2. PHYSIOLOGY

The nervous system has as its function the co-ordinating of the

activities of the organs one with another. It puts the organs into such

mutual relation that the animal reacts as a whole with speed, accuracy

and self-advantage, in response to the environmental agencies which

stimulate it. For this office of the nervous system there are two

fundamental conditions. The system must be thrown into action by

agencies at work in the environment. Light, gravity, mechanical impacts,

and so on, which are conditions significant for animal existence, must

find the system responsive and through it evoke appropriate activity in

the animal organs. And in fact there have been evolved in the animal a

number of structures called receptive organs which are selectively

excitable by different environmental agencies. Connected with these

receptive organs lies that division of the nervous system which is

termed _afferent_ because it conducts impulses inwards towards the

nervous centres. This division consists of elongated nerve-cells, in man

some two million in number for each half of the body. These are living

threads of microscopic tenuity, each extending from a receptive organ to

a central nervous mass. These central nervous masses are in vertebrates

all fused into one, of which the part which lies in the head is

especially large and complex, because directly connected with

particularly important and delicate receptive organs. The part of the

central nervous organ which lies in the head has, in consequence of its

connexion with the most important receptive organs, evolved a dominant

importance in the nervous system, and this is especially true of the

higher animal forms. This head part of the central nervous organ is

sufficiently different from the rest, even to anatomical examination, to

have received a separate name, the _brain_. But the fact of its having

received a separate name ought not to obscure the singleness and

solidarity of the whole central nervous organ as one entity. The

functions of the whole central nervous organ from region to region are

essentially similar throughout. One of its essential functions is

reception, via afferent nerves, of nervous impulses generated in the

receptive organs by environmental agents as stimuli. In other words,

whatever the nature of the agent, its result on the receptive organs

enters the central nervous organ as a nervous impulse, and all segments

of the central nervous organ receive impulses so generated. Further, it

is not known that nervous impulses present qualitative differences among

themselves. It is with these impulses that the central nervous organ

whether spinal cord or brain has to deal.

_Material and Psychical Signs of Cerebral Activity._--In the central

nervous organ the action resulting from entrant impulses has issue in

three kinds of ways. The reaction may die out, be suppressed, and so far

as discoverable lead to nothing; or the impulses may evoke effect in

either or both of two forms. Just as from the receptive organs, nerves

lead into the central nervous organ, so conversely from the central

organ other nerves, termed _efferent_, lead to various organs of the

body, especially glands and muscles. The reaction of the central nervous

organ to impulses poured into it commonly leads to a discharge of

impulses from it into glands and muscles. These centrifugal impulses

are, so far as is known, qualitatively like the centripetal impulses. On

reaching the glands and muscles they influence the activity of those

organs. Since those organs are therefore the mechanisms in which the

ultimate effect of the nervous reaction takes place, they are often

termed from this point of view _effector organs_. A change ensuing in

effector organs is often the only sign an observer has that a nervous

reaction has occurred, unless the nervous system under observation be

the observer's own.

If the observer turns to his own nervous system for evidence of

reaction, he meets at once in numberless instances with _sensation_ as

an outcome or sign of its reaction. This effect he cannot show to any

being beside himself. He can only describe it, and in describing it he

cannot strictly translate it into any term of material existence. The

unbridged gulf between sensation and the changes produced in effector

organs necessitates a separate handling of the functions of the nervous

system according as their office under consideration is sensation or

material effect. This holds especially in the case of the brain, and for

the following reasons.

_Psychosis and the Fore-Brain._--Hippocrates wrote, "It is through the

brain that we become mad, that delirium seizes us, that fears and

terrors assail us." "We know that pleasure and joy on the one hand and

pain and grief on the other are referable to the brain. It is in virtue

of it that we think, understand, see, hear, know ugliness and beauty,

evil and good, the agreeable and the disagreeable." Similarly and more

precisely Descartes indicated the brain, and the brain alone, as the

seat of consciousness. Finally, it was Flourens who perhaps first

definitely insisted on the restriction of the seat of consciousness in

higher animals to that part of the brain which is the fore-brain. A

functional distinction between the fore-brain and the remainder of the

nervous system seems, in fact, that consciousness and physical reactions

are adjunct to the fore-brain in a way in which they are not to the rest

of the system. After transection of the spinal cord, or of the brain

behind the fore-brain, psychical phenomena do not belong to the

reactions of the nervous arcs posterior to the transection, whereas they

do still accompany reactions of the nervous arcs in front and still

connected with the fore-brain. A man after severance of the spinal cord

does not possess in the strict sense consciousness of the limbs whose

afferent nerves lie behind the place of spinal severance. He can see

them with his eyes, and if the severance lie between the arms and the

legs, can feel the latter with his hands. He knows them to be a part of

his body. But they are detached from his consciousness. Sensations

derived from them through all other channels of sense than their own do

not suffice to restore them in any adequate measure to his

consciousness. He must have the sensations so called "resident" in them,

that is, referred to them, without need of any logical inference. These

can be yielded only by the receptive organs resident in the part itself,

its skin, its joints, its muscles, &c., and can only be yielded by those

receptive organs so long as the nerve impulses from them have access to

the fore-brain. Consciousness, therefore, does not seem to attach to any

portion of the nervous system of higher animals from which the

fore-brain has been cut off. In the dog it has been found that no sign

of memory, let alone intelligence, has been forthcoming after removal of

the greater part of the fore-brain.

In lower vertebrates it is not clear that consciousness in primitive

form requires always the co-operation of the fore-brain. In them the

fore-brain does not seem a _conditio sine qua non_ for psychosis--so far

as we may trust the rather hazardous inferences which study of the

behaviour of fish, &c., allows. And the difference between higher and

lowlier animal forms in respect of the fore-brain as a condition for

psychosis becomes more marked when the Arthropoda are examined. The

behaviour of some Insecta points strongly to their possessing memory,

rudimentary in kind though it may be. But in them no homologue of the

fore-brain of vertebrates can be indisputably made out. The head ganglia

in these Invertebrates may, it is true, be analogous in function in

certain ways to the brain of vertebrates. Some experiments, not

plentiful, indicate that destruction of these head ganglia induces

deterioration of behaviour such as follows loss of psychical functions

in cases of destruction of the fore-brain in vertebrates. Though,

therefore, we cannot be clear that the head ganglia of these

Invertebrates are the same structure morphologically as the brain of

vertebrates, they seem to hold a similar office, exercising analogous

functions, including psychosis of a rudimentary kind. We can, therefore,

speak of the head ganglia of Arthropods as a brain, and in doing so must

remember that we define by physiological evidence rather than by

morphological.

