 functio i 
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 arc 
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 dis- 
repute, 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 direc- 
tion. For the rest, microscopical anatomy has been brought in recent times 
to a high degree of perfection by the application of the various methods ol 
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. 1 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 



i The most fruitful of these methods may find mention here. They are : 
method of impregnation by a metal, and more particularly by silver ; and theme 
of staining by haematoxylin and methylene blue, introduced respectively by WEIGERT 
and EHRLICH. For details regarding these and other methods, see OBERS 
Anleitung beim Studium des Baues der nervosen Centralorgane, 3te Aun. i 
and EDINGER, Vorlesungen uber den Bau der nervosen Centralorgane, 6te Aun., 1900 



154 Course of Paths of Nervous Conduction [149-150 

still include numerous groups of fibres, possessing each a different functional 
significance. 1 

(3) Pathological observation is equally concerned with functional derange- 
ment and 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 
uncfergo secondary degeneration. The pathological anatomist can, there- 
fore, appeal to a law very similar to that upon which embryological investi- 
gation 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. 2 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. 3 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 micro- 
scopical structure of the central organs was furnished by the researches of STIL- 
LING. 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 I.uvs. 4 
MEYNERT, especially, rendered great services to the science ; he brought to his re- 

1 FLECHSIG, Die Leitungsbahnen im Gehirn und Ruckenmark des Menschen, 1876. 
C. VOGT, Etude sur la myelisation des hemispheres cerebraux, 1900. Cf. 5,6, below. 

2 L. TORCK, Sitzungsber. d. Wiener Akad., math.-naturw. Cl., vi., 1851, 288 ; xi. 1853, 
93. CHARCOT, Ueber die Localisationen der Gehirnkrankheiten, trs. FETZER, i., 1878, 159 
(Lefons sur le* localisations dans les maladies du cerveau, 1875) ; FLECHSIG, Ueber 
Sy sterner krankungen im Ruckenmark, 1878. 

3 GUDDEN, Arch. f. Psychiatric, ii. ( 1870, 693. FOREL, ibid., xviii., 1887, 162. VON 
MONAKOW, ibid., xxvii., 1895, i, 386. , 

4 MEYNERT, ar'. Gehirn in STRICKER'S Gewebelehre, 1871, 694 ff. (BUCK'S trans., 
650 ff.) ; Psychiatric, Pt. I. 1884. LUYS, Recherches sur le systeme nerveux cerebro-spinal, 
1865. The Brain and its Functions, 1877 and later (Internal. Sci. Series). 



Conduction in Nerves and My el 15$ 

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 re- 
search ; 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 em- 
bryonic development. It is this sheath which is responsible for the color- 
ation of the alba, so that its appearance is easily recognisable. The signs 
of secondary degeneration consist, on the other hand, in a gradual transfor- 
mation 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 uiihin 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 degenera- 
tion of motor fibres follows a centrifugal, that ot 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 de- 
terioration 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 symp- 
toms 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 quadrigeinina ; 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 stimula- 
tion 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 



1 56 Course of Paths of Nervous Conduction [151-2 

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 centri- 
fugally ; 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. 1 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 func- 
tionally 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 con- 
trary, 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 pro- 
visional manner ; the arrangement is by no means perfect, and requires to be 
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 

1 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. 



152-3] Conduction in Nerves and My el 157 

conditions of their origin, that the nerves should leave the my el in pre- 
cisely the order that is demanded by their subsequent peripheral distribu- 
tion. 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 distribu- 
tion. 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 
Deceive 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 be 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 con- 
strained 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 ten- 
dency to refer all differences of function, e.g. those obtaining between the 
various sensory nerves, to some unknown specific property of the nerve- 
fibres. 1 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. 2 At the 
present time, we may say, with better reason, that BELL'S Jiypothesis 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 pro- 
cesses 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 21 (p. 50 above) gave a schematic 
representation of the structural relations involved in the two cases. Every 

1 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. 

2 This analogy appears quite clearly as early as JOHANNES MULLER. Cf. his Lehr- 
buch d. Physiol., 4te Aufl.,i., 1844, 623. MULLER himself, 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 MULLER tells us, J. W. ARNOLD. 





158 Course of Paths of Nervous Conduction [*53-5 

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 seat, 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 (AfJ, which divides into two halves. 
The one lies within the myel, and after giving off numerous collaterals pene- 
trates with its terminal fibres into a second central neurone territory (N 2 ). 
The other is continued in the sensory nerves, and is finally lost either in ter- 
minal 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 trans- 
mission to higher neurones (N 2 ), may actually take place within the fibre 
itself. The cell Z t 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 pro- 
perties 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 stimula- 
tion of any nerve, at any point of its course, must, so long as its continuity 



T 55-6] Conduction in Nerves and My el 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 ex- 
citations (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 especi- 
ally 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 trie 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 in- 
dividual 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 versa. 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 rot 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 



160 Course of Paths of Nervous Conduction 

appear to 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 unila- 
teral 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, com- 
pensate 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, 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 hernisec- 
tion 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. 1 
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, un- 
doubtedly, the cinerea which surrounds the myetocele. 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. 2 The statement, in 
its extreme form, undoubtedly overshoots the mark. Excitation can al- 
ways be effected by summation of stimuli, or by help of poisons, like strych- 
nine, 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 

1 LUDWIG and WOROSCHILOFF, Ber. d. sacks. Ges. d. Wiss., malh.-phys. Cl., 1874, 
296. MOTT, Philos. Transact., clxxxiii. 1892, i. 

2 VAN DEEN, in MOLESCHOTT'S Untersuchungen zur Naturlehre des Menschen, vi., 
1859, 279. SCHIFF, Lehrbuch der Physiol., 1858, i. 238. PFLUGER'S Arch. /. d. ges. 
Physiol., xxviii. ; xxix. 537 ff. ; xxx. 199 ff. 



157-8] Conduction in Nerves and Myel 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 ofhr 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 ex- 
citations 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, similirly, 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. 1 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 for 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. I< 
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 

1 LuowiG~and WOROSCHILOFF, op. cit., 297. 
p. M 



1 62 Course of Paths of Nervous Conduction 

of tonus in the extensors that goes with excitation of the flexors of a limb. 1 
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, con- 
trariwise, 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. 2 
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-excita- 
tion. 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 move- 
ments 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 ob- 
served for themselves alone, after the myel has been separated from the 
higher central parts. The conclusion to be drawn frcm 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 stimula- 
tion 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 

1 H. E. HERING and C. S. SHERRINGTON, in PFLOGER'S Arch. /. d. ges. Physiol., 
Ixiii. 1897, 222. Cf. p. 93, above. 

2 Concomitant sensations with pain stimuli -are described by KOWALEWSKY (trans, 
from the Russian in HOFMANN and SCHWALBE, Jahresber. /. Physiol., 1884, 26) ; ' con- 
jugate ' sensations, with arousal of tickling, by E. STRANSKY (Wiener klin. Rundschau, 
1901, os. 24-26). In my own experience, the larynx and the external auditory meatus 
are especially liable to concomitant sensations. 



159-60] Conduction in Nerves and My el 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. 1 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 cl other sensory 
stimuli (cf. Chap. VI. 2). 

(c) Anatomical Results 

The conclusions which we have reached by way of physiological experi- 
mentation 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 'he lar^e 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 
(/, 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 hi particular of the two groups that in functional regard 

* PFLUGER, Die sensorischen Functionen des Riickenmarks, 1853, 67 8. 



164 



Course of PatJis of Nervous Conduction 



[160-1 



i-. 



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 Figg. 
20, 21 (p. 50). Fig. 67 shows the various morphological elements in theii 
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 issu- 
ing 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 neu- 
rones of this motor conduction 
cover very extensive territories. 
Indeed, it is probable that in most 
instances the entire motor conduc- 
tion involves only two neurones 
(N l and N n , Fig. 20), the one of 
FIG. 66. Cross-section through the lower which extends from the cell n of 
hilf of the myel of man, after DEITERS. For ,, ventra i cornu to the nerinherv 
the sake of clearness, the ganglion-cells are T 
drawn on a larger scale than the remaining of the body, while the other be- 

rdL^VslTku^'d VeSTornu ^vdth'the g ins with SOme ne f the fibres 
larger ganglion-cells. e Dorsal cornu, with that nm their course in the VCT1- 
the smaller ganglion-cells. /Ventral com- ^j Qr j atera j co l umn (/, m Fig. 66), 

h Do sal commissures, i Fibre-system of and ends in a cell of the cerebral 
the ventral, k that of the dorsal nerve-root. ^ nri . ov At the 




/ Ventral column. 
Dorsal column. 



m Lateral column, n 



fe M tfa same ti 

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 cornua. In these latter connexions 
we have, presumably, the substrate of reflex conduction. The sensory 
nerve-paths, on the other hand, follow a very different course. In their 



Conduction in Nerves and Myel 



case, the spinal ganglion-cell sp forms the central point of a neurone terri- 
tory, 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 /, 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 ter- 
minal ramifications stand in con- 
tact with the small cells of com- 
missure and dorsal cornu. They 
themselves are finally resolved into 
fibrillar reticula, connected by con- 
tact 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 vari- 
ous 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 from the motor. 
As a general rule, there are several 
breaks in the line ; the path con- 
sists of a number of neurone 
chains, arranged one above an- 
other. 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 
the collaterals. The whole 




in 



FIG. 67. Schema of the connexions of cells 
and fibres in the myel : combined from 
morphological plan of the system various diagrams of RAMON Y CAJAL. m 

, ~ Cells of the ventral, s cells of the dorsal 

of sensory conduction thus SUg- CQrnua> c Commissural cells, sp Cell of a 
gests a CO-ordination of parts that spinal ganglion ; h its peripheral, /its 
t , . . j central process ;' a ascending, d descending 

IS at once less Strict and more branch O f ^ n Neurites of the motor cells m. 
widely variable than is the case g Central continuation of the motor path, 
on the motor side. 

We have spoken so far only of the general properties of the mych. 
conductors, properties accruing to all nerve-fibres whose mode of origin 



i66 



Course of Paths of Nervous Conduction 



[162-3 




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 (/, 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. 63). In 
the same way, the innermost division of 
the motor ventral columns, the part bor- 
dering directly upon the ventral sulcus, 
ascends uncrossed to the oblongata, where 
it too passes over into the pryamids. 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 con- 
ditions 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 out- 
lying cuneate funicles (fun. cuneati, Fig. 68). l 

1 FLECHSIG, Ueber Systemerkrankungen im Ruckenmark, 30 ff. BECHTEREW, Die 
Leilungsbahnen im Gehirn und Ruchenmark, 1899, 17 if. 



Fun. grac. 



Fun. cun. 



Cereliellar 
i< i.il column 



Pyramidal 
ventral column 



Dorsal column 



Cerebellar 
lateral column 

Pyramidal 
lateral column 



Pyramidal ventral column 

FIG. 68. Two cross-sections of 
the myel : A from the cervical 
enlargement, B from the thoracic 
region. After FLECHSIG. 




163-4] Conduction Paths in Oblongata and Cerebellum 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 
developmen tally 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 m /el. 
Secondly, it originates new nerves : the great majority of the crania" ner es 
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 consider- 
able extent that is interpolated in the course of the principal conduction 
path. It will be understood that, under the e 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, ire extraordinarily complicated. 
A complete explication of them, in .the present' state, of our knowledge, 
is altogether out of the question. But more than fiis : it is impossible, 
as things are, to put a physiological or psycholo^ica' 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, 



1 6& Course of Paths of Nervous Conduction f 1 64-6 

more especially in our study of the conditions of conduction in the hind 
and mid brain regions. Experiments of the kind are recorded not in- 
frequently 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 present 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 inter- 
ruption 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 (/, 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 gyres and the surround- 
ing area (VC, HC, Fig. 65, p. I45). 1 The path is thus fairly well defined. 
Part of it, as is proved by the paralyses following lesion of the pyramids 

1 CHARCOT, Lefons sur les localisations, etc., 145 ff. FLECHSIG, Ueber System- 
erkrankungen. 42 ff. EDINGER, Vorlesungen, 6te Aufl., 86, 358. 



i66] 



Conduction Paths in Oblong ita and Cerebellum 



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 orgms. 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 con- 
stantly 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 promin- 
ences to form the dorsolongitudinal 
bundle (hi, Fig. 72), which in its 
further course through the pons 
makes connexions with the pontal 
nidi and more especially, as it ap- 
pears, with centres of origin of the 
oculomotor nerves and with the 
cerebellum. 1 We may accordingly 
suppose that these branches of motor 
conduction which run to the mesen- 
cephalon serve to mediate co-excita- 
tions in that region. The connexions 




FIG. 69. Course of the pyramidal paths 
in man, after EDINGER. The fibres to 
the left half of the brain are indicated by 
continuous, those to the right half by 
interrupted lines. The diagram also 
illustrates the course of secondary de- 
generation, when the area of dis ase is 
situated in the left capsula. sp Branch of 
the pyramidal path derived from the 
lateral, vp branch derived from the ven- 
tral column. Th Thalamus. Lk Lenti- 
cula. Ci Course of the pyramidal fibres 
through the capsula of the lenticula. 
Cf. the cross-section of the brain shown 
in Fig. 56, p. 130. 



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. 

1 EDINGER, op. cit., 317. RAMON Y CAJAL, Beitrag zitin Stitdhim der Medulla oblon- 
gata, 1896, 52 ff. 



170 Course of Paths of Nervous Conduction [166-7 

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 in- 
creased 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 (/, 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 cms, a structure lying in the outer 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), drawn 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. 

(c) The Regions of Origin of the Cranial Nerves and the Nidi of Cinerea 

in the Oblmgita 

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 from 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 coming 
up to the cerebrum from below, or as junctions for the important branch- 
conduction to the cerebellum, here opened for travel. The diificult ques- 
tions concerning the origin of the cranial nerves, questions that have not 
yet in every case received their final answer, are of interest for psychology 



167-8] Conduction Paths in Oblougata and Cen helium 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 with 




FIG. 70. Cross-section through the oblongata, x 4. After WERMCKE. p Pyramid. 
ci Olive. / Lemniscal tract (tract of the fillet) with fibres from the olives and from the 
funicles of the ventral column, mf Motor field (funicles from the ventral columns, 
joiniig the tegmentum later on), hi Remains of the dorsal column (also passing into 
the tegmentum). cr Restis and cerebellar peduncle. 12 Nidus and root of the hypo- 
glossus. Va Ascending root of the trigeminus. Se External, Si internal nidus of the 
acusticus. Xf Mixed root of the glossopharyngeus. Xp Dorsal, Xa ventral nidus 

of the pneumogastric. 

other nidal structures scattered throughout the oblongata. This state- 
ment 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 pneu- 
mogastric, 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. 1 

1 Cf. RAMON Y CAJAL, Medulla oblongata, 43, 122 ff. EDINGER, op. oil., 86 ff. 



172 Course of Patlis of Nervous Conduction, [168-9 

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 (/ 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 im- 
portance 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 cere- 
bellum, 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 thedentatum, more especially 
over its anterior margin. They do not appear to connect with the cinerea 
of this nidus, but radiate from its upper surface towards the cortex, where 



1 69-7 1] Conduction Patlis in Oblongata and Cerebellum 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 resth 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 fibres, 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 cere- 
bellum with the brain ganglia, by mediating a conduction to the quad- 
rigemina. 

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 



174 



Course of Paths of Nervous Conduction 



[171-2 



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 corlex effected by the medipe- 
duncles, 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 
ff. Fig. 71 shows the main 

*U if -i i . . 

features of this conduc- 
tion system. The reader 
will recognise, first of all, 
the pyramidal path, with 
its crossed branch from 
the lateral and its un- 
crossed branch from the 
ventral columns, running 
directly between myel 
and cerebral cortex (pi 
p 2 , p). He will next 
notice the other motor 
paths, derived from the 
ventral columns, and in- 
terrupted in the mesen- 
cephalic 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 pro- 
bably terminate in the 
mesencephalic region it- 
self (vv'). A considerable 
division of the sensory 
path (eg'), drawn from the 
dorsal columns, passes in 
the lemniscal decussation (k a ) to the opposite side : part of it is lost in the 




FIG. 71. Schema of the paths of conduction through 
pons and cerebellum. Cb Cerebellar cortex. N Den- 
tatum of the cerebellum. P Grey masses of the pons. 
O Olive, gn Nidi of the slender funicles. en Nidi of 
the cu^eate funicles. p Ventral column of the 
pyramids (uncrossed). p 2 Lateral column of the 
pyramids (crossed), vv' Remains of the ventral 
columns. 55' Remains of the lateral columns, g 
Slender funicles or columns of GOLL. c Cuneate 
funicles. g', c' Central continuations of these funicles. 
gi Lemnis:al path. / Conduction from olive to cere- 
bellar nidus, cs Direct cerebellar path of the lateral 
columns, r Conduction from cerebellar nidus to 
cerebellar cortex, bb' Path of the medipeduncles. 
e' Path of the prepeduncles. k t Pyramidal decussa- 
tion. k 2 Lemniscal decussation. 



172-3] Conduction Paths in Oblongata and Cerebellum 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 cms 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 (/ ), 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 (53'), 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 cerebnim 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 cms. Its development in the animal kingdom thus keeps 
even pace with the development of all these paths of conduction, and 



Course of Paths of Nen>ous Conduction 



[173-4 



T - 



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 : atro- 
phy of a cerebral lobe is ordinarily 
attended or followed by a wasting 
away of the opposite half of the 
cerebellum. The fibres of the medi- 
peduncles, probably without excep- 
tion, 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 
(la Fig. 72). These presently decus- 
sate, and come to an end in the 
rubrum of the tegmentum. In this 
way, by collection of the myelic 
columns that qome up from below, 
and of the continuations from the 
cerebellum that join them from 
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 cms. At the 
same time, the pons is broken 
through by the 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 rhom- 
boidalis (metacele), partly in the 
neighbourhood of the Sylvian 
aqueduct (mesocele), which forms 
a continuation of the central 
canal. 




Fig. 72. Cr.ss- section throrgh the pons 
of man, at the level of the root of the 
Irochlearis, a'ter STILLING. M Valvula. 
T Root of trochlearis. 5 Sylvian aque- 
duct, j Cells of origin of the fifth cranial 
nerve in the grey floor of ihe aqueduct. 
hi, v, v' , si Continuations cf the ventral 
columns. hi Dorsolongitudinal bundle. 
v Median remains of the ven'.ral columns 
on either side of the raphe. v' Anterior 
remains of the ventral columns adjoining 
the lemnis us. si Lemniscus ; continu- 
ation of the divisions cf the ventral 
col mins that s.irround the olives (capsular 
columns, fun. siliquae). si' Passage of 
the lemniscal f.bres into the roof of the 
Sylvian aqueduct. s Remains of the 
lateral columns, and format io reticularis. 
g Gelatinosa, and continuations of the 
dorsal columns. ba Prepeduncles. R 
Raphe. b Ectal, b' intermediate, b" en- 
(al cross-fibres of the pons. p-p' Con- 
tinuations of the pyramidal tracls, inter- 
mixed with cinerea and with the ascending 
continuations of the medipeduncleo issu- 
ing from it. 



J 74~5] Conduction Paths in Oblongata and Cerebellum 



177 



As a result of its cleavage by cinerea and by the cross fibres of the 
medi peduncles, 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 l 
Fig. 72) is principally made up of the continuations of pyramids, 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 (k* Fig. 71). The intercalatum 
(substantia nigra of SOMMERING : 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 f hi 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 (si si' 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 compara- 
tively small space, the whole number of conduction paths, many of which in 
their later course are widely divergent. It is, therefore, a remarkable coinci- 
dence 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. N 



I 7 8 



Course of Paths of Nervous Conduction 



[175-6 



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 comparative- 
ly few fibres 
from below, and 
sends out very 
considerable 
bundles to the 
cerebral cortex ; 
the quadr ige- 
mina 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 from 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 




FIG. 73. Vertical section through the caudex in the region of 
the pregemina ; in part alter EDINGER. A Aqueduct. B 
Prebrachium. V Pregeminum. T Thalamus. Pu Pulvinar. 
H Tegmentum. F Crusta. 5 Lemniscus. Cgm postgeniculum. 
R Rubrum. Sn Intercalatum. Py Pyramid. / Dorso- 
longitudinal funicle. O Oculomotor (third cranial) nerve. 



176-7] Conduction Paths of Cerebral Ganglia 179 

to the coronal radiation. It would appear, then, that these structures 
are terminal stations of conduction, rnalogous to the cerebral cortex, and 
not intermediate stations like the thalami and quadrigemina. 1 

(b) Conduction Paths of the Nerves of Taste and Smdl 

An important place is filled in the system of conductions that falls within 
the region we are now considering (prosencephalon, diencephalon, mesence- 
phalon) by the paths of the sensory nerves Fortunately, these are among 
the conductions that have so far been most fully investigated, and whose 




FIG. 74. Schema of the paths of conduction to the striata ; after EDINGER. Nc Cau- 
datum. Th Thalamus. V Quadrigemina. B Grey masses of the pons (intercalatum). 
hh Fibre masses passing directly from the crura to the corona. The connexions to the 
lenticula (not represented in the Fig.) are to be thought of as running straight out from 
the Th to the observer. 

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 com- 
plicated conditions that obtain in the cases of sight and hearing. The 

i EDINGER, op. cit., 272. BECHTEREW, Leitun^sbahnen im Gehirn und 
439- 



i So 



Course of Paths of NCI vons Conduction 



[177 



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 ac- 
cordingly represents, in all essential particulars, a portion of the central 
organ : cf. above, p. 47. 




FIG. 75. Schema of the origin of the gustatory nerves, after EDINGER. V Trigemirus 

(fifth cranial nerve). L Lingual branch of the trigeminus (lingualis). F Genii of the 

nervus facialis. G Glossopharyngeus. Ch Chorda tympani. VII. Geniculate or 
facial ganglion. IX Glossopharyngeal ganglion. Z Tongue. 

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 porliDn of this area in the lingualis 
(L Fig. 75), and those intended for the posterior portion in the glosso- 



177-8] 



Conduction Paths of Higher Sensory A r e;i>es 



ion 
as 

us 



pharyngeus (G). This division appears, however, to be simnly external 
Both of the gustatory nerve paths take their origin from the same masses 
of mdal nnerea 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 porti 
of the tongue, passes through its own ganglion. At the periphery, 
we shall see when we come to consider the peripheral sense apparat 
(Ch. VIII., 4), 
the two nerves 
break up into 
terminal fibrils, 
which end in and 
among the taste 
beakers, without, 
as it appears, 
coming into con- 
tact with other 
than epithelial 
terminal struc- 
tures. The 
course of the 
fibres, as shown 
synoptically in 
Fig. 75, accord- 
ingly c o r r e- 
sponds 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 




FIG. 76. Origin and termination of the olfactory nerves in 
man, after RAMON Y CAJAL. A Peripheral olfactory cells. 
h Epithelial cells lying between them, a Small intermediary 
nerve cells. B Dendrites of the glomeruli. C Central olfactory 
cells, with b their dendrites in the glomeruli. D Cells of the 
(probably) centrifugal path, with c their terminal arborisations. 
E Fibres of the olfactory tract, with / collaterals, e Free nerve 
terminations. 



182 Course of Paths of Nervous Conduction [178-80 

the olfactory mucous membrane (.4 Fig. 76), which are set between epi- 
thelial 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 jud,ge 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 glomsruli 
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 peri- 
pherally 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 regions 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. 

(c) Conduction Paths of the Acoustic Nerve 

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 thebasilar membrane (cf. Ch. VI 1 1. ,4, be- 
low), and then, in the auditory canal of the cochlea (5 Fig. 77) traverse groups 
of bipolar ganglion cells (g) which resemble the cells of the spinal ganglia 



Conduction Paths of Higher Sensory Nerves 10*3 

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 ceritralward, 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 




FIG. 77. Schema of conduction by the acoustic nerve, combined and simplified from 
HELD'S diagrams. 5 Cochlea, g Bipolar ganglion cells of the spiral ganglion. Ta 
Tuberculum acusticum. Cb Cerebellum. VA Anterior acoustic nidus. Ol Superior 
olive. R Median line of the oblongata and pons, with decussating fibres : raphe. 
RM Myel. UV Postgeminum. OV Pregeminum. SI Lemniscus. ^ H Cerebral 
cortex, rr Reflex paths to the motor nidi of muscles of eye and face, r r Reflex paths 
from the superior olives to the muscles of the body. / Uncrossed fibres, kf Crossed 

fibres. 

thus brought in contact. The ni.di in question are a somewhat < smaller 
anterior nidus, the anterior acoustic nidus (VA) 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 (/), and 
a larger number, which ascend on the opposite side of the body (kf). The 



I?4 Course of Paths of Nervous Conduction [t&O-i 

former run partly to the postgemina, and partly, having joined the lem- 
niscus, direct to the cerebral cortex. There is also, in all probability, a 
branch path from the tuberculum acusticum, which proceeds with the post- 
peduncles 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 (01) ; 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 (OF), from which 
again a centrifugal fibre system (rr) runs to motor nidi, and more espec- 
ially to the nidi of the oculomotor nerves. It would seem, therefore, that 
this quadrigeminal path is also, in part, a reflex path ; though the quad- 
rigemina 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 (0V ', UV) and 
the acoustic nidi (VA^ To), which, if we may judge from the peripheral 
direction of its neurites, conducts in the opposite direction to that' 
marked in the Figure, and would therefore represent a centrifugal path.; 
Like the centripetal path which it accompanies, it consists of a large -num.-: 
her 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 : (i) 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 gan- 
glion) 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 



182-3] Conduction Pat/is of Higher Sensory Nerves lS$ 

path, which issues from the quadrigeminal nidi, and is associate,! in its 
peripheral course with the corresponding centripetal path. 1 

This list shows us how extraordinary 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 follow- 
ing 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. 2 

(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 

jsjDecific quality, the rods and cones (5 and Z Fig. 78) -usually termed, on 
this account, vimal cells are sensory epithelia which., like tin- gustatorv 
cells, are connected only by contact with the terminal fibril^ of the optic 

_CQiiductipn. 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 (G a ), which may be re- 
garded, 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 (6\,) to which may be 
added stellate intercalary cells, found far forward in the neighbourhood of 

i This exposition follows HELD, Arch. f. Anatomic, 1893, 201 3. 
3 BECHTEREW, Die Leitungsbahnen, 169 ff. , 



186 Course of Pat/is of Nejvous Conduction [183 

the elements S and Z, and not represented in the Figure. These list two 

classes constitute to- 
gether a neurone ter- 
ritory, intervening 
between the last ter- 
minal fibiils of the 
peripheral optic con- 
duction and the large 
ganglion cells (G 2 ), 
which ma}' be con- 
sidered as the extreme 
peripheral member of 
the centripetal optic 
conduction. Between 
its limits we find, fur- 
ther, terminal arbori- 
sations of neurites (e), 
derived not from cells 
of the retina itself but 
from more centra] re- 
gions, 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 olfac- 
tory surface ; here as 
there, the structural 
relations indicate the 
existence of a centri- 
fugal secondary path, 
running alongside of 
the centripetal prin- 
cipal path. 1 




FIG. 78. Schema of the optic conduction, in part after 
VON MONAKOW. S, Z Rods and cones of the retina. 
G 2 Large nerve cells of the layer of ganglion cells. G, Bi- 
polar nerve cells, e Terminal arborisations of centrifugal 
optic fibres. OV Pregeminum. AK Pregeniculum. 
O Occipital cortex, ss Direct optic radiation, c'f 
Centrifugal optic conduction to the pregeminum. cp 
Centripetal, cf centrifugal sensory mesencephalic path. 
rr Reflex paths to the nidi (kk) of the oculomotor nerves. 



1 For a detailed account of the terminal nervous apparatus of the re in a, cf. below, 
Ch. VIII. 4. 



183-6] Conduction Paths of ffig/ier Sensory Nerves 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 cen- 
trifugal conductors. Following its course, we come upon the decussation 
of the optic nerves, the chiasma, where a distribution is made of the optic 
fibres, to the paths running further towards the central organ, that is ob- 
viously of extreme importance for the co-operation of the two eyes in bin- 
ocular vision. It is instructive, in this regard, to trace the phylog('iu.'tie 
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. 1 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 i~esults of these observations are brought 
together, in schematic form, in Fig. 79. It will be seen that the corres- 
ponding 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 ot the area of direct vision around the 
fixation point, which becomes blind only when the central disturbance 
affects both sides of the brain. 2 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 

i RAMON Y CAJAL, Die Structur des chiasma opticum. Trans. J. BRESLER, 1899. 

2 GUDDEN, Arch. f. Ophthalmologie, xxv., i. VON MONAKOW, Arch. f. Psychiatric, 
xxiii., 1892, 619 ; xxiv., 229. GILLET and VIALET, Les centres certbraux de la vision. 
1893.' 



188 



Course of Paths of Nervous Conduction 



[186 



retina of the snme side, and rracular paths frcm both retinas. It divides 
again, on both sides as is sh;wn schematically in Fig. 78, where al strac- 
tion is made from the decuss tions which we have just been disci ssing 
into two paths : the one of wl ich runs first of all to the pregenicula (AK), 
while the other passes to the p: egemina (0V). The two path; appear to have 
no connexion with each other, despite the proximi'y of cuadrigernina and 
gem'cula (see Fig. 48, p. 120) In the same way, the optic path to th* 
pregeminum runs direct to Ihis through the prelrach'a, without c ming 
into contact with the post gem inum. The first of these two paths, that 
which travels to the pregeniculum, forms the eirect optic radia'ion (ss) 



m 




FIG. 79 ! Schema of the optic decussation in man, after VIALET. L Left, R right 
retina, t Decussating fibres of the temporal half, n Uncrossed fibres of the nasal 
half, m Fibres of the centre of the retina, running to both sides. 

to the cortex of the occipital lobe. It passes over, in the .rey nidi of the 
pregeminum, into a new neurone teirritory, whose neurites break up into 
terminal fibrils in the brain cortex. On the other hand, the large pyramidal 
cells of the cortex send out neuri'es, which apparently jcin the coronal 

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 unti Riickenmark, 209 ff. On the other hand, the definitive 



1 86-7] Conduction Paths of Higher Sensory Nerves i8c 

fibres that enter the pregemina, and thus constitute a centrifugal con- 
duction 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 coming 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 centrimgally away from them, 
[f, 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 : 
(i) the centripetal sensory principal path (ss), which in the large multi- 
polar ganglion cells of the retina (G 2 ) 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 ; and 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, con- 
ducts centrifugally, and which divides into two parts : a central branch 
c'j', running from optic cortex to mesencephalon, and terminating in the 
pregemina ; and a peripheral branch cf, beginning in the pregemina and 

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 con- 
duction 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 investi- 
gators, 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 1 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., i 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, 



190 Course of Paths of Nervous Conduction r 187-8 

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 re- 
leased in the mesencephalon, without any participation of the principal 
path : released as reflexes to the oculomotor system, by way of the trans- 
ferences 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 reflections 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 cosxcita- 
tions of the other. 1 

6. Paths of Motor and Sensory Conduction to the Cerebral Cortex 
(<z) -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 cen- 
tres and to the peripheral purls of the body. We therefore ask assistance 
at this point from two other sources, physiological experiment and path- 
ological 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 de- 
rangements recorded during life with the results of post-mortem examina- 
tion. The conclusions drawn from experiments on animals may be trans- 
ferred to man only, of course, in so far as they answer the general question 

1 MONAKOW, Arch. f. Psychiatric, xx. ( 1889, 714 ff. BECHTEREW, Die Leitungs- 
bahnen, 199 S. RAMON Y CAJAL, Les nouvettes idees sur la structure du systeme nerveux, 
1894 ; Studien iiber die Hirnrinde des Menschen, i. Die Sehrinde, i oo. 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. BECH- 
TEREW, op. cit., 215. 



1 3 8-9] Conduction Patlis to Cerebral Cortex 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. 

JLxperiments on animals fall into two main classes : stimulation experi- 
ments 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 symotoms 
to be observed are phenomena of movement, twitches or contractures in 
the muscles ; abrogation experiments bring about abrogation or distur- 
bance 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 experi- 
ments. 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 rounda- 
bout way with the lines of sensory or motor conduction, or with both. 
We shall, therefore, expect a priori that not every experimental or path- 
ological change, induced over a limited area, will be followed by noticeable 
symptoms ; and that, even where such symptoms appear, they will not, as 
jijrule, consist in simple phenomena of irritability and disability such as 
arise from the excitation or transsection of a peripheral nerve. This ex- 
pectation is amply confirmed by experience. At many points, stimulation 
maybe applied without producing any symptoms whatsoever. Where it 
v / 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 ambi- 
guity of result. We shall accordingly distinguish between ccntromolor 
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. 1 These terms must 

1 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 conduc- 



192 Course of Paths of Nervous Conduction t 1 89-90 

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 men- 
tioned in this place ; a point that follows directly from the extreme com- 
plexity 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 cen Irosensory 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 particu- 
lar 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 stim- 
ulation 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 
centra] 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 

tion here and in the peripheral nerves. The other terms in current use, ' psycho- 
motor ' 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. 



190-1] Motor and Sensory Centres in Cortex of Dog 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 ani- 
mals 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 oat, inhibitory influences may continue to be 
6;xerted, either upon the entire central organ or upon distant regions, par- 
ticularly 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 mea- 
sure 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 ob- 
servations upon the human subject. 

(ft) 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 elec- 
trical 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. 1 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 

i FRITSCH and HITZIG, Arch. f. Anat. u. Physiol., 1870, 300 ff. HITZIG, Unter- 
suchungen iiber das Gehirn, 1874, 42 ff. FERRIER, The Functions of the Brain, 2nd ed., 
1886. 

P. O 



194 



Course of Pat/is of Nervous Conduction 



[191-2 



are mechanically excitable : their exact localisation is, however, impossible, 
owing to their inacessible position. 1 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. 
TFe stimulable areas are seldom more 
than a few millimetres in extent, and 
excitation of points lying between 
them is, if the stimuli are weak, un- 
accompanied 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 stimu- 
lation) or to an enhancement of ex- 
citability brought about by the pre- 
ceding 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 cor- 
tical epilepsy. 2 For the rest, the con- 
tractions 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 ex- 
pression of that retardation of the stimulation processes which is of 
universal occurrence in the central elements. 3 

The phenomena of abrogation, observed after extirpation of definite 
portions of the cerebral cortex of the dog, differ in two respects from the 




FIG. 80. Centromotor areas on the 
surface of the brain of the dog. The 
areas on the left are given, in part, 
according to FRITSCH and HITZIG; in 
part, according to the author's own 
observations ; on the right, some of 
FERRIER'S results are shown for pur- 
poses of comparison, a Neck muscles. 
a' Back muscles, b Extensors and 
adductors of the fore leg. c Flexors 
and pronators of the fore leg. d 
Muscles of the hind leg. e Facialis. 
e' Superior facial region. / Eye 
muscles, g Muscles of mastication. 



1 LUCIANI, Arch. ital. de biologic, ix., 268. 

2 FRANCK, Les centres matrices du cerveau, 1887. 

3 FRANCK and PITRES, Arch, de physiol., 1885, . 149. 
Pfluger's Arch. f. d. ges. Physiol., xxvi., 137. 



BUBNOFF and HEIDENHAIN, 



192-3] Motor and Sensory Centres in Cortex of Dog 



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 stimu- 
lation 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 excita- 
bility, but also of more remote regions. It can thus be demonstrated that 
the entire anterior 
portion of the pariet- 
al lobe, and even 
the superior portion 
of the temporal re- 
gion as well, are in 
the dogcentromotor 
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 centro- 
motor 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. 1 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 sequela? 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 

1 LUCIANI and SEPPILLI, Die Functionslocalisation auf der Grosshirnrinde, 289 ff. 
(Le localizzazioni funzionali del cervello, 1885). HITZIG, Berliner klin. Wochenschrift, 
1886, 663. 