_Cerebral Control over Lower Nervous Centres._--There accrues to the

brain, especially to the fore-brain of higher Vertebrates, another

function besides that of grafting psychical qualities upon the reactions

of the nervous system. This function is exhibited as power to control in

greater or less measure the pure reflexes enacted by the system. These

pure reflexes have the character of fatality, in the sense that, given a

particular stimulus, a particular reaction unvaryingly follows; the same

group of muscles or the same gland is invariably thrown into action in

the same way. Removal of the fore-brain, i.e. of that portion of the

central nervous organ to which psychosis is adjunct, renders the nervous

reactions of the animal more predictable and less variable. The animal,

for instance, a dog, is given over more completely to simple reflexes.

Its skin is touched and it scratches the spot, its jaw is stroked and it

yawns, its rump is rubbed and it shakes itself, like a dog coming out of

water; and these reactions occur fatally and inopportunely, for

instance, when food is being offered to it, when the dog normally would

allow no such insignificant skin stimuli as the above to defer his

appropriate reaction. Goltz relates the behaviour of a dog from which

almost the whole fore-brain had been removed. The animal lived healthily

under the careful treatment accorded it. At feeding time a little

quinine (bitter) added to its sop of meat and milk led to the morsels,

after being taken into the mouth, being at once and regularly rejected.

None was ever swallowed, nor was the slightest hesitation in their

rejection ever obtained by any coaxing or command, or encouragement of

the animal by the attendant who constantly had charge of it. On the

other hand, directly an undoctored piece had entered the mouth it was

swallowed at once. Goltz threw to his own house-dog a piece of the same

doctored meat. The creature wagged its tail and took it eagerly, then

after receiving it into its mouth pulled a wry face and hesitated,

astonished. But on encouragement to go on eating it the dog did so.

Perhaps it deemed it unseemly to appear ungrateful to the giver and

reject the gift. It overcame its reflex of rejection, and by its

self-control gave proof of the intact cerebrum it possessed.

There seems a connexion between consciousness and the power to modify

reflex action to meet the exigencies of the occasion. Pure reflexes are

admirably adapted to certain ends. They are reactions which have long

proved advantageous to the phylum of which the existent animal is the

representative embodiment. But the reflexes have a machine-like

fatality, and conscious aim does not forerun their execution. The

subject as active agent does not direct them. Yet they lie under the

control of higher centres. The cough, the eye-closure, the impulse to

smile, all these can be suppressed. The innate respiratory rhythm can be

modified to meet the requirements of vocal utterance. In other words,

the reaction of reflex arcs is controllable by the mechanism to whose

activity consciousness is adjunct. The reflexes controlled are often

reactions but slightly affecting consciousness, but consciousness is

very distinctly operative with the centres which exert the control. It

may be that the primary aim, object and purpose of consciousness is

control. "Consciousness in a mere automaton," writes Professor Lloyd

Morgan, "is a useless and unnecessary epiphenomenon." As to _how_ this

conscious control is operative on reflexes, how it intrudes its

influence on the running of the reflex machinery, little is known.

_The Cerebrum an Organ giving Adaptation and Readjustment of Motor

Acts._--The exercise of this control and the acquirement of skilled

actions have obviously elements in common. By skilled actions, we

understand actions not innately given, actions acquired by training in

individual experience. The controlling centres pick out from an

ancestral motor action some part, and isolate and enhance that until it

becomes a skilled act. The motor co-ordination ancestrally provided for

the ring finger gives an extending of it only in company with extension

of the fingers on either side of it. The isolated lifting of the ring

finger can, however, soon be acquired by training. In such cases the

higher centre with conscious effort is able to dissociate a part from an

ancestral co-ordination, and in that way to add a skilled adapted act to

the powers of the individual.

The nervous organs of control form, therefore, a special instrument of

adaptation and of readjustment of reaction, for better accommodation to

requirements which may be new. The attainment of more precision and

speed in the use of a tool, or the handling of a weapon, means a process

in which nervous organs of control modify activities of reflex centres

themselves already perfected ancestrally for other though kindred

actions. This process of learning is accompanied by conscious effort.

The effort consists not so much in any course of reasoning but rather in

the acquiring of new sensorimotor experience. To learn swimming or

skating by simple cogitation or mere visual observation is of course

impossible. The new ideas requisite cannot be constructed without motor

experience, and the training must include that motor experience. Hence

the training for a new skilled motor manoeuvre must be simply _ad hoc_,

and is of itself no training for another motor co-ordination.

The more complex an organism the more points of contact does it have

with its environment, and the more does it need readjustment amid an

environment of shifting relationships. Hence the organs of consciousness

and control, being organs of adaptation and readjustment of reaction,

will be more pronounced the farther the animal scale is followed upward

to its crowning species, man. The cerebrum and especially the cerebral

cortex may be regarded as the highest expression of the nervous organ

of individual adaptation of reactions. Its high development in man makes

him the most successful animal on earth's surface at the present epoch.

The most important part of all this adjustment in his case, as he stands

now, consists doubtless in that nervous activity which is intellectual.

The mentality attached to his cerebrum includes reason in higher measure

than is possessed by the mentality of other animals. He, therefore, more

than they, can profitably forecast the future and act suitably to meet

it from memory of the past. The cerebrum has proved itself by his case

the most potent weapon existent for extending animal dominance over the

environment.

_Means and Present Aims of Physiological Study of the Brain._--The

aspects of cerebral activity are therefore twofold. There is the

contribution which it makes to the behaviour of the animal as seen in

the creature's doings. On the other hand there is its product in the

psychical life of the animal. The former of these is subject matter for

physiology; the latter is especially the province of psychology.

Physiology does, however, concern itself with the psychical aspect of

cerebral functions. Its scope, embracing the study of the bodily organs

in regard to function, includes the psychic as well as the material,

because as just shown the former inextricably interlace with the latter.

But the relation between the psychic phenomena and the working of the

brain in regard to any data of fundamental or intimate character

connecting the two remains practically as unknown to us as to the Greek

philosophers. What physiology has at present to be content with in this

respect is the mere assigning of certain kinds of psychic events to

certain local regions of the cerebrum. This primitive quest constitutes

the greater part of the "neurology" of our day, and some advance has

been made along its lines. Yet how meagre are really significant facts

will be clear from the brief survey that follows. Before passing finally

from these general considerations, we may note that it becomes more and

more clear that the brain, although an organ than can be treated as a

whole, is complex in the sense that separable functions belong in some

measure to its several parts.

The means principally adopted in studying the functions of the

brain--and it must be remembered that this study in its present phase is

almost exclusively a mere search for localization--are four. These are

the physiological, the clinico-pathological, the histological and the

zoological. The first named proceeds by observing the effects of

artificial excitation, chiefly electric, of various parts of the brain,

and the defects produced by destruction or removal of circumscribed

portions. The clinico-pathological proceeds by observing the

disturbances of body and mind occurring in disease or injury, and

ascertaining the extent of the disease or injury, for the most part

_post mortem_. The histological method examines the microscopic

structure of the various regions of the brain and the characters and

arrangement of the nerve-cells composing it. The zoological follows and

compares the general features of the brain, as represented in the

various types of animal creation.

It is on the functions of the fore-brain that interest now mainly

focuses, for the reasons mentioned above. And the interest in the

fore-brain itself chiefly attaches to the functions of its cortex. This

is due to several causes. In man and the animals nearest him the cortex

forms by far the larger part of the whole cerebral hemisphere. More than

any other part it constitutes the distinctively human feature. It lies

accessible to various experimental observations, as also to traumatic

lesions and to the surgeon's art. It is composed of a great unbroken

sheet of grey matter; for that reason it is a structure wherein

processes of peculiar interest for the investigation in view are likely

to occur. To make this last inference more clear a reference to the

histology of nervous tissue must be made. The whole physiological

function of the nervous system may be summed up in the one word

"conduction." This "conduction" may be defined as the transmission of

states of excitement (nerve-impulses) along the neural arcs composing

the system. The whole nervous system is built up of chains of

nerve-cells (neurones) which are nervous conductors, the chains often

being termed arcs. Each neurone is an elongated cell which transmits

nerve-impulses from its one end to its other, without so far as is known

modifying the impulses in transit, unless in that part of the nerve-cell

where the nucleus lies. That part of the neurone or nerve-cell is called

the perikaryon or cell-body, and from that part usually many branches of

the cell (each branch being a nerve-fibre) ramify. There is no evidence

that impulses are modified in transit along a branch of a nerve-cell,

but there is clear evidence of manifold modification of nerve-impulses

in transit along the nerve-arcs of the nervous system. These nerve-arcs

are neurone-chains. In them one neurone continues the line of conduction

where the immediately foregoing neurone left it. That is, the neurones

are laid in conductive series, the far end of one apposed to the near

end of its precursor. The place of juxtaposition of the end of one

neurone against the beginning of another is called the _synapse_. At it

the conduction which has so far been wholly intra-neuronic is replaced

by an inter-neuronic process, in which the nerve impulse passes from one

neurone to the next. The process there, it is natural to think, must be

physiologically different from that conductive process that serves for

transmission merely within the neurone itself. It may be that to this

inter-neuronic conduction are due the differences between conduction in

nerve-_arcs_ and nerve-_trunks_ (nerve-fibres) respectively. Significant

of the former are changes in rhythm, intensity, excitability and

modifications by summation and inhibition; in fact a number of the main

features of nervous reaction. These characters impressed upon conduction

in nerve arcs (neurone-chains) would therefore be traceable to the

intercalation of perikarya and synapses, for both these structures are

absent from nerve-trunks. It is therefore probably to perikarya and

synapses that the greater part of the co-ordination, elaboration and

differentiation of nervous reactions is due. Now, perikarya and synapses

are not present in the _white_ matter of the central nervous organ, any

more than they are in nerve-trunks. They are confined exclusively to

those portions of the central organ which consist of _grey_ matter (so

called from its naked-eye appearance). Hence it is to the great sheet of

grey matter which enfolds the cerebrum that the physiologist turns, as

to a field where he would expect to find evidences of the processes of

cerebral co-ordination at work. It is therefore to items regarding the

functions of the great sheet of cerebral cortex that we may now pass.

_The Cerebral Cortex and its Functions._--The main question which vexed

the study of the physiology of the cerebral hemispheres in the 19th

century was whether differences of function are detectible in the

different regions of the hemisphere and especially in those of its

cortex. One camp of experimenters and observers held that the cortex was

identical in function throughout its extent. These authorities taught

that the various faculties and senses suffer damage in proportion to the

amount of cortex removed or injured, and that it is a matter of

indifference what may be the particular region wherein the destruction

takes place. Against this an opposed set of observers held that

different regions perform different functions, and this latter

"differential" view was raised in two wholly dissimilar forms in the

first and last quarters of the 19th century respectively. In the first

quarter of the century, a school, with which the name of Gall is

prominently associated, held that each faculty of a set of particular

so-called "faculties," which it assumed constituted intelligence, has in

the brain a spatially separate organ proper to itself. Gall's doctrine

had two fundamental propositions. The first was that intelligence

resides exclusively in the brain: the second, that intelligence consists

of twenty-seven "faculties," each with a separate local seat in the

brain. The first proposition was not new. It is met with in Hippocrates,

and it had been elaborated by Descartes and others. But Bichat in his

_Anatomie generale_ had partly wandered from the gradually established

truth and referred the emotions to the visceral organs, returning to a

naive view popularly prevalent. Gall's first proposition was probably

raised especially in reaction against Bichat. But Gall's proposition was

retrograde from the true position of the science of his time. Flourens

and others of his contemporaries had already shown not only that

intelligence was resident exclusively in the brain, but that it was

resident exclusively in that part of the brain which is the fore-brain.

Now Gall placed certain of his twenty-seven intellectual faculties in

the cerebellum, which is part of the hind-brain.

_Phrenology._--As to Gall's second proposition, the set of faculties

into which he analysed intelligence shows his power of psychological

analysis to have been so weak that it is matter of surprise his doctrine

could obtain even the ephemeral vogue it actually did. Among his

twenty-seven faculties are, for instance, "_l'amour de la progeniture,

l'instinct carnassier, l'amitie, la ruse, la sagacite comparative,

l'esprit metaphysique, le talent poetique, la mimique_," &c. Such

crudity of speculation is remarkable in one who had undoubtedly

considerable insight into human character. Each of the twenty-seven

faculties had its seat in a part of the brain, and that part of the

brain was called its "organ." The mere spatial juxtaposition or

remoteness of these organs one from another in the brain had, according

to Gall, an influence on the constitution of the mind. "_Comme l'organe

des arts est place loin de l'organe du sens des couleurs, cette

circonstance explique pourquoi les peintres d'histoire ont ete rarement

coloristes_." All these "faculty-organs" were placed by Gall at the

surface of the brain. "This explains the correspondence which exists

between craniology and the doctrine of the functions of the brain

(cerebral physiology), the single aim of my researches." Gall wrote that

he found the bump of pride (_la bosse de l'orgueil_) as far down in the

animal series as the goat. Broussais traced the "organ" of veneration as

far down as the sheep. Gall found the bump of murder (_bosse du

meurtre_) in the carnivora. Later it was traced also in herbivora.