FIG. 8 1. Centromotor region of the surface of the brain 
of the dog. After LUCIANI. 




196 Course of PatJis of Nervous Conduction [ I 93~4 

by supposing that the excitable zones stand in closer relation to the peri- 
pheral 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 
~T>y any means in complete muscular paralyses. J[n general, there is in- 
hibition 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. 1 

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 uncer- 
tainty which attaches to sensory symptoms, place serious obstacles in the 
path of investigation. There are, however, two points in which the dis- 
turbances 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 im- 
pressions 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. 2 All 
these phenomena indicate that the functions of perception have in such 

1 On this point, cf. in particular GOLTZ, Ueber die Verrichtungen d f s Grosshinis, 
1881, 36, 119 ff. 

2 GOLTZ, PFLUGER'S Arch. f.d. ges. Physiol., xxvi., 170 ff. ; xxxiv., 487 ff. LUCIANC 
and SEPPILLI, op. cit. (German), 50 ff. 



194-5] Motor and Sensory Centres in Cortex of Dog 



197 



cases been abrogated or disturbed, but that the removal of the cer.tro- 
sensory 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 conduc- 
tion 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 decussa- 
tion 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 




FIG. 83. Auditory centre of the dog. After LUCIANI. 




FIG. 84. Olfactory centre of the dog. After LUCIANI. 



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 occ : pital lobe, though less marked disturbances may be 



198 Course of Paths of Nervous Conduction [i95-6 

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 prin- 
cipal 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 cer- 
tainty : it probably lies on the two opposite surfaces of the intercerebral 
fissure in the anterior region of the parietal lobe. 1 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 centro- 
motor area for the general muscular system of the body ; it can accord- 
ingly be illustrated from Fig. 81, which we have already employed in our 
previous discussion. This coincidence suggests the hypothesis that a dis 
tribution 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. 

(c) Motor and Sensory Cortical Areas in the Monkey 

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 2 and FERRiER, 3 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 

* SCHITSCHERBACK, Physiol. Centralblatt, v., 1891, 289 ff. 
8 HITZIG, Untersuchungcn iiber das Gehirn, 126 ff. 

FERRIER, The Functions of the Brain, 1886, 235 ff. 



196-7] Motor And Sensory Centres in Cortex of Monkey 199 

they extend as far as the superior portion of the subfrontal and medi- 
frontal gyres. More exact determinations of the points were then made 
in further investigations by HORSLEY and SCHAFER/ and by HORSLEY 
and BEEVOR. 2 Fig. 85 gives the results obtained by the two latter writers 
on a Bonnet Monkey (Macacus). 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 




ZurucKzieheiider Zunge 

FIG. 85. Centromotor cortical areas in the brain of a Bonnet Monkey (Macacus sinicus) . 
After HORSLEY and BEEVOR. 

The index follows the Figure in two vertical columns, beginning on the left, and 
reading from above downwards. Where a column sub-divides, the titles run from left 
to right within it. Movement of head to opposite side ; Turning ot eyes and head to 
same side ; Turning of eyes ; Tongue ; Hip ; Knee ; Hallux ; Small toes ; Ankle ; 
Shoulder, protraction and retraction ; Elbow ; Wrist ; Fingers ; Index finger ; Eye 
muscles ; Thumb ; Angle of mouth, elevation ; Tongue protruded ; Angle of mouth, 
retraction ; Larynx ; Pharynx ; Mastication ; Mouth open ; Tongue retracted. 

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, 



1 SCHAFER, Beitrdge zur Physiologie, C. LUDWIG gewidmct, 1887, 269 ff. 

8 HORSLEY and BEEVOR, Philos. Transactions, 1890, clxxix., 205 ; clxxxi., 



129. 



2OO Course of Paths of Nervous Conduction [ 1 97~& 

together, more particularly, with concomitant movements of other, function- 
ally co-ordinated muscle groups upon the opposite side. Thus, stimula- 
tion 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 orang-utan, 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 differ- 
ence 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 SCHAFER, 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 
be 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 obsejryers agree on these essential points, that the cortical surface 
of the occipital brain constitutes the visual centre, and that of the tem- 
poral 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. 1 

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 

1 H. MUNK, Ueber die Functionen der Grosshirnrinde, ate Aufl., 1890. Berichte 
der Berliner Akademie, 1892, 679 ; 1893, 759 ; J 895, 564 ; 1896, 1131 ; 1899, 936. 



198-9] Motor and Sensory Centres in Cortex of fttonkey 2ot 

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 con- 
duction 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. 1 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 repre- 
sentation, 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 signifi- 
cance 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 sur- 
rounds the central on all sides (A}. The centre is supposed, on the one hand, 
to correspond to the spot of clearest vision of the eye 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 

* Uebcr die Verrichtungen des Grosshirns, Abth. i.-vii. In PFLIJGER'S Arch. f. d. 
go:. Physiol., 1876-1892. 



202 



Course of PatJis of Nervous Conduction 



[199 



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 




FIG. 86. Sensory regions on the surface of the brain of the dog. After MUNK. 7. Dor- 
sal view. II. Lateral view (left hemisphere). A Visual area. A' Central region of the 
visual area. B Auditory area. B' Region for the perception of articulate sounds. 
C / Tactual area. C Region for the fore leg, D for the hind leg, E for the head 
F for the eyes, G for the ear, H for the neck, J for the trunk, a g Points of motor 
excitability (see the explanation of Fig. 80). 




FIG. 87. 



Sensory regions on the surface of the brain of the monkey. 
the meanings given under Fig. 86. 



The letters have 



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. 1 MUNK found, also, that these results of extirpation were con- 

1 Cf. p. 1 88, above. 



I99~ 201 ] Motor and Sensory Centres in Cortex of Monkey 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 epuivalent 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. 1 Similar eye movements were also observed by E. A. SCHAFER 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. 2 Movements of 
this kind, following upon stimulation of central sensory surfaces, may be inter- 
preted 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 trans- 
mission,' 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 (?). Destruction of a limited area ?' lying at the centre of this 
larger area (Fig. 86, //.), with retention of the surrounding parts, is said to 
abrogate only the perception of articulate sounds, i.e. to occasion mental deaf- 
ness ; 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 e'ar 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, ), the 
series ending with neck and trunk (H, /). These regions, in agreement with 
the results of other observers, coincide with those described above as the centro- 
motor 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, /., 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, how- 
ever, been challenged from many quarters. The physiological and psycho- 

1 On the relation of eye movement and retinal sensation, see below, Ch. XIV. 2. 
* E. A. SCHAFER, Proc. of the Royal Soc., 1887, 408. BAGINSKY, Arch. /. Physiol., 
1891, 227 ff. 



204 Course of Path': of Nervous Conduction [20 1-2 

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. 1 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. 2 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 field of vision have found confirmation in the experiments of other 
observers : 3 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 con- 
tractions, 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 appear- 
ance after the lapse of time. 4 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 

1 CHRISTIANI, Zur Physiologic des Gehirns, 1885, 31 ff. GOLTZ, in PFLUGER'S 
Arch. /. d. ges. Physiol., li., 570 ff. 

2 GOLTZ, in PFLUGER'S Arch. /. d. ges. Physiol., xxxiv., 459, 487 ff. CHRISTIANI, 
op. cit., 138 if. 

3 FERRIER, in Brain, 1881, 456; 1884, 139. LOEB, in PFLUGER'S Arch. /. d. ges. 
Physiol., xxxiv., 88 ff. LUCIANI and SEPPILLI, op. cit. (German), 145. 

4 For the criteria to be applied, cf. NOTHNAGEL, Topische Diagnostik der Gehirn- 
krankheitcn, Einleitung. 



202-3] Motor and Sens. iy Centres in Human Cortex 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 centro- 
motor 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 a 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 




FIG. 88. Motor cortical areas of the cerebral hemisphere of man. After 
VON MONAKOW. FS Sylvian fissure. FR Central fissure. F lt F. 2 , F s Super- 
frontal, medifrontal and subfrontal gyres. T lt T 2 , T 3 Supertemporal, medi- 
temporal and subtemporal gyres. S. marg. Marginal gyre. ang. Angular 
gyre. P lt P 2 Parietal and subparietal gyres. FP Interparietal fissural 
complex. Po Occipital fissure. The remaining names are to be read across, 
from left to right, and from above downwards. Trunk ; upper leg ; lower 
leg ; foot ; toes. Shoulder ; elbow. Head ; wrist. Eyes ; fingers ; thumb. 
Tongue ; upper and lower facial muscles. Mouth ; platysma (a muscle 
lying beneath the skin, at the side of the neck, and extending from chest 
and shoulder to face). Larynx ; mastication. 

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. 1 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 

1 NOTHNAGEL, Topische Diagnostik, 438 ff. H. DE BOYER, Etudes cliniques sur les 
ttsions corticales, 1879. EXNER, Untersuchungen uber die Localisation dcr Functionen 
in dcr Grosshirnrinde des Menschen, 1881. VON MONAKOW, Gehirnpathologie, 1897. 
282 ff. 



2O6 



Course of PatJis of Nervous Conduction 



[203-4 



movements, to which may be added, later on, contractures due to the 
action of muscles not affected by the lesion. 1 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 prin- 
cipal centrosensory areas in the human brain. First and foremost, the 

Tast&pMf* 

77/777^7 




FIG. 89. Sensory areas on the outer surface of the human brain, after FLECHSIG. 
Tastsphdre. Tactual area. Sehsphare. Visual area. Horsphdre. Auditory area. 



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 

1 FERRIER. Localisation der Hirn^rkrankungen, 12 ff. (Localisation of Cerebral 
Disease, 1878). NOTHNAGEL, Topische Diagnostik, 549. VON MONAKOW, Gehirn- 
pathologie, 376 ff. 



204-5] Motor and Sensory Centres in Human Cortex 



207 



of the pregeminum and geniculum of the opposite side has followed upon 
destruction of the one occipital lobe. 1 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. 2 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 spsak 
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 

Tastsphare 




Riechsphare 



FIG. 90. Sensory areas on the median surface of the human brain, after FLECHSIG. 
The conjectural limits of the olfactory area are indicated by small open circles. Tast- 
sphdre, Sehsphdre, as before. Riechsphdre, Olfactory area. 

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. 3 We may therefore suppose that symptoms 

1 NOTHNAGEL, Topische Diagnostik, 389. LUCIANI and SffFFfLU. op. cit. (German), 
167 ff. VON MONAKOW, Gehirnpathologie, 445 ff. 

2 VON MONAKOW, Arch. f. Psychiatric, xxiv., 229 ff. Cf. also the conditions found to 
obtain in the brain of LAURA BRIDGMAN, a deaf-mute, who became blind in early child- 
hood. It may be remarked, further, that the whole development of this brain sugges 
an elaborate system of vicarious functioning, especially in the sensory region. 

SON, Amer. Journal of Psvch., iii., 1890, 293 ; iv., 1892, 503 ff. 

3 Cf. the cases described byFiJRSTNER (Arch. f. Psychiatne, vm. 162 ; ix., 90) and 
REINHARD (ibid,, 147), and by VON MONAKOW, Gehirnbalhologie, 468 ff. 



208 Course of Paths of Nervous Conduction [205-6 

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, be 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. 1 

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, derange- 
ment 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 dis- 
tinguished by their symptoms as aphasia, word-deafness, agraphia, word- 
blindness, etc. are of extremely complex character. And we must accord- 
ingly 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 prin- 
cipal terminus of the acoustic path (Fig. 89) is the posterior section of the 
supertemporal gyre (7\ 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). 2 

The demonstration of the centres for the fibres of the gustatory and 
olfactory nerves presents considerable difficulty, though for other reasons. 

1 Cf. the following Chapter, and the account of visual ideas given in Ch. XIV. 

2 WERNICKE, Der aphasische Symptomencomplex, 1874. KAHLER and PICK, Bei- 
trage zur Pathologic und pathologischen Anatomic des Centralnervensy stems, 1879, 24, 
182. LUCIANI and SEPPILLI, op, cit. (German), 217 ff. VON MONAKOW, Gehirn- 
pathologie, 506 ff. 



206-7] Motor and Sensory Centres in Human Cortex 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 deter- 
mine 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 ot 
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 dis- 
tribution 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. 1 

. 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 agree- 
ment 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 sensa- 
tion 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 ob- 
servations 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. 2 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 

1 FLECHSIG, Gehirn und Seele, 2te Aufl., 1896, 61 ; Die Localisation der geistigen 
Vorgdnge, 1896, 34. 

2 EXNER, op. cit., 63 ff. LUCIANI and SEPPILLI, op. cit. (German), 321 ff. On dis- 
turbances of the sensations of movement, consult further NOTHNAGEL, Topische 
Diagnostik. 465 ff. VON MONAKOW, Gehirn pathologie, 362 ff, 

p. r 



2io Course of Paths of Nervous Conduction [207-9 

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 ob- 
servations, 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 develop- 
ment 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 cort oal areas. 
According to the observations of FERRIER, MUNK and LUCIAXI, 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 agree- 
ment between the results. The only difference of any considerable moment 
is that thejierangements of function resulting from cortical lesions are as 
a rule more serious in man than they are in the animals. And this differ- 
ence 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. 1 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 
1 On this point see below, Ch. VI., 5 and 6, 



209-10] Motor and Sensory Centres in Human Cortex 



21 I 



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 restora- 
tion 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 observa- 
tions, 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 pre- 
carious, 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 in- 
timate connexion with it, have been under observation for a long period of 
time. 1 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 (ex- 
ternal and internal sensations of touch, pain, and organic sensations) may be 
regarded as sufficiently well established. The localisation was first suggested 
by TURCK, 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. 2 In these cases, and still more in cases of destruction of the' central 
gyres themselves, the symptoms are, however, invariably complicate'd by the 
simultaneous appearance of motor paralysis in the corresponding regions of the 
body. For the rest, the hemianaesthetic 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 sensa- 
tions, 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. 3 

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 \\ith 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, 4 which has been accepted 
by MEYNERT, S H. MUNK, 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 

1 See below, Ch. VI., 7. 

2 CHARCOT, Lemons sur Us localisations, etc. (Vorlesungen uber die Localisation der 
Gehirnkrankheiten, 120 ff.). NOTHNAGEL, Topische Diagnostik, 581 f< 

3 VON MONAKOW, Gehirnpathologie, 364 ff. 

* Arch. f. experimentelle Pathologic, iii., 1874, 171. 
s MEYNERT, Psychiatrie, 1884, 145. 

* Arch, f. Physiol., 1878, 171 ; Ueber die Functionen der Grossh''rnr:udr, 44. 



212 Course of Paths of Nervous Conduction [210-1 

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 
_deerper-iying 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 con- 
sists 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 surrounding 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, but a general concept, gained 
by abstraction from a large number of concrete facts. And the concrete in- 
dividual 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. Besides, 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 trans- 
mission from brain cortex to central conduction paths. 1 

There is one other fact that we may mention, in conclusion, as of import- 
ance 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 con- 
duction 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 

1 Cf. with this WUNDT, Zur Frage der Localisation der Grosshirnrinde, in Philos 
Slttdien, vi., 1891, i ff. 



2 1 1 -2] Association Systems of Cerebral Cortex i 1 3 

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. 1 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 
Jiimself 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. 2 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. 3 
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. ' 

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 
tofection 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 ' 
on 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. 4 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, 

1 FLECHSIG, Die Localisation der geistigen Vorgdnge, 13 ff. EDINGER, Voriesungen., 
6te Aufl., 161 ff. 

2 FLECHSIG, Neurol. Centralblatt, 1898, no. 21. 

3 Cf. DEJERINE, Zeitsch. f. Hypnotismus, v., 1897, 343 ; O. VOGT, ibid., 347 ; 
SIEMERLING, Berliner klin. Wochenschrift, xxxv., 1900, 1033. See also p. 217, below. 

* MEYNERT, in STRICKER'S Gcwebelehre, 117 (POWER'S translation, ii.,48if.}; 
Psychiatrie, 40. 



Course of Paths of Nervous Conduction 



[212-3 



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 peri- 
pheral 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 func- 
tional unity of separate cortical areas. 
The association system like the 
projection s} T stem, may be divided 
into various component systems, dis- 
tinguished in this case partly by the 
direction of connexion, partly by the 
distance separating the connected re- 
gions of the cortex. We thus obtain 
the following subordinate systems of 
association fibres : 

(i) The system of transverse com- 
missures. This is principally com- 
posed of the callosum or great commis- 
sure, 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. 1 27). 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 hemi- 
spheres. The callosal fibres cut across the coronal radiations at all points 
except in the occipital region, where the two sets of fibres separate into 




FIG. 91. Systems of transverse asso- 
ciation fibres ; schematic cross-section 
Hirough the prosencephalon in the 
region of the precommissure. After 
EDINGER. Bk Callosal radiations. 
Ca Fibres of the precommissure. 



Association Systems of Cerebral Cortex 215 

distinct bundles (;', Fig. 58, p. 135 ; cf. also Fig. 57, p. 134). The con- 
nexion effected by the callosum between symmetrical parts of the cortex 
is fullest, as might be conjectured from its marked increase of size in trans- 
verse 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 




FIG. 92. Systems of longitudinal association fibres. After EDINGER. F Frontal 
P parietal, O ocripital, T temporal lobe. Bk Callosum. Is Region of the insula. 
Cg Londitudinal fibres of the callosal gyre (cingulum). fu Uncinate fascicle, ft Longi- 
tudinal fascicle, fa Arcuate fascicle. w Intergyral fibres (fibrae propriae). 



several compact bundles of this kind, devoted more especially to the con- 
nexion of the frontal with the temporal lobe (uncinate and arcuate 
fascicles) and of this latter with the extremity of the occipital lobe (longi- 
tudinal fascicle). 