Broussais added apologetically that "the herbivora cause a real

destruction of plants."

Gall's doctrine enjoyed enormous vogue. He himself had the gifts and the

demerits of quackery. His doctrine possessed, apart from its falsity,

certain other mischievous qualities. "_Que ces hommes si glorieux, qui

font egorger les nations par millions, sachent qu'ils n'agissent point

de leur propre chef, que c'est la nature qui a place dans leur coeur la

rage de la destruction_." One of his scientific opponents rejoined,

"Nay, it is not that which they should know. What they should know is

that if providence has allowed to man the possibility of doing evil, it

has also endowed him with the power to do good." The main cause of the

success of phrenology (q.v.) has been no doubt the common desire of men

to read the characters and hidden thoughts of others by external signs.

Each bump or "bosse" on the cranium was supposed to indicate the

existence and degree of development of one or other of the twenty-seven

"faculties." One such "bosse" showed the development of the organ of

"goodness," and another the development of the organ of "murder." Such

an easy means to arrive at information so curious delighted many

persons, and they were not willingly undeceived.

_Modern Localization Doctrines._--The crude localization of the

phrenologists is therefore too clumsy to possess an interest it might

otherwise have had as an early expression of belief in cerebral

localization, a belief which other labours have subsequently justified,

although on facts and lines quite different from these imagined by Gall

and his followers. Patient scientific toil by the hands of E. Hitzig and

D. Ferrier and their followers has slowly succeeded in obtaining certain

facts about the _cortex cerebri_ which not only show that different

regions of it are concerned with different functions, but, for some

regions at least, outline to some extent the kind of function exercised.

It is true that the greater part of the cortex remains still _terra

incognita_ unless we are content with mere descriptive features

concerning its coarse anatomy. For several scattered regions some

knowledge of their function has been gained by physiological

investigation. These scattered regions are the _visual_, the _auditory_,

the _olfactory_ and the _precentral_.

The grey matter of the cerebral cortex is broadly characterized

histologically by the perikarya (nerve-cells bodies) which lie in it

possessing a special shape; they are pyramidal. The dendrite fibres of

these cells--that is, their fibres which conduct _towards_ the

perikarya--are branches from the apex and corners of the pyramid. From

the base often near its middle arises one large fibre--the axone fibre,

which conducts impulses away from the perikaryon. The general appearance

and arrangement of the neurones in a particle of cortical grey matter

are shown in fig. 15, above. The apices of the pyramidal perikarya are

turned towards the free surface of the cortex. The figure as interpreted

in terms of functional conduction means that the cortex is beset with

conductors, each of which collects nerve-impulses, from a minute but

relatively wide field by its branched dendrites, and that these

nerve-impulses converge through its perikaryon, issue by its axone, and

are carried whithersoever the axone runs. In some few cells the axone

breaks up into branches in the immediate neighbourhood of its own

perikaryon in the cortex. In most cases, however, the axone runs off

into the subjacent white matter, leaving the cortex altogether. On

reaching the subjacent white matter it mingles with other fibres and

takes one of the following courses:--(1) to the grey matter of the

cortex of the same hemisphere, (2) to the grey matter of the cortex of

the opposite hemisphere, (3) to the grey matter of the pons, (4) to the

grey matter of the bulb or spinal cord. It is noteworthy that the

dendrite fibres of these cortical neurones do not transgress the limits

of the grey cortex and the immediate neighbourhood of the perikaryon to

which they belong; whereas the discharging or axone fibre does in the

vast majority of cases transgress the limits of the grey matter wherein

its perikaryon lies. The cortical neurone therefore collects impulses in

the region of cortex just about its perikaryon and discharges them to

other regions, some not cortical or even cerebral, but spinal, &c. One

question which naturally arises is, do these cells spontaneously

generate their impulses or are they stirred to activity by impulses

which reach them from without? The tendency of physiology is to regard

the actions of the cortex as reactions to impulses communicated to the

cortical cells by nerve-channels reaching them from the sense organs.

The neurone conductors in the cortex are in so far considered to

resemble those of reflex centres, though their reactions are more

variable and complex than in the use of the spinal. The chains of

neurones passing through the cortex are more complex and connected with

greater numbers of associate complex chains than are those of the spinal

centres. But just as the reflex centres of the cord are each attached to

afferent channels arriving from this or that receptive-organ, for

instance, tactile-organs of the skin, or spindles of muscle-sense, &c.,

so the regions of cortex whose function is to-day with some certainty

localized seem to be severally related each to some particular

sense-organ. The localization, so far as ascertained, is a localization

which attaches separate areas of cortex to the several species of sense,

namely the visual, the auditory, the olfactory, and so on. This being

so, we should expect to find the sensual representation in the cortex

especially marked for the organs of the great distance-receptors, the

organs which--considered as _sense_ organs--initiate sensations having

the quality of projicience into the sensible environment. The organs of

distance-receptors are the olfactory, the visual and the auditory. The

environmental agent which acts as stimulus in the case of the first

named is chemical, in the second is radiant, and in the last is

mechanical.

_Olfactory Region of Cortex._--There is phylogenetic evidence that the

development of the _cortex cerebri_ first occurred in connexion with the

distance-receptors for chemical stimuli--that is, expressed with

reference to psychosis, in connexion with olfaction. The olfactory

apparatus even in mammals still exhibits a neural architecture of

primitive pattern. The cell which conducts impulses to the brain from

the olfactory membrane in the nose resembles cells in the skin of the

earthworm, in that its cell-body lies actually amid the epithelium of

the skin-surface and is not deeply buried near or in the central nervous

organ. Further, it has at its external end tiny hairlets such as occur

in specially receptive-cells but not usually in purely nervous cells.

Hence we must think that one and the same cell by its external end

receives the environmental stimulus and by its deep end excites the

central nervous organ. The cell under the stimulation of the

environmental agent will therefore generate in itself a nervous impulse.

This is the clearest instance we have of a neurone being actually

excited under natural circumstances by an agent of the environment

_directly_, not indirectly. The deep ends of these olfactory neurones

having entered the central nervous organ come into contact with the

dendrites of large neurones, called, from their shape, mitral. In the

dog, an animal with high olfactory sense, the axone of each olfactory

neurone is connected with five or six mitral cells. In man each

olfactory neurone is connected with a single mitral cell only. We may

suppose that the former arrangement conduces to intensification of the

central reaction by summation. At the same time it is an arrangement

which could tend to smother sharp differentiation of the central

reaction in respect to locality of stimulus at the receptive surface.