(3) The system of intergyral fibres (fibrae propriae, 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 



2l6 Course of Paths of Nerrous Conduction [ 21 3~^ 

cerebral cortex may be again divided into three classes, according to their 
mode of origin and termination. They may (i) 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.' l 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 con- 
siderable 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 centro- 
sensory 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 symp- 
toms of abrogation are decisive : the disturbances set up are of a com- 
plicated nature, and their compensation may later be effected, within 
wide limits, by vicarious functioning of other parts. This result is in har- 
mony with the further fact that there is no region of the brain surface 
1 FLECHSIG, Gehirn und Seele, 2te Aufl., 1896; Neurol. Cenlralblatt, 1898, no. 21. 



21^-6] Association $y steins of Ctrebral Cortex 

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 associa- 
tion centres. Itjias been found, e.g., that the association centres, together 
wjih^thejnain 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 denned 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 hypo- 
thesis ; while DEJERINE, VON MONAKOW, SIEMERLING, O. VOGT and others 
pronounced the 1 distinction altogether impracticable. 1 There are no cortical 
areas^jay these authorities, to which projection fibres cannot be traced ; just 
as there are, by general admission, none which are not supplied with association 
Ibres. 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- 

1 RAMON Y CAJAL, Die Structur des chiasma opticum, 56. EDINGER, Vorlesitngen, 
6te AufL, 228. HITZIG, Les centres de projection et d' association, Rapport lu au xiii. 
CongrJs internat. de Med. a Paris, Le Nevraxe, i., 1900 (criticism by FLECHSIG, ibid., ii., 
and reply by HITZIG, 1900). VON MONAKOW, Monatsschrift f. Psychiatric, viii., 1900, 
405. O. VOGT, Journ. de physiol, et pathol. gen., 1900, 525. See above, p. 213. 




Course oj Paths of tiervous Conduction [216-7 

sidcred 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 lias been displaced far forwards ; so that, 
iu 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 peri- 
phery, 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 



2 i 7~8j Stru. ture of Cerebral Cortex 

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 




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l^A v ;i;:^:-r,.i-.*> 



KwSl 
ft&tfMi 

i^v.'^i-i^M 



lM,W ' 

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rlfV|r,.;!i;/| 

;(:!, i. <r ; i 

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FIG. 93. Section through the postcentral 
gyre. After RAMON Y CAJAL. i Plexi- 
form layer. 2 Small, 3 intermediate 
pyramidal cells. 4 Large pyramidal 
cells of the outer layer. 5 Small pyra- 
midal and stellate cells. 6 Large pyra- 
midal cells of the inner layer. 7 Layer of 
spindle-shaped and triangular cells. 8 
Deeper-lying portion of this layer. 




mmm^ 

M;&m(0r : ti 




FIG. 94. Section through the occipital 
cortex of man. After RAMON Y CAJAL. 
i Plexiform layer. 2 Small, 3 inter- 
mediate pyramidal cells. 4 Layer of 
large stellate cells. 5 Small stellate cells. 
6 Small pyra nidal cells, with ascending 
neurites. 7 Giant pyramidal cells. 8 
Pyramidal cells with deflected ascending 
neurites. 9 Spindle cells. 



220 Course of Paths of Nen'dus Conduction t 21 7~$ 

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 consider- 
ably 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 what- 
ever 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 par- 
ticularly 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 (.4 Fig. 95), in par- 
ticular, 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 pyra- 
midal cells are dendritic in character. The stoutest of them leaves the 



219-20] 



Structure of Cerebral Cortex 



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 centri 
fugal conduction, as indicated by the arrows in the Fig., beginning 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 longi- 
tudinal 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 outer- 
most 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 messncephalon. 
Putting these facts together, we 
may describe the structure of the 
motor cortex as follows. (i) 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 dis- 
charges 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 centro- 
motor 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 




FIG. 95. Individual cells and fibre con- 
nexions from the centromotor region. 
After RAMON Y CAJAL. A Large pyra- 
midal cells, with upward trending dend- 
rites and downward trending neurites. B 
Intercalary cells, representing, perhaps, 
links in the chain of association fibres. D 
Terminal ramifications of upward trend- 
ing neurites. 



222 



Course of Paths of Nervous Conduction 



[220-1 



terminal station of paths of sensory conduction. Its position in the motor 
cortex is, as we have said, comparatively insignificant. It may be distin- 
guished from the other constituents by its plexiform structure, the fibres run- 
ning 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 path- 
ological observation 
place the centre for 
the general sense in 
this part of the cor- 
tex. 

We now turn, by 
way of contrast, to the 
' visual ' cortex, as a 
typical illustration of 
a pre-eminently sen- 
sory region. We ' are 
at once struck by the 
marked difference in 
the course of the fibres. 
Plexiform formations, 
with fibres running in 
all directions hori- 
zontally, therefore, as 
well as vertically and 
obliquely are very 
strongly preponderant, 
while the longitudinal 
fibre masses character- 
istic 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, through- 
out the centromotor region. We notice, in particular, the longitudinal 
centrifugal fibres, connected with the pyramidal cells (A). There are, 




FIG. 96. From the layer of stellate cells of the visual cor- 
tex. After RAMON Y CAJAL. A Pyramidal cell. B Inter- 
calary cell. C Stellate cells. 



221-2] Structure of Cerebral Cortex 223 

further, the centripetal fibres, ascending to the cortex from deeper lying 
cell groups ; and, lastly, the supposed association fibres with their inter- 
calary cells (B). We must accordingly infer that the visual cortex dis- 
charges^centromoto^ 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. 1 

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 signifi- 
cant differences. At the same time, there are several layers which, with 
their characteristic elements, attain to very different degrees of develop- 
ment 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 im- 
portance 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 con- 
duction paths ; the latter characteristic of the sensory regions, and there- 
fore, 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 



1 RAMON Y CAJAL, Studien uber die Hirnrinde des Menschen, German trans, by 
BRESLER. Heft I : Die Sehrinde ; Heft 2 : Die Bewegungsrinde, 1900. Comparative 
Study of the Sensory Areas of the Human Coitex, in Decennial I olume of the Clark Uni- 
versity, 1899, 311 ff. 



224 



Course of Paths of Nervous Conduction [222-3 



cells with limited neurone territory, set longitudinally and connected with 
longitudinal fibre systems, which may perhaps be looked upon as the sub- 
strate 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 whethe'r the different regions of the' cerebral corte'x 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 ; * and he has been followed by GOLGI 2 and KOLLIKH R. 3 
Other investigators, and more especially RAMON Y CAJAL,* 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 them- 
selves furnished conclusive proof of the extraordinary degree of similarity 
obtaining in the structure of the various regions, and have 1 shown that the 
differences are in every case merely relative differences in the number of par- 
ticular elements and in the development of the layers. Nay more : since they 
have made it probable that the different centromotor, sensory and ' associa- 
tive ' 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 
Junctions 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. jThey 
rather force us lO_ the conclusion that the different modes of cortical activity 
are founded not upon the specific character of the structural elements, but 
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. 



1 MEYNERT, Vierteljahrsschrift /. Psychiatric, i., 97, 198 ; ii., 88. Also in STRICKER'S 
Gewebelehre, ii., 704 ff. (POWER'S trans., ii. 381.) 

a GOLGT, Sulla ftna anatomia degli organi centrali, 1886. 

3 KOLLIKER, Gewebelehre, 6te Aufl., ii., 807 ff. 

4 RAMON Y CAJAL, Studien iiber die Hirnrinde des Menschen, Heft i., 5 ff. Cf. also 
FLECHSIG, Die Localisation der geistiqen Vorgdnge, 82 ff. 



224-5] Principle of Manifold Representation 22$ 

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 supple- 
mented by direct physiological analysis of the functions themselves. At 
the same time, it is evident that we may look to the conditions of con- 
duction 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 physio- 
logical 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 inter- 
pretation 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 Q 



226 Principles of Central Conduction [225 

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 con- 
ditions) it may be stimulated, or its excitation inhibited, without the inter- 
ference 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 cere- 
bellum, and in the association centres, with which these regions stand in 
connexion. Now it is by no means necessary that the \vhole 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 peri- 
pheral 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 pro- 
gression 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. 

(6) 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 peri- 
pheral 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 con- 
nexions reaches its final term in the cerebral cortex. Every part of the 
cortex, however diverse its proximate connexions and however distinct 



226-7] Principle of Ascending Complication 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 ' associa- 
tive,' 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 proba- 
bility, 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 psycholo- 
logical grounds, could hardly beheld 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 histo- 
logical 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. 

(c) The Principle of the Differentiation of Directions of Conduction 
At this point the question naturally arises, whether or not the investiga- 
tion of nervous conduction has furnished any evidence of specific differences 



228 Principles of Central Conduction [227-8 

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 prob- 
ably be inferred from the anatomical and physiological relations of the 
conduction paths. This is the difference in direction of conduction, con- 
nected 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 note- 
worthy that these cells always occur in situations where the functional 
requirements do not include a differentiation of the directions of con- 
duction, 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 physiolo- 
gically 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 con- 
ditions) in which directions the processes are conducted ; but it cannot 



228-9] Differentiation of Directions and Central Colligation 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 qualita- 
tive differences in the central elements can be demonstrated by morpho- 
logical methods or inferred from the mechanics of innervation. 

(d) The Principle of the Central Colligation of Remote Functional Areas. 

Theory of Decussalions 

We have yet to mention a circumstance from which, in very many 
cases, the principle of the manifold representation of peripheral areas un- 
doubtedly 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 con- 
duction 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 cere- 
bral cortex) where the centres for these movements lie close together ; so 
that a suitable co-ordination can be effected, for voluntary as for involun- 
tary, purely reflex movements, along well-trodden paths of cross con- 
nexion 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- 



230 Principles of Central Conduction [229-30 

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 com- 
pound 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 dis- 
posal, 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 decussa- 
tion ; 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 com- 
pensate 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 



230-1] 



Theory of Decussations 



231 



of an extended object which remain invisible to the other. The hypo- 
thesis 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 mam- 
mals, 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 be of the object abc 
(Fig. 97). It follows at once, from 
the fact of inversion, that the two 
retinal half-images a/3 and fly are 
out of their right positions : for if 
we regard fly as the direct con- 
tinuation of a/3, fl should be joined 
to fl, and not y 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 sus- 
tain in the external object (a' (3', 

ay>. 

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 con- 
duction paths ; that it was fol- 
lowed, 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, corre- 
lated with these. 1 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 
1 RAMON Y CAJAL, Die Structur des chiasma opticum, 22 ff. 




FIG. 97. Schema of the act of binocular 
vision in a vertebrate with laterally placed 
eyes and total decussation of the optic 
nerves. 



232 Principles of Central Conduction [2 31 -2 

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 remark- 
ably 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 demon- 
strated. Even in cases of what is called cortical hemianopsia in man, 
its existence has never been suspected. On the other hand, it is note- 
worthy that this cortical hemianopsia is much more obscure in its symp- 
toms 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 diminu- 
tion, not with complete abrogation of sensitivity to light. 1 This, however, 
is just the opposite of what we should expect, if an undisturbed repro- 
duction 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 we see objects upright, despite the optical inversion of their images, 
than would be given with this hypothesis of a vertical decussation. Wher- 
ever 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 
1 VON MONAKOW, Gehirnpatholo^ie, 450 



232-3] 



Theory of DecnssationS 



233 




b 

FIG. 98. Relation of the position of the image 
on the retina to the movements of the eye. 



in the direction 0.8, 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 re- 
tinal 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. Intrinsi- 
cally, 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 
jter^med, with RAMON Y CAJAL, ' panoramic ' (as opposed to ' stereo- 
scopic ') vision. It covers a wide range ; but it mediates only a super- 
ficial 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 sym- 
metrically 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 inner- 
vation of the left rectus internus, whose direction of pull is indicated by 



234 Principles of Central Conduction [ 2 33~4 

the dotted line 2 , must be so co-ordinated with the innervation of the 
right rectus internus, *i, that it promptly relieves its predecessor, to give 
way in its own turn to the innervation of the left externus, e 2 . 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 interims 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 arrange- 
ment 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 ; 
jt 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 refer- 
ence 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 e v will 
first be innervated from a central point e'. As the internus t\ comes into 
action, the central innervation will move from e' to *'. Next, upon the 
entrance of the object into the visual field of the left eye, it will pass without 
interruption to 2 , i", and thence, finally, to e v e". If there were no decussa- 
tion, 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 *' e e" i", and the innervation would first of all travjl 
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 



234-5] 



Theory of DecussationS 



235 



cc. 



arrangement which controls movement naturally determines as well the 
localisation of the resting eyes ; so that be 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 posit- 
ing an organisation that extends 
over the entire sensorimotor sys- 
tem and is determined by the 
necessary co-ordination of par- 
ticular sensory and motor points. 
The hypothesis has the further 
advantage that it, and it alone, 
can adequately explain the obvi- 
ous connexion between the change 
from total to partial decussation 
of the optic fibres, on the one 
hand, and the passage from pano- 
ramic 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 




FIG. 99. Schema of the act of binocular 
vision in man and in animals with a com- 
mon field of vision. 



236 Principles of Central Conduction [ 2 35~6 

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 con- 
tact 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 ay 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 move- 
ment 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 sym- 
metrically 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 sym- 
metrically 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 



] Theory of Decussations 237 

inlerni are represented in the brain, too, on the inside, nearer the median 
plane, and the musculi externi on the outsido. 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, repre- 
sented by the inner portions of the retinas, and governed, of course, by 
the conditions of panoramic vision and the special laws of eye movemsnt 
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 co-ordination 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 dispossd in the essential matter of sensori- 
motor connexions. 

There are many other instances of the decussation of coniuction 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 



238 Principles of Central Conduction [ 2 37~8 

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 move- 
ments 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 pre- 
ponderantly 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 conse- 
quence 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. Second- 
arily, it is true, recourse is had to the subsidiary hypothesis (mentioned above, 
pp. 20 1 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 



238-9] Theory of Decussations 239 

sufficiently refuted, even apart from the complex nature of the disturbances con- 
sequent 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 often- 
times 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 in- 
volved 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 sus- 
tained 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. 1 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 con- 
trol 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 
1 OGLE, Medico- chirurgical Transactions, liv., iS/i, 279. 



240 Principles of Central Conduction [239-40 

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 fore- 
front 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. 1 Since, then, the more developed half of the brain must corre- 
spond 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 prac- 
tice imposed by outside conditions upon the bodily organs and their central 
representatives. 

1 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 (Anatomic 
comparee du systeme nerveux, ii., 242). GRATIOLET'S results are, however, questioned 
by ECKER (Arch. f. Anthropologie,iii.,2i$). W. BRAUNE (Arch. f. Anatomic, 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 obser- 
vations accord with those made by BROCA and P. BERT upon the differences in temper- 
ature found in man at different parts of the head ; the left half of .he frontal region is 
on the average warmer than the right, and the frontal regio.i as a whole warmer than 
the occipital (P. BERT, Soc. de biologic, 19 Janv., j?^9j- 



240-1] 



CHAPTER VI 

The Physiological Function of the Central Parts 

I. 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 under- 
standing 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 in- 
fluence : 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. B 



242 Physiological Function of Central Parts [241-2 

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 approxi- 
mates 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 pro- 
cesses 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 pro- 
cesses is evinced by the dependence of the n-tlrx 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. 

1 See above, pp. 43, 99. 



242-3] Reflex Functions 

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 pro- 
ceeds, 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 centro- 
sensory areas of the cerebral cortex. 1 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 sen- 
sory 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 corre- 
sponding to them, are stimulated, the result resembles that of summation 
of stimulations within the same sensory area : that is to say, the inter- 
ference 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 inter- 
pretation 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 combina- 
tion of circumstances which, naturally, brings it about that jhe nerve 
trunk possesses a relative minimum of reflex excitability. 2 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. 3 

1 H. E. HERING and SHERRINGTON, in PFL<JGER'S Arch. f. d. ges. Physiol., Ixviii., 

1897, 222. 

2 See above, p. 88, * See above, Ch. iii., pp, 80, 94, flf. 



244 Physiological Function of Central Parts [ 2 43~4 

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 posi- 
tion of equilibrium in which the excitatory and inhibitory forces counter- 
act 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 anta- 
gonistic 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. 1 As regards intensity and dis- 
tribution, 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 re- 
flexes with increasing intensity of stimulus (discussed above, p 162) admits 
of exception ; the general and relatively constant conditions of reflex con- 
duction are cut across by the more variable influences arising from the reci- 
procal regulation of sensations and movements. 2 

\b) Melencephalic (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 im- 
portant part in various physiological functions. We may mention the 

* MOTT and SHERRINGTON, Proc. of the Royal Soc., Ivii., 1895, 481. C. BA.STIAN, 
ibid., Ivii., 89. 

* SHERRjNgiQN, j n THOMPSON YATES' Labor. Reports, i., 45, 175. 



544~S] Reflex Functions 

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 re- 
lation 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 determin- 
ants, and its influence is correspondingly restricted. Others, again, like 
the movements of swallowing, appear to be pure reflexes ; they are abro- 
gated 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 bila- 
teral, 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 re- 
gular 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 pha- 
ryngeal muscles. In fine, then, it is probable that all these oblongata re- 
flexes, 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 



246 physiological Function of Central Parts [245-6 

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 some- 
times stands in connexion with a second sensory path, from which, accord- 
ingly, 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 cesophageal 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 expira- 
tion ordinarily proceeds. They are distinguished by the fact that stimu- 
lation 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. 1 Further : 
the secondary sensory path from the expiratory centre to the mucous mem- 
brane 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, especi- 
ally 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 move- 
ments and heart beat. The heart is supplied by two sorts of nerve paths, 
which affect the sequence of beats in opposite ways : accelerating nerves, 

1 These and the other mimetic reflexes are of great psychological importance, and 
are accordingly discussed under the heading of expressive movements in Part IV., Ch. 
xvu. 