Considering the diffuse way in which olfactory stimuli are applied in

comparison, for instance, with visual, the exact localization of the

former can obviously yield little information of use for locating the

exact position of their source. On the other hand, in the case of visual

stimuli the locus of incidence, owing to the rectilinear propagation of

light, can serve with extraordinary exactitude for inferences as to the

position of their source. The adaptation of the neural connexions of the

two organs in this respect is therefore in accord with expectation.

The earliest cerebral cortex is formed in connexion with the

neurone-chains coming into the central nervous organ from the patch of

olfactory cells on the surface of the head. The region of cerebrum thus

developed is the so-called olfactory lobe and hippocampal formation. The

greater part of the cerebral hemisphere is often termed the _pallium_,

because as its development extends it folds cloak-wise over the older

structures at the base of the brain. The olfactory lobe, from its

position, is sometimes called the _pallium basale_, and the hippocampal

formation the _pallium marginale_; and these two parts of the pallium

form what, on account of their phylogenetic history, Elliott Smith well

terms the _archipallium_. A fissure, the limbic fissure, marks off more

or less distinctly this archipallium from the rest of the pallium, a

remainder which is of later development and therefore designated by

Elliott Smith the _neopallium_. Of the archipallium, the portion which

constitutes the olfactory lobe is well formed in the selachian fish. In

the reptilian cerebrum the hippocampal region, the pallium marginale,

coexists in addition. These are both of them olfactory in function. Even

so high up in the animal scale as the lowest mammals they still form one

half of the entire pallium. But in the higher apes and in man the

olfactory portion of the pallium is but a small fraction of the pallium

as a whole. It is indeed so relatively dwarfed and obscured as to be

invisible when the brain is regarded from the side or above. The

olfactory part of the pallium exhibits little variation in form as

traced up through the higher animals. It is of course small in such

animals as Cetaceans, which are _anosmatic_. In highly osmatic such as

the dog it is large. The _uncus_, and _subiculum cornu ammonis_ of the

human brain, belong to it. Disease of these parts has been accompanied

by disturbance of the sense of smell. When stimulated electrically (in

the rabbit) the olfactory pallium occasions peculiar torsion of the nose

and lips (Ferrier), and change, often slowing or arrested, of the

respiratory rhythm. P.E. Flechsig has shown that the nerve-fibres of

this part of the pallium attain the final stage of their growth, that is

to say, acquire their sheaths of myelin, early in the ontogenetic

development of the brain. In the human brain they are myelinate before

birth. This is significant from the point of view of function, for

reasons which have been made clear especially by the researches of

Flechsig himself.

The completion of the growth of the nerve-fibres entering and leaving

the cortex occurs at very various periods in the growth of the brain.

Study of the development of the fibres entering and leaving the various

regions of the pallium in the human brain, discovers that the regions

may be conveniently grouped into those whose fibres are perfected before

birth and those whose fibres are perfected during the first post-natal

month, and those whose fibres are perfected after the first but before

the end of the fourth post-natal month. The regions thus marked out by

completion before birth are five in number, and are each connected, as

also shown by collateral evidence, with one or other particular species

of sense-organ. And these regions have another character in common

recognizable in the nerve-fibres entering and leaving them, namely, they

possess fibres projected to or from parts of the nervous system

altogether outside the cortex itself. These fibres are termed

"projection" fibres. Other regions of the cortex possess fibres coming

from or going to various regions of the cortex itself, but do not

possess in addition, as do the five primitive cortical fields, the

fibres of projection. So that the facts established by Flechsig for the

regions of pallium, which other evidence already indicated as connected

with the sense-organ of smell, support that evidence and bring the

olfactory region of cortex into line with certain other regions of

cortex similarly primarily connected with organs of sense.

It will be noted that what has been achieved by these various means of

study in regard to the region of the cortex to which olfactory functions

are attributed amounts at present to little more than the bare

ascertainment of the existence there of nervous mechanisms connected

with olfaction, and to the delimiting roughly of their extent and of

their ability to influence certain movements, and in man sensations,

habitually associated with exercise of the olfactory organ. As to what

part the cortical mechanism has in the elaboration or association of

mental processes to which olfaction contributes, no evidence worth the

name seems as yet forthcoming. In this respect our knowledge, or rather

our want of knowledge, of the functions of the olfactory region of the

cortex, is fairly typical of that to which we have to confess in regard

to the other regions of the cortex, even the best known.

_Visual Region of the Cortex._--There is a region of the cortex

especially connected with vision. The _optic nerve_ and _tract_

constitute the second link in the chain of neurones joining the retina

to the brain. They may therefore be regarded as the equivalent of an

intraspinal tract connecting the deep ends of the afferent neurones from

the skin with higher nervous centres. In the bony fishes the optic tract

reaches the grey matter of the optic lobe, a part of the mid-brain, to

which the so-called anterior colliculus is equivalent in the mammalian

brain. In the optic lobe the axones of the neurones of the optic tract

meet neurones whose axones pass in turn to the motor neurones of the

muscles moving the eyeballs, and also to other motor neurones. But in

these fish the optic tract has no obvious connexion with the fore-brain

or with any cerebral pallium. Ascending, however, to the reptilian brain

is found an additional arrangement: a small portion of the optic tract

passes to grey matter in front of the optic lobe. This grey matter is

the lateral geniculate body. From this geniculate body a number of

neurones extend to the pallial portion of the cerebrum, for in the

reptilian brain the pallium is present. The portion of pallium connected

with the lateral geniculate body lies above and behind the olfactory or

archipallium. It is a part of what was mentioned above as neopallium.

In the mammalian brain the portion of the optic tract which goes to the

optic lobe (_ant. colliculus_ of the mammal) is dwarfed by great

development of the part which goes to the geniculate body and an

adjoining grey mass, the pulvinar (part of the optic thalamus). From

these latter pass large bands of fibres to the occipital region of the

neopallium. In mammals this visual region of the cortex is distinguished

in its microscopic features from the cortex elsewhere by a layer of

myelinate nerve-fibres, many of which are the axones of neurones of the

geniculate body and pulvinar. Thus, whereas in the bony fishes all the

third links of the conductive chain from the retina lead exclusively to

the final neurones of motor centres for muscles, in the mammal the

majority of the third links conduct to grey matter of the cortex

cerebri.