246-8] Reflex Functions 24? 

which increase the rapidity of heart beat, and inhibitory nerves, which 
diminish it, or bring the organ to a complete standstill. Both may be re- 
flexly excited ; but the centre for the accelerating fibres is intimately con- 
nected 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 fluctu- 
ations 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 
inner vation. The vessels are governed, like the heart, by motor and inhi- 
bitory 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 pressor 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 hyperaemia 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 



248 Physiological Function of Central Parts [248-9 

support one another. An intensive cold stimulus applied to the surface I 
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 inner- 
vation effects, due perhaps to special ganglion cells lying within them, per- 
haps 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. 1 

In all probability it is the nerve nidi of the oblongata, with their inter- 
current 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 be much more easily explained on 

1 For the relative autonomy of the cardiac movements, see especially T. W. ENGEL- 
MANN, in PFLOGER'S Arch. f. d. ges. Physiol., Ivi., 1894, 149 ; for the autonomous func- 
tions of the movements that fall within the sphere of the spinal reflexes, see ^OLTZ and 
EWALD, ibid.. IxiiU, 1897, 362. 



249-5] fcejlex functions 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 con- 
traction of the pupil. Both reflexes are bilateral, though with weak excita- 
tions 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 tensor tympani, which presumably accompany every sound stimula- 
tion : 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 
palp ebr arum 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 



250 Physiological Function of Central Parts 

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 mani- 
fold 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. 1 

(c) Purposiveness of the Reflexes. Extent of Reflex Phenomena 
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 
be 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 con- 
sciousness. Worded in this way, however, the question is evidently mis- 
leading. 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 

NOTHNAGEL. in ViRCHow's Archw., xliv., 4. BINSWANGER, Arch, f. Psychiatr., 
xix., 759. 



251-2] keflex Functions 2$i 

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 move- 
ment. 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 move- 
ment released in the central organ by the action of sensory stimuli. Sup- 
pose, 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 stimula- 
tion. 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 denomi- 
nated 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 stimu- 
lating object that is pressing upon the skin ; for in this instance the move- 
ment 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 



, 

Physiological Function of Central Parts 

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 psychi- 
cally 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 stimula- 
_t[oji is denominated a reflex, whether psychical intermediaries are brought 
into pjay_pr not. From this point of view, the voluntary action is some- 
times 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 move- 
ments. 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 sensori- 
motor protoplasmic substance of the body. The centralisation of the 
psychical functions in the brain, that is, goes pari passu with their decen- 






2 53~4] Automatic Excitations 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. 1 



3. Automatic Excitations 
(a) Automatic Excitations in My el and OUon^ata 

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 'j)sychically 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 b)' 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 dis- 
criminate 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, 2 and the sphincter muscles of bladder and intestine, 
where similar results have been observed. 3 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 

1 See above, Ch. i., pp. 29 ff., and below, Chs. xvii., xviii., on Will and Consciousness. 

2 GOLTZ and FREUSBERG, in PFLUGER'S Archiv. f. d. ges. Physiol., xiii., 1876, 460, 

3 MASIUS, Bulletin de I'academie de Beige, xxiv., xxv., 1867, 1868, 



254 Physiological Function of Central Parts [254- 5 

no change in muscular tension, apart from the concomitant twitch and its 
elastic after-effects. 1 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 excep- 
tion, 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 breath- 
ing or, indeed, by any circumstance that prevents the elimination of the 
oxidised constituents of the tissues. The presence in the dyspnceic 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 accumula- 
tion 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 com- 
pletes the circle of self-regulating functions, whereby the process of respira- 
tion 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 

1 HEIDENHAIN, Physiol. Studien, 1856,9. WUNDT, Lehre von derMuskelbewegung, 



2 5 5~6] Automatic Excitations 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 pressor 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 there- 
fore 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 excita- 
tion of the inspiratory muscles, constriction of the blood-vessels and inhibi- 
tion 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 res : 
piratory 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 



256 Physiological Function of Central Parts [ 2 56-/ 

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. 1 The dyspnceic blood may, however, occasion muscular convulsions 
of the same kind, though less widely diffused, by stimulation of the myel. 2 

(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 unim- 
portant. 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 

1 KUSSMAUL and TENNER, in MOLESCHOTT'S Untersuchungcn zur Naturlehre des 
Menschen, in., 1^57, 77. 

LUCHSINOER, in PFLCGER'S Arch. /. d. ges. Physiol., xiv., 1877, 383. 



2 57~8] Automatic Excitations 257 

the foundation, upon which the complex psychophysica] 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. 1 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 c ntral 
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 dyspnceic 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. 2 

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 ; 3 for changes in the innervation of heart and vessels are 
frequently observed in cases of mental disease. 4 Now all the chronic 
forms of insanity are connected with more or less serious modifications of 

1 Mosso, Ueber den Kreislauf des Blutes im menschlichen Gehirn, 1881, 74 ff. 

2 Cf. with this the discussion of the psychology of dreams, Part V., Ch. xx. The 
excitatory influence of the dyspnceic 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. 

3 WERNICKE, Lehrbuch d. Gehirnkrankheiten, ii., 10. KRAEPELIN, Psychiatric, 6te 
Aun., i., 54. 

4 WOLFF, Allg. Zeitschr. f. Psychiatric, xxvi., 273. ZIEHEN, Sphygmographische 
Untersuchungen bei Geisteskranken, 1887. 

P. S 



25 8 Physiological Function of Central Parts [258-9 

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 hallucina- 
tions are accompanied by changes in the subjective sensations, muscular 
'anTorganic, 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, how- 
ever, 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. 1 



4. Functions of the Mesencephalon and Diencephalon 

( a ) Functions of the Mesencephalon and Diencephalon in the Lower Verte- 
brates 

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 func- 
tional differences ; so that it is far more dangerous in the case of mesence- 
phalon 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 disturb- 
ance 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 

1 For this psychological aspect of mental derangement, and for an account of sleep 
and of similar states (hypnotism), see Part v., Ch. xx. 



259-60] MesencepJialon and Diencephalon 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. 1 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 mesen- 
cephalic 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. 2 It was then applied, 
systematically, by GOLTZ, in work upon the frog ; 3 and has been used by 
CHRISTIANI 4 and, still more recently by GOLTZ 5 again for mammals, and 
finally by J. STEINER 6 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- 

1 Here belong more particularly the experiments of LONGET (A natomie et physiologic 
du systeme nerveux, 1842 ; German trans, by HEIN, i., 385) ; SCHIFF (Lehrbuch d. Physiol., 
i., 342) ; VULPIAN (Physiol. du systeme 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 an- 
tiquated ; 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 stimula- 
tion, without regard to morphological relations and the course of the conduction paths. 

2 FLOURENS, Recherches exper. sur les fonctions du systeme nerveux, 2nd ed., 1842. 

3 GOLTZ, Beitrdge zur Lehre von den Functionen der Nervencentren des Frosches, 1869. 
* CHRISTIANI, Zur Physiologie des Gehirns, 1885. 

s GOLTZ, Der Hund ohne Grosshirn, in PFLUGER'S Arch. f.d. ges. Physiol., li., 1892, 
57. 

6 J. STEINER, Die Functionen des Centralnervensy stems, Abth. 1-4, 1885-7900. 



260 Physiological Function of Central Parts [260-1 

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. 1 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. 2 
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 indis- 
tinguishable from a normal animal. 3 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. 4 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 

1 J. STEINER, Die Fun:tionen des Centralnervensystems, ii., 211 flf. 
1 GOLTZ, Die Functionen der Nervencentren des Frosches, 65. 

SCHRADER, in PFLOGER'S Arch. f. d. ges. Physiol., xli., 1887, 75. 
SCHRADER, in PFLOGER'S Arch. f. d. ges. Physiol.. xliv., 1888, 175. 



262-3] Mesencephalon and tiiencepliatoit 26 1 

obstacles when stimulated to movements of escape, and of cccasionally 
executing what appear to be spontaneous movements. 1 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. 2 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 intro- 
duction of food into the gullet) went on without disturbance from the begin- 
ning. The progress of time brought with it, however, a much more com- 
plete 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 dien- 
cephalic 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 func- 
tions. 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 prosen- 
cephalon : 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, localiss 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, 

1 CHRISTIANI, Zur Physiologie des Gehims, 25. 

2 GOLTZ, in PFLUGER'S Arch. f. d. ges. PhysioL, li., 570 3. 



262 Physiological Function of Central Parts [263-4 

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 
j any kind in the psychical behaviour of the animals. At a somewhat higher 
I 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 func- 
tions 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 condi- 
tioned 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 prosen- 
cephalon. According as we incline to the one or the other of these inter- 
pretations, will our estimate of the functional significance of the mesence- 
phalic 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 inter- 
ferences by operation, and especially interferences with the central organs, 
are known to affect the functions for a certain period of time. Still, it is 



264-5] Mesencephaton and Diencephalon 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 com- 
pensation is observable, v/hereas in the fishes recovery sets in at once. 
And lastly, there are numerous phenomena, drawn from all kinds of sources, 
which prove 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 interpolation 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 orgins, so long, that is, as they stand 



264 Physiological Function of Central Parts [265-6 

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 ol 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 ordin- 
arily follow upon its direct removal or impairment. The most striking of 
these, upon the sensory side, is the blindness which, in mammals, in accord- 
ance 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. 1 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 (Reitbahnbewe- 
gvmg, mouvement de manege) 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 veterbrates, 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. 2 
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, tne direction of curvature corresponding to that of 
the circus movements. 3 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 

1 BECHTEREW, in PFLOGER'S Arch. f. d. ges. Physiol., xxxiii., 413, 
* STEINER, Die Functionen des Centralnervensy stems, iii., 79 ff. 
SCHIFF, Lehrbuch d. Physiol., ii.. 343. 



266-7] Mesencephaton and Dienccphaloit 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 symp- 
toms are always ambiguous, and the results are correspondingly doubtful. 1 
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 from the deeper lying centres to the prosen- 
cephalon ; 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 excita- 
tions 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 anato- 
mical 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 stat'ons 
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 ssnse departments v with the excep- 
tion of smell) and of the movements correlated with them. It follows that 
the mesencephalon and diencephalon, in proportion to their degree ol 
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 develop- 
ment 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 

1 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 Archiu, Iviii., 429 and Ixii., 203. 



266 Physiological Function of Central Parts [267-8 

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 origin- 
ally directed. Physiologists, from FLOURENS to GOLTZ, made these experiments 
with the primary intention of deriving from the resulting symptoms of abroga- 
tion 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 GOI.TZ 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 sub- 
traction. 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 differ- 
ences 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 inverte- 
brates 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 experi- 
ments 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, unf6rtunately, 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 move- 
ment 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 



268-9] Mesencephalon and Dicncephaton 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, through- 
out 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 mer,en- 
cephalon 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. 1 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 move- 
ment 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 
ma)' 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 dien- 
cephalon, 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 dien- 
cephalon. 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 
1 STEINER, op. cit., ii., 106 ; iii., 126 ; iv., 54 ff, 



268 Physiological Function of Central Parts [269-70 

them is lost, a portion of its functions can, as a general rule, be taken over by 
another. 

F.xperiments on completely decerebrised animals, especially the higher 
mammals, are of extreme importance, not only for the functions of the mesen- 
cephalic and diencephalic region, but also for the more general question of the 
functional representation of h'g'ier 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. 1 
The animal was deprived of its left hemisphere in two experiments, performed 
on the 2/th of June and the I3th of November, 1889 ; the entire right hemi- 
sphere was removed on the i/th of June, 1890. It was killed, with a view to 
post mortem examination, on the 3ist 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 subse- 
quent 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 pre- 
pared 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 insensi- 
tive 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 'recog- 
nition ' 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 
1 GOLTZ, in PFLOGER'S Arch. f. d. ges. Physiol., li., 1520 ff. 



2/o-i] MesenccpJial.n and Diencephalon 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 move- 
ments 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 interpreta- 
tion. 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 meaning 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 



270 Physiological Function of Central Parts [271-2 

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 
anaesthesia or by motor disturbances or by both combined. Sometimes, 
it is true, affections of the thalami run their course without any sign of dis- 
turbance whatsoever : 1 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 impera- 
tive 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. 2 The determining factors 
in this result are apparently two : on the one hand, the voluntary suppres- 
sion of the symptoms, and, on the other, the greater scope of the automatic 
regulations and functional substitutions that, in the human brain, coun- 
teract the disturbances in question. Both factors indicate that, while the 
basal functions of this region of the human brain correspond to those dis- 
charged 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 after removal 
of the prosencephalic parts can hardly be regarded as probable. 

(c) Striatum and Lenticula 

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 speci- 
fically 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 

NOTHNAGEL, Topische Diagnostik. 204 ff. VON MONAKOW, Gehirnpatholoeie, 586 ff. 
WERNICKE, Lehrbuch der Gehirnkrankheiten, i., 370. 



2 7 2 ~3J Cerebellum 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 what- 
ever. NOTHNAGEL found, further, that mechanical or chemical stimulation 
of the striatum of the rabbit occasioned hurried running movements. 1 
MAGENDIE observed the same result after complete removal of the striatum. 2 
Anaesthesia, on the contrary, does not appear to be a consequence of injury 
to these structures. 3 For the rest, the intensive disturbances that ordi- 
narily 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.* 

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 parts, with the oblongata, with the mesencephalon 
and cliencephalon, 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 cere- 
bellar functions. And, on the other, we ate 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. 

1 NOTHNAGEL, in VIRCHOW'S Archiv, Ivii., 209. 

2 MAGENDE, Lemons sur les fonctions du systdme nerveux, i., 2&o. Ci. also SCHIFF 
Lehrbuch d~. Physiol,, i., 340. 

3 NOTHNAGEL, Topische Diagnostik, 263 ft. VON MONAKOW, Gehirnpalhologie, 584. 
* FLECHSIG, Plan dfs menschl, Gekirns, 41. 



272 Physiological Function of Central Parts [-73-4 

These symptoms themselves consist, for the most part, in motor distur- 
bances. 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 de- 
rangement. 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 deflection. 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. 1 

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- 

1 LUCIANI. // cervelctto. 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, Ixviii., 33), FERRIER (Functions of 
the Brain, 174), and BECHTEREW (PFLUGER'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. 



275-5] Cerebellum 273 

turbances ; they become exceedingly complicated, and interpretation is 
correspondingly difficult. 1 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. 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 accom- 
panies 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. 3 And for the same reason we may suspect that this organ 
is involved in the dizziness produced by certain toxic agencies. 4 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 be- 
come familiar with a sensory apparatus of the former kind in the ampullae 
and canals of the labyrinth of the ear. 6 On the other hand, w.e 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 thejtwo 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. 6 These 
relations to the vestibular division of the labyrinth are, perhaps, 
our best means of accounting for the influence of the cerebellum 

1 LUCIANI, op. cit. 32. LADAME, Hirngeschwulste, 93. WERNICKE, Gehirnkrank- 
heiten, iii., 353. VON MONAKOW, Gehirnpathologie, 624. 

2 In a case in whch 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, Anatomic et 
physiologic du systeme 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. Psychiatric, 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.). 

3 PURKINJE, in KUST'S Magazin d. Heilkunde, xxiii., 1827, 297. HITZIG, Das 
Gchirn, 196 ff. Der Schwindel, 1898 (off-printed from NOTHNAGEL'S Pathologic, ix.), 
36 ff. 

4 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. 

s See Part III., Ch. xiii. 

6 BECHTEREW, Die Leitungsbahnen im Gchirn und Rilckenmark, 361. 

' P. T 



274 Physiological Finiction of Central Parts [ 2 75~7 

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 de- 
monstrable subjective or objective causes, these may ordinarily be traced 
back to one genera! 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 be 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. 1 
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 
tin- voluntary action is directed exclusively upon the end to be 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 (i) be paralytic 
phenomena, i.e. they may be occasioned by a partial abrogation of voluntary 
movements: They may (2) appear as purely anaesthetic symptoms. They 
ma XJ^-g ns i s L Ldisturjmnces 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 

VON LEYDEN and GOLDSCHEIDER, Die Erkrankungen des Ruckenmarks. In 
NOTHNAGEL'S Handbuch d. Pathologic, x., 149. 



2 77~8] Cerebellum 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.' 1 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 un- 
certain, 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, 2 whose views 
have held their own, down to the most recent times, among physiologists 
and clinicians. But, first, this definition is too indeterminate to char- 
acterise 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 cere- 
bellum 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 
anaesthesia, and yet differ from them on the important point that abroga- 
tion of sensations never makes its appearance among the cerebellar pheno- 
mena. 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 

1 LUSSANA, Journal de la physiol., v., 418 ; vi., 169. LUSSANA and LEMOJGNE, 
Fisiologia dei centri nervosi, 1871, ii., 219. 

2 FtOUREjsts, Recherches experimentales, 2nd ed., 28, 



276 Physiological Function of Central Parts [278-9 

movements must always proceed from a sort of reflection. The imme- 
diacy and certainty of movement shown by the uninjured animal are 
either lost or, if they may in some measure be rega : ned, 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 operation. The cerebellum 
appears to be intended for the direct regulation of voluntary mrjements 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 post- 
peduncles 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. 1 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 them- 
selves 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 physio- 
logists 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 func- 
tional importance, a position attested, in any case, by the structure and volume 

1 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 

Such disturbances have, as a matter of fact, been observed by BECHTEREW 

PFLOGER 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). 



279.1 Cerebellum 

of the organ renders the exact determination of the nature of the ' co-ordina- 
tions ' or ' regulations ' effected by the cerebellum a matter of extreme diffi- 
culty ; 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 orga,n, 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 indefmiteness, LUCIANI undertook to analyse the pheno- 
mena, 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 
s-ymptoms 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). 1 It- has been objected, with some justice, to this 
characterisation, that the symptoms which it discriminates are, in part at 
least, closely interconnected : atony and asthenia, e.g., always occur together. 2 
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, how- 
ever, the emphasis must fall, without any question, upon that member of the 
triad \\hich 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 com- 
plete 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 obser- 
vations, 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 

1 LUCIANI, II cerveletto, ( erm. trans., 282. 

2 FERRIER, in Brain, xvii., 1894, i fit. 



Physiological Function of Central Parts [280-1 

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. 1 It finds 
striking confirmation in the statements made by KAHI.ER and PICK, from the 
pathological standpoint, concerning the relation of other forms of ' ataxia,' 
as it is termed, to the cerebellar symptoms. 2 HiT7,iG, too, in his interpretation 
of cerebellar dizziness, seems to take up a very similar position. 3 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-ordination's" 
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 circum- 
stances, 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 any- 
where, 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 
Jcveloped a central mechanism, automatic in function, whose office it is to 
transmit the first, and only the first, discharging impulses to an auxiliary centre ; 
and that this auxiliary centre is endowed with self-regulating apparatus, again 

1 Edition of 1874, 220. 

187Q K 58. LER ^ PKK ' Bcitrd S e **rPathol. u. pathol. Anal. d. Ccntralncrvensystems, 
" Hrrzio, Der Schwindel, 42 Q. 



28I-2J 



Cerebellum 



automatic in function, which adapt each several movement to the sense impres- 
sions 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 cerebsllar 
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, we 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 sensor}' 
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 centro- 
motor 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. 1 

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 prosence- 
phalon, has persuaded several authors to attribute to it a share in what are called 

1 Cf. the discussion of the voluntary actions, Part IV., Ch. xvii. 



280 Physiological Function of Central Parts [282-3 

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 
i authors, records of cases that had not been properly investigated, and observa- 
. . .. the s is] ion oi - ; : d< ' ision for s il :li irre Lstibly upon the- 
reader, the whole forming a mass of evidential matter that should be impressive 
solely by its bulk, would of itself forbid our devoting any attention to their 
writings, even if we did not find upon every page the mark of inveterate pre- 
possession. 1 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 2 
and LussANA, 3 have regarded as possible this relation of the cerebellum to the 
sexual functions ; though their standpoint, in making this admission, has gener- 
ally 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, how- 
ever, that they have become anything more than mere conjectures.' Moreover, 
the argument from the indemonstrability of the opposite can be rebutted, 
in the present case, by a sufficient number of positive instances, both experi- 
mental 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. 5 The statistics 
of cerebellar tumours in man have, also, failed to yield the slightest confirmation 
of the phrenologists' view. 6 Finally, the symptomatology of cerebellar affec- 
tions, so far as it is given objectively, on the ground of observations, affords no 
hint of sexual reference. 7 

6. Functions of the Cerebral Hemispheres 

(a) Phenomena of Abrogation after Partial Destruction of the Prosen- 

cephalon 

Our knowledge of the functions of mesencephalon and diencephalon 
is, it will be remembered, mainly derived from observations of the psycho- 
physical activities left intact after removal of the prosencephalon (pp. 259 
ff.)- These same observations may, of course, be turned to account for a 

1 GALL, Anatomic et physiologic du systems nerveux, iii., 1818, 85. COMBE, On the 
functions of the cerebellum, 1838. 

2 R. WAGNER, Gotlinger Nachrichten, 1860, 32. 
1 LUSSANA, Journal de la physiologic, v., 140. 

1 On phrenology in general, see below, 6. 
^ ^ LUCIANI, // cervelello, Germ, trans., 198. Cf. also FERRIER, Functions of the Brain, 

8 LADAME, Hirngeschwulste, 99. 
kf> %Sif" NOTHNAGEL - Topische Diagnostik, 78 ff. VON MONAKOW, Gehirnpatho- 



283-4] Cerebral Hemispheres 2Sl 

tlicory 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 inhibi- 
tory 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 hemi- 
spheres 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 lim'ted 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 funct'ons. 1 

These abrogation phenomena and their compensations are of pecul'ar 
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 opera- 
tion, in very much the same way as if they were normal animals. 2 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 extremi- 
ties, it preferred as a rule to use the muscles of the same side. The per- 

1 LADAME, Hirngcschwtilste, 186 f. NOTHNAGEL, Topische Diagnostik der Gehirn- 
krankheiten, 435 ff. VON MONAKOW, Gehirnpathologie, 376 ff. 

2 GOLTZ, in PFLUGER'S Arch. f. d. ges. Physiol., xlii., 1886, 484. 



2 2 Physiological Function of Central Parts [284-5 

ceptions of sight and hearing had also become uncertain, though they were 
by no means destroyed. 1 GOLTZ afterwards made experiments, with like 
result, upon a monkey (Rhesus), which was almost entirely deprived of one 
hemisphere. 2 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 derangerrienf7 Tlie 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 im- 
pairment of the perceptual functions (cf. p. 196 above). In both cases, 
whether the frontal or the occiptal 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 dis- 
like 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 hypersesthesia ; in the latter, they became 
apathetic, probably in consequence of their partial anaesthesia ; quarrel- 
some dogs were rendered, to all appearance, good-tempered, though it is true 
that they were also uninterested. 3 

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, oi 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 induce 1 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 decussa- 
tion 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. 4 Unfortunately, however, in no one of these cases, which 
all belong to the older medical literature, was the patient subjected to any 

1 LOEB, in PFLCGERS' Arch. f. d. ges. Physiol., xxxiv., 1884, 67. 
1 GOLTZ, ibid., Ivi., 1899, 411. 

3 GOLTZ, ibid., xxxiv., 1884, 450 ; xlii., 1888, 439. 

4 LONGET, Anatomic et physiologic du systtme nerveux, Germ, trans, by HEIM, i., 
539 ff- 



Cerebral Hemispheres 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 com- 
plicated kind, and their psychological analysis is accordingly todtdefective 
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 func- 
tional analysis is possible. 1 

All these observations, which refer to the consequence of more or less 
extensive cerebral lesions, are, it is clear, of but comparatively slight im- 
portance 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 com- 
plete 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 prosence- 
phalon, 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 le- 
sions. 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- 

1 Cf. the discussions of the centres of vision, speech and apperception, 7, below. 



Physiological Function of Central Parts L 286~7 

Iluences of compensation are incalculable. On the other hand, the symp- 
toms 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 dience- 
phalon. 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 prosence- 
phalon do not afford a safe basis for judgment, since it remains uncertain 
whether and how far they themselves owe their origin to some compensa- 
tion 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, arid 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 stale, of purposive 
reaction to tactual and visual stimuli, but also adapt their movements, like 
normal animals, to external impressions. They avoid obstacles, they re- 
cover their equilibrium by balancing, etc. ; nay more, they apparently exe- 
cute 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 
si-nsory 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 be 
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 pros- 
encephalon, is a matter of extreme difficulty. In any event, however, the 



287-8] 



Cerebral Hemispheres 



285 



distinction, so far as it is practicable at all, must also be left over for the 
detailed psychological analysis of the processes involved. 1 

(c) Results from Comparative Anatomy and Anthropology 

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. Compara- 
tive anatomy shows that the mass of the cerebral lobes, and more especially 





FIG. 100. Normal small brain, with fair 
fissural development. 



FIG. 101. 



Brain of the mathematician, 
C. FR. GAUSS. 



their superficial ridging by fissures and gyres, increase with increasing in- 
telligence 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, 
injhejsrnallest, 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 abun- 
dance of fissures. 2 The physical organisation is another factor of great 

1 See below, Parts IV., V. 

2 LEURET and GRATIOLET, Anatomic comparee du systeme nerveux, ii., 290. OWEN- 
Anatomy of Vertebrates, iii. 



286 Physiological Function of Central Parts [288-9 

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 
"f males of Teutonic descent between the ages of thirty and forty may be set 
;it 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 797 grammes. Still greater is the discrepancy if we consider, not the 
wright of the brain, but its superficial development, conditioned upon the 
number of gyres. Thus H. WAGNER gives for man a surface of 2,196 to 1,877, 
for an orang-utan a surface of 535.5 sq. cm. 1 Among the lower races of 
man the brain has also been found, as a rule, to be both smaller and less 
convoluted. 2 And numerous observations go to show that, within the 
same race and nationality, eminently gifted individuals possess large and 
richly convoluted hemispheres. 3 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.* 



HUSCHKE, Schddel, Him und Seele, 60. H. WAGNER, Massbestimmungcn der 
Oberfldche des grossen Gehirns, 1864, 33. 

2 TIEDEMANN, Das Him des Negers mil dem des Europders und Orang-Utangs ver- 
lichen, 1837. BROCA, Memoires d' anthropologie , 1871, 191. 

GALL and SPURZHEIM, Anatomic et physiologic du 'sysleme nerueux, ii., 251. 

R. WAGNER, to whom we owe these reproductions, and those of certain other 

ams of eminent men (Dirichlet, C. Fr. Hermann, etc.), was himself somewhat hesitant 

vmg this conclusion (Goltinger gel. Anz., 1860, 65 ; Vorstudien zu einer wissen- 

lorpholooie und Physiologic des Gehirns, 1860, 33). C. VOGT however, justly 

that it follows, without any question, from WAGNER'S own figures, if we select 

icm the illustrations that really apply to individuals of acknowledged mental 

eminence See also BROCA, Memoires d' anthropologie, 155. For the rest it need 

>e pointed out that, here as elsewhere, the concurrent factors of race height, age, 

taken into consideration. A normal Hottentot brain in the skull of an 

European would, as GRATIOLET observed, spell idiocy. 



289-9] Cerebral Hemispheres ; Localisation 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 de- 
parture of the^system of ' phrenology '_ founded by FRANZ JOSEPH GALL, y^ 
Unfortunately, however, the physiological and psychological premisses upon 
which GAIL 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, he 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 demon- 
strated, does not obtain, at any rate to the extent required. GALL distin- 
guished 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 metaphysique, 
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 GALI.'S suggestion, as has been 
expressly acknowledged by BOUILLAUD, to whom it is due. 1 At the same 
time, we must not forget that even in this instance, where a pronouncement 
of GALL'S has recceived a certain measure of confirmation from the facts, 
there is really an essential difference between what was actually dis- 
covered, viz., the anatomical seat of central derangements of speech, and 
the phrenological ' organ of language.' The two can, in truth, be identi- v 
fied only if we force a phrenological interpretation upon the phenomena of 

1 BOUILLAUD, Recherches cliniques propres a demontrer que la perte de la parole corre- 
sponde a la lesion des lobules anterieures du cerveau et a confirmer I 'opinion de M. GALL 
etc. In Arch. gfn. de med., viii., 1825. 



2 88 Physiological Function of Central Parts [290-1 

aphasia, which in the light of actual analysis they will not bear. Cf. 
below, ' ?& 

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 pyschological theories 
that, at bottom were no less questionable than the ' internal senses ' 
of the phrenologists. Thus FI.OURENS, whose views long held undisputed 
sway in physiolog)', insisted strenuously upon the unity and indivisibility 
oT the cerebral functions, and argued from it that their organ must be 
similarly indivisible. This opinion was largely determined by the analogy 
7)f~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 cere- 
bral 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.' 1 

This doctrine, of the specially -psychical funct'on of the prosencephalon 
and of the indivisibility of that function, presently became untenable. 
It was overthrown partly by the pathological observations on the conse- 
quences 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 evi- 
dently a reactive effect of the advance in knowledge of the ana'tomy 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 languag-, pcetic talent, etc., are br.ni?hed from 

1 FLOURENS, Rccherches expfr. sur les fonctions da sysfdme nerveitx, 2nd ed. 1842. 



291-3] Cerebral Hemispheres : Localisation 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 j ustif y 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 excita- 
tions brought in along the sensory conduction paths release the specific 
sensations. The centromotor regions are counted among thess sensory 
centres, on the further assumption that a voluntary action may be adequately 
denned 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 mor- 
phologically 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 con- 
duction ; the latter take up excitations proceeding from the sensation 
cells, and thereby acquire the capacity of renewing these excitations, a pro- 
cess 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, 

u 



290 Physiological Function of Central Parts [293~4 

the essential activity of the cerebrum consists in the function of the asso- 
ciation centres, while the sensory centres serve, on the whole, simply to 
take up the sense impressions, in the order in which they affect the peri- 
pheral 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 sens? : 
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 an- 
tithesis of sensation cells and idea cells, which naturally leads to a corre- 
sponding 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 asso- 
ciation 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 tin. i 
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 phy- 
sjological 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 dif- 
ferent cortical regions in the psychical functions, though they have given 
up the hypothesis of the functional equivalence of all the parts of the cere- 



2 94~5] Cerebral Hemispheres: Localisation 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 oppo- 
nents 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 peri- 
pheral 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 inter- 
pretation of the experiments on animals, which, as we know, are pre-emi- 
nently 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 sen- 
sations and ideas are absolutely distinct conscious contents belongs to 
the older spiritualistic psychology, which taught that ' ideas,' as contra- 
distinguished 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 



292 Physiological Function rf Central Parts [295-6 

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, qr_that the sensations which enter into our 
ideas differ in any other way than by their intensity, duration, and frag- 
mentary character from the sensations aroused by external sensory stimuli. 1 
.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 unnatu- 
ral 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 conscious- 
ness 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 depart- 
ment. Hence the paths that run between different sensory centres could, 
at most, be taken as the conjectural substrate only of what are called ' com- 
plications,' 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 ' asso- 
ciation 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 psy- 
chology of association, very much as the sense-centre theory, in its distinction 
l>etween sensations and ideas, is dominated by the old spiritualistic view 
of mind. The psychology of association seeks, as we know, to derive con- 
cepts, judgments, complex intellectual and affective processes all and sundry, 

1 See Ch. vii. below. 



296-7] Cerebral Hemispheres ; Localisation 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-pperation of the sensory centres with which they are connected by asso- 
ciation 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 pro- 
portion 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 repre- 
sentatives are undoubtedly right in insisting upon the multiplicity of rela- 
tions 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, jhis_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 ex- 
periments 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 influ- 
ence is in most cases greatly underestimated. What we rather need is, 
evidsntly, an analysis of the individual central functions, in the light of ob- 
servations of pathological defects, carefully colbcted 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, 



/ ' . . I 

294 Physiological Function of Central Parts [297-8 

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 even, 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 them- 
selves comes up as a further problem, whose solution will effectively supple- 
ment, 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, VESAI.US was especi- 
ally instrumental in spreading the opinion that the brain is tlfe scat of the mental 
activities. For a long time, however, the old doctrine of ARISTOTLE and GALEN, 
that made thejieart 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 
ot 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.* 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 
fferently worked out by different authors. 2 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 

\ his doctrine of irritability, whose chief significance lay in the fact that it 
erred the capacities of sensation and movement to different kinds of tissue 
>rmer to the nerves, the latter to the muscles and other contractile elements.^ 
DESCARTES. Les passions de I'dme. I. See above, p. 124. 

the list in HALLER, ElementaJ>hysiolof>ia;,iv. 1762 397 <_ 

^^ dOCtrine " f ir " tabiliiy in ^author's 



298-9] Cerebral Hemispheres : Localisation 295 

The source of these capacities HAI.LER finds in the brain. This organ is connected 
with the mind and the psychical functions only_in__so far as it is the sensoriuni 
commune, or the place where all activities of sense arc exercised and whence all 
muscular movements take their origin. The sensoriuni extends over the whole 
substance of cerebrum and cerebellum.* It is therefore certain that every 
lierve 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. 2 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 
jas_they split up within it into finer and finer branches. 3 

The position thus won was never lost by physiology. Nevertheless, the 
endeavours after a physiological localisation of the mental faculties were con- 
stantly 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. 4 The phrenology 5 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. TJtie_ individual 
capacities localised in the brain were, accordingly, also termed ' internal 'c^__ 
jjgnses. GALL distinguished twenty-seven ; his pupil SPURZHEIM increased 
the number to thirty-five. 6 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." 7 His main argument for the analogy 
of the ' internal senses ' with the external sense organs is derived from ana- 
tomical investigations ; as every sensory nerve is a bundle of nerve fibres, so 
is the whole brain a collection of nerve bundles. 8 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 

1 Elem. physiol., iv., 395. 

2 Ibid., 397. 

3 " Hypothesin esse video et fateor," he cautiously adds. Ibid., 399. 

4 GALL and SPURZHEIM, Anatomic et physiologie du systeme nerveux, i., 1810. Cf. 
also the same authors' Recherches sur le systeme nerveux, etc., 1809 (contains the memoir 
presented to the French Institute and the report of the Commissioners). jG_ALi/s__tiYQ_ 
main services to brain anatomy are his introduction of the method of dissection from 

upwards, and his demonstration of the universally fibrous character of the alba. 

5 GALL'S system is set forth in detail in vols. ii.-iv. of the Anatomie et physiologie. 

6 COMBE, System of Phrenology, 1825 (Germ, trans, by HIRSCHFELD, 1833, 101 f.). 
' Anat. et physiol., iv., 341. 

8 Ibid., i., 271 ; ii., 372. 



396 Physiological Function of Central Parts [299-300 

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 
jnore enlightened position occupied by HALLER. The great Swiss already lias 
Jan inkUngj)f the true principle, that the peripheral organs of the body must in 
i -/i _2!n_e_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 lan- 
guage," GALL hit upon the right path. And in spite of all his errors, this fact 
lias 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. 1 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 for 
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 ? 
('.ranted 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 physiognomies of LAVATI-K 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 ' tha". GALL pretends to have discovered, as P. J. MOBIUS has undertaken 
to do in the interests of the ' organ of mathematical ability.' 2 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 MOBIUS has found 
confirmed in the case of three hundred mathematicians, admits ol 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 endow- 
ment 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 di; covered in highly developed brains of poets, philosophers, philo- 

86o * P> J- MOBII.^ Franz Joseph Gall, in SCHMIDT'S Jahrbuch d. Medicin, cclxii., 1899, 
P. J. MOBius, Ueber die Anlage zur Mathematik : mil 5 1 Bildnissen, 1900. 