The application of electric stimuli to the surface of the cortex does

not for the greater part of the extent of the cortex evoke in higher

mammalian brains any obvious effect; no muscular act is provoked. But

from certain limited regions of the cortex such stimulation does evoke

muscular acts, and one of these regions is that to which the neurones

forming the third link of the conductive chain from the retina pass. The

muscular acts thus provoked from that region are movements of the

eyeballs and of the neck turning the head. In the monkey the movement is

the turning of both eyeballs and the head away from the side stimulated.

In short, the gaze is directed as to an object on the opposite side. The

newer conductive chain traceable through the cortex does therefore,

after all, like the older one through the optic lobe, lead ultimately to

the motor neurones of the eye muscles and the neck, only it takes a

longer course thither and is undoubtedly much more complex. What gain is

effected by this new and as it were alternative and longer route, which

takes a path up to the cerebral cortex and down again, we can only

conjecture, but of one point we may rest well assured, namely, that a

much richer inter-connexion with other arcs of the nervous system is

obtained by the path that passes via the cortex. The functional

difference between the old conductive circuit and the new can at present

hardly indeed be stated even in outline. A natural inference might be

that the phylogenetically older and less complex path is concerned with

functions purely reflex-motor, not possessing sensation as an attribute.

But fish, which possess only the older path, can be trained to seize

bait of one colour and not of another colour, even against what appeared

to be an original colour-preference in them. Such discrimination

individually acquired seems to involve memory, though it may be

rudimentary in kind. Where motor reaction to visual stimuli appears to

involve memory--and without memory the training could hardly be

effective--some germ of consciousness can hardly be denied to the visual

reactions, although the reactions occurred in complete absence of a

cortical path and indeed of a visual cortex altogether.

Removal of the visual pallium in the tortoise produces little or no

obvious defect in vision; but in the bird such a lesion greatly impairs

the vision of the eye of the side opposite to the lesion. The impairment

does not, however, amount to absolute blindness. Schrader's hawk, after

removal of the pallium, reacted to movements of the mice with which it

was caged. But the reactions were impaired: they lacked the sustained

purpose of the normal reactions. The bird saw the mice; that was

certain, for their movements across its field of vision made it turn its

gaze towards them. But on their ceasing to move, the reaction on the

part of the bird lapsed. Neither did their continuing to move excite the

attack upon them which would have been the natural reaction on the part

of the bird of prey towards its food. The bird apparently did not

recognize them as prey, but saw them merely as moving objects. It saw

them perhaps as things to which mental association gave no significance.

Similarly, a dog after ablation of the occipital lobes of the cortex is

able to see, for it avoids obstacles in its path; but if food is offered

to it or the whip held up to it, it does not turn towards the food or

away from the whip. It sees these things as if it saw them for the first

time, but without curiosity, and as if it had no experience of their

meaning. It gives no hint that it any longer understands the meaning of

even familiar objects so long as these are presented to it through the

sense of vision. Destruction of the visual cortex of one hemisphere

alone produces in the dog impairment of vision, not as in the bird

practically exclusively in the opposite eye, but in one lateral half of

each eye, and that half the half opposite the hemisphere injured. Thus

when the cortex destroyed is of the right cerebral hemisphere, the

resultant visual defect is in the left half of the field of vision of

both eyes. And this is so in man also.

In man disturbances of sensation can be better studied because it is

possible to obtain from him his description of his condition. The

relation of the _cortex cerebri_ to human vision can be summarized

briefly as follows. The visual cortex is distinguishable in higher

mammals by a thin white stripe, the stripe of Gennari, seen in its grey

matter when that is sectioned. This stripe results from a layer of

nerve-fibres, many of which are axones from the neurones of the lateral

geniculate body and the pulvinar, the grey masses directly connected

with the optic nerve-fibres. In the dog, and in such monkeys as the

Macaque, the region of cortex containing this stripe traceable to optic

fibres forms practically the whole occipital lobe. But in the man-like

apes and in man this kind of cortex is confined to one region of the

occipital lobe, namely, that of the calcarine fissure and the _cuneus_

behind that. This region of cortex thus delimited in man is one of

Flechsig's areas of earlier myelinization. It is also one of his areas

possessing projection fibres; and this last fact agrees with the

yielding by this area, when under electrical stimulation, of movements

indicating that impulses have been discharged from it into the motor

neurones of the muscles of the eyes and neck. Evidence from cases of

disease show that destruction of the cortex of the upper lip of the

calcarine fissure, say in the right half of the brain, causes in man

impairment in the upper right-hand quadrant of both retinae: destruction

of the lower lip of the fissure causes impairment in the lower

right-hand quadrants. Destruction of the calcarine region of one

hemisphere produces therefore "crossed hemianopia," that is, loss of the

opposite half of the field of vision. But in this hemianopia the region

of central vision is always spared. That is, the piece of visual field

which corresponds with the yellow spot of the retina is not affected in

either eye, unless the calcarine regions of both hemispheres are

destroyed. This central point of vision is connected therefore not with

one side of the brain only but with both.

The impairment of sight is more severe in men than in lower animals.

Where the destruction of the visuo-sensory cortex in one calcarine

region is complete, a candle-flame offered in the hemianopic field

cannot even be perceived. It may hardly excite a reflex contraction of

the pupil. In such cases the visual defect amounts to blindness. But

this is a greater defect than is found in the dog even after entire

removal of both occipital lobes. The dog still avoids obstacles as it

walks. Its defect is rather, as said above, a complete loss of interest

in the visual images of things. But a dog or monkey after loss of the

visual cortex hesitates more and avoids obstacles less well in a

familiar place than it does when entirely blind from loss of the

peripheral organ of vision. In man extensive destruction of the visual

cortex has as one of its symptoms loss of memory of localities, thus, of

the paths of a garden, of the position of furniture, and of accustomed

objects in the patient's own room. This loss of memory of position does

not extend to spatial relations ordinarily appreciated by touch, such as

parts of the patient's own person or clothing. There is nothing like

this in the symptoms following blindness by loss of the eye itself.

Those who lose their sight by disease of the retina retain good memorial

pictures of positions and directions appreciated primarily by vision.