300- 1 ] Cerebral Hemispheres : Localisation 297 

logians, e'c., 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, 
MAGEXDIE and FLOUREXS. ! The views which these investigators developed 
of the significance of the central organs plainly represent a reaction against the 
phrenological doctrines. In MAGEXDIE this spirit shows itself in the strict 
congruity of theory with observed facts. 2 FLOUREXS had the more decisive 

"TrnTuence TTjSbTTThe 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 ceiebellum as 
the centre of conduction of voluntary movements, and the cerebral lobes as 
the seat of intelligence and will. 3 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 tioeu-i vital, 
destruction of which means instantaneous death. JThe higher central regions, 

' on the other hand, discharge the functions assigned to them uniformly through- 
out 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, there- 
fore, that the smallest fragment of these organs can function for the whole. 
In his view of the cerebral hemispheres as the properly psychical centres, FLOU- 
REXS 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 experi- 
ence, FLOUREXS substituted for it the total mass of the cerebral hemispheres. 
FLOUREXS' doctrines thus came, in virtue of their spiritualistic prominence, 
into sharp conflict with the more materialistically coloured ideas of the phreno- 
logists : a circumstance that was not without import for the issue of the struggle. 
Their acceptance in scientific circles was, however, chiefly due to the fact that 
they gave a fairly accurate representation of the observational data in experi- 
ments on animals. It was not realised that the same psychological difficulties 
attach to them as to the phrenological theory of organs. Nevertheless, intelli- 
gence 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 

1 A criticism of the teachings of phrenology from the standpoint of comparative 
anatomy is to be found in LEURET, Anatomie compares du systeme nerveux, i. ; a criticism 
based upon the writer's physiological experiments, in FLOURENS' Examen de la phreno- 
logie, 1842. 

2 MAGENDIE, Lecons sur les fonctions du systeme nerveux, 1839. 

3 FLOURENS, Recherches exper. sur les fonctions du systeme nerveux, 2nd ed. 1842. 



298 Physiological Functions of Central Parts [302-3 

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. 1 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 BOUII.LAUD on the anatomical substrate of aphasia. 2 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 GoLXZ. 3 So, in the controversy carried on principally between 
H. MUNK and GOLXZ, 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 phreno- 
logy ; 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 ele- 
mentary 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 

() 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 com- 
plicated course of their paths and the number of central areas which these 

1 Cf.,e.g., BURDACH, Vom Bau und Leben des Gehirns, iii. ARNOLD, Physiologic, i., 
836. HUSCHKE, Schddel, Him und Seele, 174. 



* BROCA, Sur la siege de lafaculU du language, 

* Cf. especially his discussions in PFLCGER'S 



s Arch. /. d. ges. Physiol., xx., 1879, 10 ff. 



303-4] Psyc/iophj'sical Analysis: Visual Centre 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 con- 
duction. The connexions with certain sensorimotor and regulatory centres, 
in particular, centres like the pregemina, cerebellum, etc., can, in the pre- 
sent state of our knowledge, be referred only quite generally to the interac- 
tions 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 ana- 
tomical 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 in- 
stance, 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 dis- 
placed to the periphery of the body. Hence it iv 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 photo- 
chemical 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). 1 

It is at this point that the process of vision proper begins. The photo- 
chemical 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 alwa}'s 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 me- 
diating 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 physiolo- 
gies is not able to satisfy these conditions. How inadequate it is will be 
1 Cf. Part II., Ch. viii. below. 



Physiological Function of Central Parts [3 O 4~S 

seen at o:ice from a consideration of Fig. 78 (p. 186), although the Figure 
do?s 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 (A K), by nodal points, at which, in all probability, connex- 
ions are also madewith other paths. Further: as we said above(pp.i88,23off.), 
thedccussationof 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 centri- 
fugal motor innervations proceeding from the occipital cortex. This re- 
lation 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 mesen- 
cephalic region, where it enters, in the pregeminum (0V) into two connex- 
ions. 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 con- 
ditions laid down by the requirements of binocular (panoramic or stereo- 
scopic) vision. The second is a sensory connexion ; for the suppos3dly 
centrifugal terminations of the opticus in the retina (c /) 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 con- 
jecture that, in both cases, the paths will take a crossed course ; for it is but 
natural to regard the centrifugal system c /, c' f as the substrate of the co- 
excitations, 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 
corn?xion, not only with its sensory continuation of, but also with the motor 
path rr, so that the regulation of ocular movements by light impressions 
( an 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 



305-6] Psycliophysical Analysis : Visual Centre 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 com- 
ponents 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 excita- 
tion, 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 neigh- 
bouring 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 depart- 
ments of the optic conduction, the mesencephalic and prosen cephalic- 
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 couise 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 



3O2 Physiological Function of Central Parts [306-7 

visual centre in the occipital cortex (pp. 260 fL). 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 com- 
plex 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 direct- 
ly 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 com- 
plexity, even if we were not constrained to posit this complication of con- 
ditions by our psychological analysis of the visual processes. 1 

(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 sub- 
stance, 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 intelli- 
gence 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 derange- 
ment of speech, or even by complete abrogation of the speech functions, with- 
out 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 re- 
presentation in the brain cortex is without exception bilateral, has the pecu- 
liarity that its development is exclusively unilateral. Since the majority 
of mankind are right-handed, it is situated, as a result of the decussation 

1 See below. Part III., Ch. xiv., on visual ideas. 



307-8] Psychophy steal Analysis : Speech Centres 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 




FIG. 102. Position of the speech centres in the cortex of the left cerebral hemisphere. 
M Motor, A acoustic, O optic speech centre. S Centre for writing movements. The 
remaining letters have the same meaning as in Fig. 65, p. 145. 

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 symp- 
toms 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 pheno- 
mena 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 articu- 
lations : 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, Jt occupies the supertemporal gyre, more 



304 Physiological Function of Central Parts [308-6 

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 con- 
dition 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 con- 
sists 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 



309-10] P sychophy steal Analysis : Speech Centres 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 ' : 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 blind- 
ness ' : words can be spoken, and can also be heard, understood and re- 
membered ; 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 disordeis ; 
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 sub- 
cortical disturbances. The latter are also termed intercortical or conduc- 
tive disturbances, on the assumption that, while the cortical lesions involve 
the speech centres themselves, the subcortical injuries interrupt the con- 
nexions mediated between different centres by association fibres. 1 

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 syn- 
ergy of the different centres, pathologists are accustomed, in order to ex- 
plain 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. 2 The little circles M and S (A ) denote the primary, motor 

1 WERNICKE, Der aphasische Symptomencomplex , 1874. 

2 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 un- 

V, * 



* 



306 Physiological Function of Central Parts [310-1 

and sensory-acoustic, speech centres; E and are the secondary centres 
co-ordinated with them, E that of the movements of writing, that of visual 
word-pictures. Besicfes these centres, however, all schematic representa- 
tions 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 

^ # "W^ that this centre C, together 

/ \ ^ ' i ^ with the name attached to 

9. / "*/,'' ,' it, is in realitv only an in- 

X' /*> ^ s' 

X s 70 definite expression for the 

manifold relations in which 
the various speech centres 
must stand with all the cor- 

f -3 v /_*'' tical 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, 
FIG 107. Schema of the speech centres and their . , , ,, . 
connexions. After WKRNICKE and LICHTHEIM. 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 cir- 
cumscribed 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 sub- 
sisting 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 asso- 
ciations made. It is, however, generally assumed that the conductions may 

broken lines. A plan resembling LICHTHEIM'S, and factually accordant v.ith it, but 
somewhat more complicated in outward appearance, had been drawn some years earliei 
by KUSSMAUL : Die Stdrunen der Sprar.he. 1877, 183. 



311-3] Psychophysical Analysis : SpeecJi Centres 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 excita- 
tions 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 con- 
ditions 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 5 to C is unimpaired, will be rightly understood. Further, if the con- 
duction 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 5, the path which is supposed to run in the alba of the 
insula, is now out of function. Similar consequences follow from an inter- 
ruption of conductio i between M and E, S and 0, and E. A break at 12, 
e.g., would mean that the writing of words to dictation has become im- 
possible, though printed or written words can still be copied from sight, pro- 
vided that the conduction 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- 






308 Physiological Function of Central Parts [3 ! 3~4 

gether unexplained. 1 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 inter- 
ruptions 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 unim- 
paired. But phenomena of this nature do not occur at all, in a pure form ; 
the symptoms usually ascribed to the disturbance in question are para- 
phasic, and appear to be of more complex origin. 2 The incongruity thus 
made apparent between the schema and the 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 asso- 
ciated with the acoustic word symbol, and the acoustic word symbol itself 
so frequently precedes articulation as a constituent of the total word com- 
plication, that the association may be much more probably referred to the 
indirect path CS 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 articu- 
lation 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 kind 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 qiiali- 
iaivve 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 

' '' " ; in , m : isn - , ith , mcrete sensible idc-.ct. The 
amnesic symptom that sets in most easily, and that is therefore the first to 

1 Cf. with this discussion the critical remarks in my Vdlkcrbsycholosie, i.. i, 1900, 
495 ff- 

a S. FREUD, Zur Auffassung der Aphasien, 1891. STORKING, Vorlcsuneen iiber 
Psychopathologie, 1900, 127 ff. 



;] PsycJiophy steal Analysis : Speech Centres 309 

appear in old age, is, accordingly, forgetfulness of proper names. Next in 
order come the ideas of concrete objects : chair, house, table, etc. _Some- 
what 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. 1 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 can be thought only by help of the cor- 
responding 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 ex- 
traordinary 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 discqyer_^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 attri- 
butes 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 
1 KUSSMAUL, Storungen der Sprache, 163 f< 



310 Physiological Function of Central Parts 

him. 1 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^ubcortical 
aphasia. In reality, however, instances of associative compensation are ex- 
Tfemely 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 pre- 
supposed 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 deter- 
minate 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 from 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 be termed, in their entirety, the 
psychological structure of verbal ideas. We may, further, limit our enquiry 
to the constitution oi consciousness thai characterises thr 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 ap- 
preciation 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 

1 GRASHEY'S case. Archiv f. Psychiatric, 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., i ff.). Cf. the detailed discussion in my 
Volker psychologic, i., i, 1900, 502 ff., and in SwKKitiG.Vorlesungen uber 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. /. 
Psychiatric, xxviii., i ff., and C. BASTIAN, Aphasia and Other Speech Defects, 1898. 



3 1 6-7 J Psychophysicat Analysis : Speech Centres 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 5, 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 compli- 
cated 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 con- 
stituents must vary very considerably with the constitution of the individual 
consciousness, with the stage of practice attained, and with the specific con- 
ditions 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, how- 
ever, so closely associated with the terms a m that these may always be per- 
ceived, 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 u 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 dicta- 
tion, 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 con- 
stituents L S. 1 

Finally, there are two further phenomena, connected with this variation 

1 Cf. the fuller psychological discussion of the processes of association in Part V., 
Ch. xix. below. 



312 Physiological Function of Central Parts [3 ! 7-8 

in the intimacy of associative combinations, that require a special mention. 
They bring out, in a striking way, the quite extraordinary variety of ten- 
dency 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 audi- 
tory 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 i 
particular idea is for the moment beyond reach, in cases i.e. where we are i 
" 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 con- 
nexion 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 move- 
ments of writing easily arouse the articulate^ sensations of the organs of 
speech, but these require special conditions if they are to touch off the move- 
ments ; 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 



318-319] 



Psycliophysical A nalysis . Speech Centres 



arc 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 om 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 con- 
struct 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. 1 This schema, it must be remembered, is not a 




1. 



FIG. 104. Schema of the associations of a complete verbal idea. 

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 associa- 
tions between them may be regarded as relatively stable connexions. There 

1 This schema is taken from, my Volkerpsychologie, i., i, 1900, 519 ff. (section on 
the psychical structure of word ideas). The reader is referred to this passage for further 
explanation_of the Figure. 



y 



u<" i . (i . _____ 

314 Physiological Function of Central Parts [3I9--2O 

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, tjie_jexpression ' speech centre ' 
cannot, under any circumstances, designate a central organ, in the sense in 
whlchlhis 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 localisa- 
tions, 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 thi speech centres holds, in reality, ol the ' ^cnsory centres ' 
Jis 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 



320-l] Psychophysical Analysis : Apperception Centre 315 

and movements, that work together in the functional whole, the con- 
nexions still admitting of continual adaptation to external conditions. 

(c) The Apperception Centre 

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 
(FigTSS, pr205ynFatEoIbgicarobservations 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. 1 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. __ 
\1 combines the animal passions of a man with the intellectual activities of a 

X^ * ~ *- . - . - " - - - ._... -. 

/ ,_child," so writes the attending physician. 2 In other cases, decay of memory, 

inability to concentrate the attention, entire loss of will-power, etc., are 
quoted as characteristic symptoms. 3 These results agree with the observa- 
tion that the_j3a.thological 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, 4 and with the general law that intellectual 
development keeps even pace throughout the animal kingdom with the 
development of the prosencephalon. 5 It is also said that highly de- 
veloped human brains are often characterised by an especially abundant 
formation of secondary gyres and fissures in the frontal lobes. 6 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. 7 

1 Cf. the cases collected by CHARCOT and PITRES (Revue mensuelle, Nov., 1877), 
FERRTER (Localisation of Cerebral Disease, 1878), and DE BOYER (Etudes cliniques, 
40, 54) ; also BIANCHI, in Brain, xviii., 1895, 497. 

2 The report is printed by FERRIER, op. cit. 

3 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. 

4 MEYNERT, Vierteljahrsschrift f. Psychictrie, 1867, 166. 

5 FLATAU and JACOBSOHN, Handbuch d. Anat. u. vergl. Anat. d. Centralnerven- 
systems d. Sdugcthiere, i., 536 ff. MARCHAND, Die Morphologie des Stirnlappens, in 
Arbeiten des paihol. Institute zu Marburg, ii., 1893. 

6 H. WAGNER, op. cit. 

7 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. 




316 Physiological Function of Central Parts [321-2 

On the ground of these facts, it has been suggested by various investi- 
gators, among others by MEYNERT, HITZIG, FERRIER and FLECHSIG, that 
the frontal brain stands in intimate relation to the functions of the ' intelli- 
gence.' 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 dis- 
turbances of an intrinsically elementary character, but manifesting them- 
selves^ 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 impor- 
tance for the interrelation of the central functional areas. And the objection 
that lesions of the frontal brain have occasionally been observed to pass with- 
out permanent moral or intellectual injury is not decisive : 1 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 reg'on without causing 
the most serious intellectual defects." * ' 

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 : 3 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 ordinarly term the state of ' attention.' 
Now the objective component of this complex process, the ' clarification ' 

1 ZIEHEN, LeitfaJen der physiol. Psychologic, ste Aufl., 195 : Introduction to Physio- 
logical Psychology, 1895, 231. 



a VON MONAKOW, Gehirnpatholoeie. 492. 
a Part IV.. Ch. xvii. ; Part V., Ch. xviii. 



X 



322-4] Psycho physical A na lysis : Apperception Centre 317 

of a contents, is surely suggestive in the highest degree of determinate phy- 
siological 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 physio- 
logical substrate. And it is evident that this substrate may very well con- 
sist of certain simple processes, consistent with the general principles of 
nerve mechanics ; whereas the _idea of ' intelligence ' is altogether so com- 
plicated 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 
PJiysiological change, when the sensation becomes more obscure, is a corre- 
sponding 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 intensificationof 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 excitations 
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 



Physiological Function of Central Parts 



[324-5 



as elsewhere, by certain excitations that are conducted to the centre ; 
but their liberation is at the same time influenced by that incalculable mani- 
fold 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. 



\y. thus n rd tpperception is the one elementar) process^indisgensaMe 
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 far 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 interpre- 
tation 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 excita- 
tions from the primary sensory centres ; 
and the other centrifugal, la, gf, etc., 
that carries, conversely, to subordinate 
centres the inhibitory impulses pro- 
ceeding from AC. We then have, ac- 
cording as these impulses are trans- 
mitted to sensory or motor centres, 
either the apperception of sensations or 
the execution of voluntary movements. 
In the former case, it is other sensa- 
tions, 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 as- 
sumed 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 




FIG. 105. Schema of the hypothetical 
connexions of the apperception centre. 
SC Visual centre. HC Auditory centre. 
S Central fibres of the opticus. H Cen- 
tral fibres of the acusticus. A, O Sen- 
sory, L, B motor intermediate centres. 
MC Direct motor centre. M Central 
motor fibres. AC Apperception centre. 
ss', hh' Centripetal paths to AC. la 
gf, etc. Centrifugal connexions of AC. 



325-6] Psychophysical Analysis: Apperception Centre 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 psycho- 
logical 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 con- 
nexions 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 con- 
duction 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 inter- 
mediaries 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 sensa- 
tions, 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 SC and HC, clearer or more obscure. The 
processes in O and A will thus have the significance of resultants, which corre- 
spond 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 L, in which an act of apperception 
releases (by the paths g f r s, y < p o-) a determinate motor excitation, corre- 
sponding to the sensory excitations brought up from SC and HC (by ss' ' , M'), 
or from O and A (by eh, e/c) ; or else an unmediated activity on the part of the 
two elements, phonetic utterance and script form, releases (by the paths e f, 
0) 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 / r s, <pcr) 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 
and all paths of connexion between subordinate centres are represented by 



320 Physiological Function of Central Paris [3 2 6~7 

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, (i) The impressions are not conducted 
farther. Then the sensations remain in the state of mere perception or in- 
distinct apprehension. (2) An individual impression a is apperceptively 
enhanced by inhibition of the impressions bed: 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 
eta) 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 
KC and Aa, the resulting word idea and the phonetic utterance are apperceivcd. 
(4) The processes described under (3) combine with (a) a conduction of the 
resultants from A by way of L to MC (along e< and <pcr) : involuntarily pro- 
nunciation of the word designating an apperceived idea ; (6) a conduction from 
AC by way of /. to MC (along y< and <f>pc-~) : 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 : involun- 
tary association of the word idea to the script form. (5) If the original impres- 
sion a is the script form of a word, we have the following possibilities : (a) direct 
apperception, again, by means of an inhibition la ; (6) 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, *e and Aa : 
apperception of a visual and of the corresponding auditory word idea (the ordin- 
ary process in reading) ; and so on. For the rest, this Figure, too, necessarily 
leaves out of account aU 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 con- 
sciousness, 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) Tiie 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 



327-8] Principles of Connexion of Elements 321 

contents and meaning, but each one again, in all probability, closely releted 
to the others. 