Cases of disease are on record in which loss of visual memory has

occurred without hemianopia. Visual hallucinations referred to the

hemianopic side have been observed. This suggests that the function of

visual memory in regard to certain kinds of percepts must belong to

localities of cortex different from those pertaining to other visual

percepts. The area of cortex characterized by the stripe of Gennari

occupies in man, as mentioned, the calcarine and cuneate region. It is

surrounded by a cortical field which, though intimately connected with

it by manifold conducting fibres, &c., is yet on various grounds

distinct from it. This field of cortex surrounding the visuo-sensory of

the calcarine-cuneate region is a far newer part of the neopallium than

the region it surrounds. Both in the individual (Flechsig) and in the

phylum (Bolton, Campbell, Mott) its development occurs far later than

that of the visuo-sensory which it surrounds. Flechsig finds that it has

no "projection" fibres, that is, that it receives none of the optic

radiations from the lower visual centres and gives no centrifugal fibres

in the reverse direction. This field encompassing the visuo-sensory

region differs from the latter in its microscopic structure by absence

of the lower layer of stellate cells and by the presence in it of a

third or deep layer of pyramidal cells (Mott). Its fibres are on the

average smaller than are those of the visuo-sensory (W.A. Campbell).

This zonal field is small in the lower apes, and hardly discoverable in

the dog. In the anthropoid apes it is much larger. In man it is

relatively much larger still. The impairment of visual memory and visual

understanding in regard to direction and locality is said to be observed

in man only when the injury of the cortex includes not only the

calcarine-cuneate region but a wide area of the occipital lobe. From

this it is argued that the zonal field is concerned with memories and

recognitions of a kind based on visual perceptions. It has therefore

been termed the _visuo-psychic_ area. It is one of Flechsig's

"association-areas" of the cortex.

Adjoining the antero-lateral border of the just-described _visuo-psychic

area_ lies another region separate from it and yet related to it. This

area is even later in its course of development than is the

visuo-psychic. It is one of Flechsig's "terminal fields," and its fibres

are among the last to ripen in the whole cortex. This terminal field is

large in man. It runs forward in the parietal lobe above and in the

temporal lobe below. Its wide extent explains, in the opinion of Mott,

the displacement of the visuo-sensory field from the outer aspect of the

hemisphere in the lower monkeys to the median aspect in man. To this

terminal field all the more interest attaches because it includes the

angular gyrus, which authorities hold to be concerned with the visual

memory of words. Study of diseased conditions of speech has shown that

the power to understand _written_ words may be lost or severely impaired

although the words may be perfectly distinct to the sight and although

the power to understand _heard_ words remains good. This condition is

asserted by many physicians to be referable to destruction of part of

the angular gyrus. Close beneath the cortex of the angular gyrus runs a

large tract of long fibres which pass from the visual cortex (see above)

to the auditory cortex (see below) in the superior temporal gyrus and to

the lower part of the frontal lobe. This lower part of the frontal lobe

is believed--and has long been believed--to be concerned intimately with

the production of the movements of speech. A difficulty besetting the

investigation of the function of the angular gyrus is the fact that

lesion of the cortex there is likely to implicate the underlying tract

of fibres in its damage. It cannot be considered to have been as yet

clearly ascertained whether the condition of want of recognition of seen

words--"word-blindness"--is due to cortical injury apart from

subcortical, to the angular gyrus itself apart from the underlying

tract. Word-blindness seems, in the right-handed, to resemble the

aphasia believed to be connected with the lower part of the frontal

lobe, in that it ensues upon lesions of the left hemisphere, not of the

right. In left-handed persons, on the contrary, it seems to attach to

the right hemisphere.

_Auditory Region of the Cortex._--Besides the two great organs of

distance-receptors, namely, the nose and eye, whose cerebral apparatus

for sensation has just been mentioned, those of a third great

distance-receptor have to be considered. The agents of stimulation of

the two former are respectively chemical (olfactory) and radiant

(visual); the mode of stimulation of the third is mechanical, and the

sensations obtained by it are termed auditory. Their cerebral

localization is very imperfectly ascertained. Electric stimuli applied

to a part of the uppermost temporal gyrus excites movements of the ears

and eyes in the dog. Destruction of the same region when executed on

both hemispheres is argued by several observers to impair the sense of

hearing. To this region of cortex fibres have been traced from the lower

centres connected with the nerve-fibres coming from the cochlea of the

ear. From each cochlear nerve a path has been traced which passes to the

_insulae_ and the above-mentioned _temporal_ region of cortex of both

the cerebral hemispheres. The insula is a deeper-seated area of cortex

adjoining the uppermost temporal convolution. To it Flechsig's

chronological studies also impute a connexion with the nerves of the

ear. Early myelinization of fibres, presence of ascending and descending

"projection" tracts to and from lower centres outside the cortex,

calibre of fibres, microscopic characters of its cortical cells, all

those kinds of indirect items of evidence that obtain for the visual

cortex likewise mark out this insular-temporal area as connected fairly

directly with a special sense-organ, as in fact a sensory field of the

cortex; and the suspicion is that it is auditory. Clinical observation

supports the view in a striking way, but one requiring, in the opinion

of some, further confirmation. It is widely believed that destruction of

the upper and middle part of the uppermost temporal convolution produces

"word-deafness," that is, an inability to recognize familiar words when

heard, although the words are recognized when seen.

More precise information regarding this auditory region of the cortex

has recently been obtained by the experiments of Kalischer. These show

that after removal of this region from both sides of the brain in the

dog the animal shows great defect in answering to the call of its

master. Whereas prior to the operation the animal will prick its ears

and attend at once to the lightest call, it requires after the removal

of the auditory regions great loudness and insistence of calling to make

it attend and react as it did. This is the more striking in view of

other experimental results obtained. Kalischer trained a number of his

dogs not to take meat offered them except at the sound of a particular

note given by an organ pipe or a harmonium. The dogs rapidly learned not

to take the food on the sounding of notes of other pitch than the one

taught them as the permissive signal. This reaction on the part of the

animal was not impaired by the removal of the so-called auditory regions

of the cortex. Kalischer suggests that this reaction taught by training

is not destroyed by the operation which so greatly impairs the common

reaction to the master's call, because the former is a simpler process

more allied to reflex action. In it the attention of the dog is already

fastened upon the object, namely the food, and the stimulus given by the

note excites a reaction which simply allows the act of seizing the food

to take place, or on the other hand stops it. In the case of answering

the call of the master the stimulus has to excite attention, to produce

perception of the locality whence it comes, and to invoke a complicated

series of movements of response. He finds that destruction of the

posterior colliculi of the mid-brain, which have long been known to be

in some way connected with hearing, likewise destroys the response to

the call of the master, but did not destroy the trick taught to his dogs

of taking meat offered at the sound of a note of one particular pitch

but not at notes of other pitch given by the same instrument.