AnatomicaHjL_reg a J'4 e ^' 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 indi- 
cates 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 dements 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 bear 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. Itjneans, 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 psycho- 
logical 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 



322 General Principles of Central Functions [328-9 

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 con- 
nexions, 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 physio- 
logical signification, cannot but suggest the hypothesis that, wherever the 
physiological functions of the central elements have acquired a specific colour- 
ing, 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 sensa- 
tion, 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 them- 
selves nervous in character, as they are in the olfactory mucous membrane 
and perhaps in the retina of the eye, or whether they simply represent epi- 
thelial 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 



329-30] Principle of Original Indifference 323 

among the central elements. The first is this : that a fairly long con- 
tinuance 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. 1 The second phenomenon is this : that the functional dis- 
turbances occasioned by central lesions may be compensated without dis- 
appearance 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 func- 
tional 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 con- 
duction ; and even these are apparently confined to the more highly dif- 
1 See p. 53, above ; and cf. Part II., ch. viii., below. 



General Principles of Central Functions 

ferentiated nerve cells (pp. 42 f-)- Physiologically, the principle of in- 
difference 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. 

(c) The Principle of Practice and Adaptation 

' Practice,' in the ordinary acceptation of the term, consists in the per- 
fecting 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-oper- 
ating 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. 
\\V 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. 



331-2] Practice and Adaptation; Vicarious Function 



obverse of the practice processes is seen, on the other hand, in the 
decrease and ultimate abrogation of functions which, as the result of func- 
tional 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 enhance- 
ment of a given function. It may lead up to new combinations of ele- 
mentary 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 
J_adap_tation.' Adaptation, that is. _can never be anything else than a 
result of elementary practice processes. At the same time, it is a more com- 
plex process, since it consist?, by its very nature, in a number of concomi- 
tant 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 sig- 
nificance for the nervous functions are those in which newly practised 
elements take the place of others, whose activity has been suspended undei 
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 
physiolog'cal 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 dis- 
charge, 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, 



326 General Principles of Central Functions [33 2 ~4 

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 acquisition of novel 
functions. The first form is conditioned upon the original functional inter- 
dependence 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, sub- 
stitution 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 state- 
ment 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 func- 
tion, 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 dis- 
turbances in cases of injury to the cerebellum, or of lesion of the dience- 
phalic 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 mon 
particularly in decerebrised animals (pp. 259 ff.). For both forms of substi- 
tution, 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 



334~5] Principle of Vicarious Function 

higher plane and in more compl'cated form, the same relations ofsensory 
and motor conditions that characterise the mesencephalic portion of the 
visual centres (cf. Fig. 78, p. i85). 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 bsyond the reach of 
compensation by vicarious function. And, on the other hand, the dis- 
turbances 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 assump- 
tion 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 degenera- 
tion and decay (Fig. 22, p. 53). Suppose, e.g., that the ' speech centres ' 
of the right hemisphere were entirely functionless : it would then be im- 
possible 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 con- 
ductions. 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 



328 General Principles of Central Functions [335~6 

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 fromJtluVpoint 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 
Tn 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 con- 
tradistinction 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- 



Principle of Relative Localisation 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 adapta- 
tion, implies a dependence upon external and internal conditions which 
allow the elementary functions a certain freedom of exchange, the localisa- 
tion 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 localisa- 
tion 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 oi 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 ana- 
tomical 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 refuta- 
tion, point for point, under compulsion by the facts. 

(i) In opposition to the principle of the connexion of elements many physio- 
logists, 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 hypo- 
thesis 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. 1 Some- 

1 MEYNERT, Vierteljahrsschrift f. Psychiatrie, i., 1867, 80. H. MUNK, Ueber die 
Functionen der Grosshirnrinde, Einleitung, 9. 



330 General Principles of Central Functions [337-8 

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. Neverthe- 
less, 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 tne 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. 1 

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 lcgi:al consequence of the strict interpretation of the law 
<>l 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 tathe 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, 

1 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. 



338] Summary : the Older and the Newer Views 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 indiffer- 
ence, which by the great majority of the central elements has in all probability 
been retained as a permanent possession. The live 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 ex- 
plained, first, by this very interpendence 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. 



Index 



ABROGATION experiments, 191 

phenomena, motor and sensory. 
242, 259, 264 f., 271 f., 280 i., 
283 f. 

Absolute localisation, principle of, 330!. 
Accommodation, centre for, 190, 301 
^>Acid reaction of cinerea, 55 
Acoustic nidi, 183 

speech centre, 303 

Acusticus, origin and course of, 182 f?. 
Terminations of, in cochlea, 182 
Conductions of, i84f. 

connexion of cochlearis and ves- 

tibularis, 185 
Adaptation, principle of, 233 f. 

of visual centre to retinal im- 

pressions, 301 
After bra.in, 107 f. 
Aggregation, 62, 64 
Aggregate state, 63 
Agraphia, 208, 304 
Alba, 39, 55, in f. 
Albicans, 119, 125, 127, 132 
Alexia, 305 

-"Alkaline reaction of alba, 55 
Amblyopia, 211, 274 
Amnesia, forms of, 304 ff. 

total and partial, 304 
~ / Amnestic aphasia, 304 
Amoeba, 29, 33 
Amoeboid movements, 33 f. 

of dendrites, 43, 54 
Amorphous sustentacular substance, 39 
Amphioxus, nervous system of, 117,260 
Amygdala, 1 29 f . 
Analysis, causal, of mental phenomena, 6 f. 

Methods of functional, 241 f. 

Illustrations of, 298 ff. 
Anaesthesia, 160, 211, 274^ 
Anastomosis, 51 
Angular gyre, 146 
Animal and vegetable functions, 34 

impulses. See Impulse 

psychology, aim of, 5 

Relation of, to ethnic, 5 

Relation of, to generic 

comparative, 6 
Animals, experiments on, 191 
Antiphrenologtcal physiology, 293 
Ape cleft, 146 



> Aphasia, 208, 287, 296, 298, 304 ff. 
^Apperception, 316 

centre, 315 ff. 
Arbor, 121 f., 127, 141 
Arsenic, 52 

Ascending complication, principle of,226 f. 
Assimilation, 84 f. 
'Association centres, 217 f. 

Theory of, 290 ff. 

fibres, 213 ff. 

function of, 214, 292 
and projection fibres, 213 f. 
^ Astasia, 277 

Asthenia, 279 

Atactic aphasia, 303 

Ataxia, 278 

Atony, 277 

Atrophy, significance of, for investiga- 
tion of conduction paths, 

154 f- 

of a cerebral lobe, 176 

of an occipital lobe, 20^ 

- of pregeminum and geniculum,2O7 
Atropin, 77 
Attention, 316 
Auditory centre, 197, 202, 206 ff. 

Connection of, with speech, 3 10 

f-. 33 

Aula, 113 f., 126, 131, 133 
> Automatic movements, 151 

Centres for, 253 ff. 

respiration and heart beat, 

254 ff. 

imperative movements, 264, 270 
Autonomy of elements, principle of, 329 
Axis cylinder. See Neurite 



BACTERIA, manifestations of life in, 30 
Base of brain, 125, 132, 139, 143, 146 
Bell's law, is6f. 
Bigemina, 107, 109 f., 123 
Function of, 258 ff. 
Blindness. See Cortical, Mental, Word 

blindness 

-Blood cells, 30, 54 

>r /Bodily substrate of mental life, subject 
/ of Part T ., 12, 27 ff. 

Differentiation of, 33 ff. 

General problems in investi- 

gation of, 105 f. 



333 



334 



Index 



Body, 18 

Relation of. to mind, 20, 101 fl. 

to spirit, 20 
Brain, simplest form of, 36 

Development of, 106 ff., ii2ff. 
Development of outward form 
of, 137 . 
Law of growth of, 140, 147 ff. 

Law of fissuration of, 147 

Parts of, 106, 109, n8ff. 

Conduction paths of, 167 ff. 
Functions of, 244 ff. 

Three vesicles of, 106 f. 

Five Divisions of, 107 ff. 

Ventricles of, 109 ff. 

surface, law of growth of, r \o, 

145 ff- 

ventricles. See Ventricl ,-s 
vesicles, 106 f., 1 13 f. 

Broca's convolution, 301 

CALCAR. 129, 132, 146 

Callosal gyre, 114, 127, 135 f., 139, 142 f. 

Callosum, 113, 127, 130, 134 ff. 
Tapetum of, 12?, 135 
Rostrum and splenium of, 133 
Function of, 214 

Defect of, and atrophy of occip- 

ital lobe, 215 
Canalisation, 100 
Candicans. See Albicans 
Capacity, nervous, 7$ 
Capsular columns (fun. siliquae), 118 
(aud.it urn. iJot. 

Conduction in, 178 f. 

Function of, 270 f. 
Caudex, 107 

Causality, physical and psychical, 8 ff. 
Cellifugal conduction in neurite, 42 
Cellipetal conduction in dendrites, 42 
Cells, organic, 30, 33 

Differentiation of, 33 fl. See 

Nerve cell 
> Sensation, 289 

CentraPcoTITgation oT remote functional 
areas, principle of, 229 ff. 

fissure, 144 f. 

organs, morphological develop- 

ment of, 104 ff. 

Course of conduction paths 

in, 150 ff. 

Physiological function of, 

241 ff. 
Centres, ideational, 289 

Nervous, first appearance of, 36 f . 

Influence of, on processes of 

excitation, 85 ff., 243 

Nutritive function of, 96 

Cortical, 190 ff. 

Centromotor and centro- 

sensory, 191, 193 ff., 198 ff. 
204 ff. 



. 



Centres, Nervous. Munk's direct sensory 
and mental, 203 

Visual, 196 f, 200, 206 
Auditory, 198, 200, 208, 211 

Olfactory, 198, 209 

Gustatory, 198, 209 

for sensations of touch and 

movement, 198, 200, 209 f. 

relation of centres for touch, 

movement and gensral sense 
to motor centres, 198, 200, 
209, 21 1 
for sensation and idea, 289 

Functional development of, 

2 12 f. 

Functional interaction of, 293 
Psychical, 218, 290 
Sensation, 289 

Centrifugal sensory paths, 151, 159, 182, 

184, 1 86, 189 

Cerebellar lateral column, 166, 174 
peduncles, 120 ff. 

Conduction in, 172 ff. 

Nidi of, 176 

Function of, 273 

Cerebellum, development of, 106 ff, 109, 
110, 112, 121 

Position and parts of, 121 ff, 125. 

127, 128 

Conformation of, 140 f. 

Conductions of, 167 ff. 172 ff. 

Structure of, 175 

Function of, 271 ff. 

Relation of, to acusticus, 273, 2^6 

to sex functions, 280 

to intelligence, 279 f. 
Cerebrin, 54 

Cerebrum, 107, 126 ff, 138 ff. 

Conductions in ganglia of, 178 f. 
Cervical enlargement of myel, 117 
Chiasma, 187 ff, 230 ff. 
Child psychology, aim of, 5 

Relation of, to ethnic, 5 

Relation of, to individual, 
5 ft, 

Cholesterin, 55 

Cholin, 55 

Chorda tympani, i8of. 

Cilia, as motor organs, 34 

as tactual organs, 34 
Ciliata, 29 

Cinerea, 39 

Chemical constitution of, 55 

Three forms of, 112 

Nidi of, 112 1 18 f, 122, 126, 127 

Conduction in, 15 if, 160 ff. 

Irritability of, 160 f. 

Resistance in, 161 

Transference of excitation in, 

161 ff. 

Clarke's columns, 116 
Classification of mental phenomena. 12 ff. 



Index 



335 



Claustrum, I2pf. 

Coelenterates, 35, 37 

Collaterals, 41 

Commissural system of prosencephalon 



Commissures, dorsal, of myel, 1 16 

White and grey ventral, of myel, 1 1 6 
Medicommissure, 123, 125 
Postcommissiire, 124, 127 
Precommissure, 114, 126, 132, 214 

See Callosum 

Comparative psychology, also called x 
generic, 6 

Relation of, to animal and 
ethnic, 6' 

Complex mental formations, 13, 15 f. 
Conarium. See Epiphysis 
Concept, conceptual feeling, 311. 
Conduction, anatomical conditions of, 36, 

48 ff. 

from motor nerve to muscle, 67 ff. 
Reflex, 85 ff. 151 
Transverse and longitudinal, 87 

Centripetal and centrifugal, 151 
Principal and auxiliary, 1 50 ff . 
Law of isolated, 151 

investigated by physiological 

method, 152 f. 

by anatomical, 153 f. 

by pathological, 154 

Waller's law of, 155 

in nerves and myel, 155 ff. 
in cinerea, 160 f. 

in oblongata, 167 ff. 

Branch, to cerebellum, 167, 169 ff. 
Direct, to cerebrum, 168 f, 173 
in pons and cerebellum, 172 ff. 

to and from the quadrigemina and 

thalami, 178 f. 

to striata, 179 

in nerves of taste, i8of. 

of smell, 181 f. 

in acusticus, 182 ff. 

in opticus, 185 ff. 
Terminations of, in cortex, 190 ff. 

of projection and association 

systems, 213 ff. 

Principles of central, 225 ff. See 

Intercentral paths 

Central, principles of, 225 ff. 

Manifold representation, 225 f. 

Ascending complication, 225 f. 

Differentiation of directions, 

227 ff. 
Colligation of remote func- 

tional areas, 229 ff. 
Connective tissue, sustentacular, 39 
Connexion of elements, principle of, 320 ff. 
/ Conscious contents as processes, 4 
Consciousness, criteria and limits of, 27 ff. 

Primitive form of, 34 

Dissociation of, 54 



Conservation of energy, 60 ff. 

Application of principle of, to 

mechanics of nerve, 65 ff. 
Constrictions of primitive sheath, 45 
Contact of dendrites and collaterals, 46 
49, 51. See Neurone theory 
Contractile vesicle, 34 
^^Contractility, earliest evidence of mental 

function, 29 ff. 
an intrinsic character, 59 
Contraction, muscular, as measure of 
nervous excitation, 58 f. 

after stimulation of motor nerve 

67 

Isotonic, isometric and auxo- 

tonic curves, 68, 72 

increased by practice, 75 
and phenomena of fatigue, 77 
Superposition of, in tetanus, 76 
thermal and electrical conse- 
quences of, 80 

in reflex excitation, 85 ff. 

in enhanced reflex excitability, 

88 ff . " 

in investigation of conduction 

paths, 153 

Convolution, of cerebrum, 138 ff, 145 ff. 
of cerebellum, 140 f, 147 
Two conditions of, 147 ff. 

Corneal sheath, 45 

Cornua of myel, 115 f. 
of paracele, 131 

Corona, 130, 168 

Terminations of, in neighbourhood 

of central gyres, 168, 194, 212 
(cf. 198, 200, 205, 210) 

Relation of, to association system, 

214, 217 
Cortex, cerebellar, 140 f, 175 

Cerebral, outward form of, 138 ff. 
Conduction paths to, 190 ff. 

Centromotor and centro- 

sensory areas of, 191 ff. 

Intercentral paths to. 191 

Association system of, 213 ff. 

Projection system of, 213 ff. 
Structure of, 218 ff. 

Relation of structure and 

function in, 220 ff. 

Reflexes of, 252 

Automatic excitations of, 

256 ff. 

Function of, 256 ff, 280 ff. 

Substitutions in, 28.| 

as seat of intelligence, 284 
Cortical blindness, 201 

centres, methods for demonstration 

of, 190 ff. 

Motor and sensory, in dog, 

193 ff. 

Motor and sensory, in monkey, 

198 ff. 



336 



index 



Cortical centres, motor and sensory 
man, 204 ff. 287 ff. 

Visual, 298 ff. 

Speech, 302 ff. 

Apperception, 315 ff. 

deafness, 201 
epilepsy, 194 

stimulation, latent period of, 194 
Coughing, reflex centres for, 245 ff. 
Criteria of mind, 27 f. 

Crura, 123. 125, 128 

Nidi of, 123 

Decussations in, 170, 174 f. 

Motor and mixed paths of, 177 
Functions of, 177 

Crusta, 119, 123 

Conduction in, 176!. 
Cuneate funicles, 118 f. 

Conduction in, 166, 170 
Cuneus, 143 f. 146 

Curare, 47 

Current of action, 80 

Duration of, with momentary 

stimulation of nerve, 83 
Custom, source of psychological know- 
ledge, 5 
Cylindrical columns (cminentiae leretes), 120 

DECUSSATIONS, in myel, i59ff. 

in oblongata, 168 f, 170, 172 

in pons and cerebellum, 173, 174 ff. 

of olfactorius, 182 

of ncusticus, 184 

of opticus, 187 ff. 

of motor paths to cortex, 195, 205 

of sensory paths to cortex, 197 

in the transverse commissural 

system, 214 
Theory of, 230 ff. 
Deafness. See Cortical, Mental, Word 

deafness 

Degeneration of nerves, 52 f, 54 f, 168, 325 
Deiters' cells, 44 
Dendrites, 41 
Dentatum, 118 

Conduction in, 173 
Depressor nerves, 247 

Derangement of central areas, 159, 160 f, 
187, 191, 196, 201, 206 ff, 253 
257, 264, 270 f, 273 

Diacele, 109. See Ventricles 

Diencephalon, structure of, 107, 124 ff. 
Function of, in lower vertebrates, 
258 ff. 

in man, 269 ff. 
Differentiation, of mental functions and 

their substrate, 33 ff. 
of animal and vegetable functions, 

34 
of the three germinal layers, 34 f. 

of nerve cells, 36 f. 

Of directions, principle of, 227 ff. 



in Dimensions of mental processes, 6 
/ Disgregation, work of, 60 
S Dissimilation, 84 f. 
> Dissociation, work of, 60 f, 65 

of consciousness, 54 
Dizziness, 273 ff. 
Dorsal columns of myel, 116 

of oblongata, 118 

Conduction in, 166, 170 

Continuations of, in cero- 

bellum and pons, 173 f. 

Cornua of my