_Other Senses and Localization in the Cortex Cerebri._--Turning now to

the connexion between the function of the cortex and the senses other

than those of the great distance-receptors just dealt with, even less is

known. Disturbance and impairment of skin sensations are observable both

in experiments on the cerebrum of animals and in cases of cerebral

disease in man. But the localization in the cortex of regions specially

or mainly concerned with cutaneous sensation has not been made

sufficiently clear to warrant statement here. Still less is there

satisfactory knowledge regarding the existence of cortical areas

concerned with sensations originated in the alimentary canal. The least

equivocal of such evidence regards the sense of taste. There is some

slight evidence of a connexion between this sense and a region of the

hippocampal gyrus near to but behind that related to smell.

As to the sensations excited by the numerous receptors which lie not in

any of the surface membranes of the body but embedded in the masses of

the organs and between them, the _proprioceptors_, buried in muscles,

tendons and joints, there is little doubt that these sensations may be

disturbed or impaired by injury of the _cortex cerebri_. They may

probably also be excited by cortical stimulation. But evidence of

localization of their seat in, and their details of connexion with, the

cortex, is at present uncertain. Many authorities consider it probable

that sensations of touch and the sensations initiated by the

proprioceptors of muscles and joints (the organs of the so-called

muscular sense) are specially related to the post-central gyrus and

perhaps to the pre-central gyrus also. The clearest items on this point

are perhaps the following.

Besides the regions instanced above, in the limbic (olfactory),

occipital (visual), and temporal (auditory) lobes, as exhibiting

precocity of development, there is a region showing similar precocity in

the fronto-parietal portion of the hemisphere. This is the region which

in the Primates includes the large _central fissure_ (sometimes called

the fissure of Rolando). To it fibres are traced which seem to continue

a path of conduction that began with afferent tracts belonging to the

spinal cord, and tracts which there is reason to think conduct impulses

from the receptor-organs of skin and muscles. The part of the cortex

immediately behind the _central fissure_ seems to be the main cortical

goal for these upward-conducting paths. That _post-central_ strip of

cortex would in this view bear to these paths a relation similar to that

which the occipital and temporal regions bear to afferent tracts from

the retina and the cochlea. There are observations which associate

impaired tactual sense and impaired perception of posture and movement

of a limb with injury of the _central region_ of the cortex. But there

are a number also which show that the motor defect which is a

well-ascertained result of injury of the _pre-central_ gyrus is

sometimes unaccompanied by any obvious defect either of touch or of

muscular sense. It seems then that the motor centres of this region are

closely connected with the centres for cutaneous and muscular sense, yet

are not so closely interwoven with them that mechanical damage inflicted

on the one of necessity heavily damages the other as well. There is

evidence that the sensory cortex in this region lies posterior to that

which has been conveniently termed the "motor." These latter in the

monkey and the man-like apes and man lie in front of the central

fissure: the sensory lie probably behind it. A.W. Campbell has found

changes in the cortex of the post-central convolution ensuing in the

essentially sensory disease, _tabes dorsalis_, a disease in which

degeneration of sensory nerve-fibres of the muscular sense and of the

skin senses is prominent. He considers that in man and the man-like apes

the part of the post-central gyrus which lies next to and enters into

the _central fissure_ is concerned with simpler sensual recognitions,

while the adjoining part of that convolution farther back is a "psychic

region" concerned with more complex psychosis connected with the senses

of skin and muscle. His subdivision of the post-central gyrus is based

on histological differences which he discovers between its anterior and

its posterior parts and on the above-described analogous differentiation

of a "sensory" from a "psychic" part in the visual region of cortex.

It will be noted that although certain regions of the cortex are found

connected closely with certain of the main sense organs, there are

important receptive organs which do not appear to have any special

region of cortex assigned to their sensual products. Thus, there is the

"vestibular labyrinth" of the ear. This great receptive organ, so

closely connected in function with the movements and adjustment of the

postures of the head and eyes, and indeed of the whole body, is

prominent in the co-ordination necessary for the equilibrium of the

body, an essential part of the fundamental acts of progression,

standing, &c. Yet neither structural nor functional connexion with any

special region of the cortex has been traced as yet for the labyrinthine

receptors. Perceptions of the position of the head and of the body are

of course part of our habitual and everyday experience. It may perhaps

be that these perceptions are almost entirely obtained through sense

organs which are not labyrinthine, but visual, muscular, tactual, and so

on. The labyrinth may, though it controls and adjusts the muscular

activities which maintain the balance of the body, operate reflexly

without in its operation exciting of itself sensations. The results of

the unconscious reflexes it initiated and guided would be perceptible

through other organs of sense. But against this purely unconscious

functioning of the labyrinth and its nervous apparatus stands the fact

that galvanic stimulation of the labyrinth is accompanied by well-known

distinctive sensations--including giddiness, &c. Moreover, the prominent

factor in sea-sickness, a disorder richly suffused with sensations, is

probably the labyrinth. Yet there is marked absence of evidence of any

special and direct connexion between the _cortex cerebri_ and the

labyrinth organs.

Also there is curiously little evidence of connexion of the cortex with

the nervous paths of conduction concerned with pain. As far as the

present writer can find from reference to books and from the clinical

experience of others, "pain" is unknown as an _aura_ in cortical

epilepsy, or at most is of equivocal occurrence.

The preceding brief exposition of some of the main features of the

localization of function in the _cortex cerebri_, gradually deciphered

by patient inquiry, shows that the scheme of partition of function so

far perceptible does not follow the quaint lines of analysis of the

phrenologists with their supposed mental entities, so-called

"faculties." On the contrary it is based, as some of those who early

favoured a differential arrangement of function in the cerebrum had

surmised, on the _separateness of the incoming channels from peripheral

organs of sense_. These organs fall into groups separate one from

another not only by reason of their spatial differentiation at the

surface and in the thickness of the body, but also because each group

generates sensations which introspection tells us are of a species

unbridgeably separate from those generated by the other groups. Between

sensations of hearing and sensations of sight there is a dissimilarity

across which no intermediate series of sensual phenomena extend. The two

species of sensations are wholly disparate. Similarly there is a total

and impassable gap between sensations of touch and sensations of sight

and sound. In other words the sensations fall into groups which are

wholly disparate and are hence termed species. But within each species

there exist multifold varieties of the specific sensation, e.g.

sensations of red, of yellow, &c. We should expect, therefore, that the

conducting paths from the receptive organs which in their function as

sense-organs yield wholly disparate sensations would in so far as

subserving sensation diverge and pass to separate neural mechanisms.

That these sense-organs should in fact be found to possess in the cortex

of the cerebrum separate fields for their sensual nervous apparatus is,

therefore, in harmony with what would be the _a priori_ supposition.

But, as emphasized at the beginning of this article, the receptive

organs belonging to the surfaces and the depths of the body and forming

the starting-points for the whole system of the afferent nerves, have

two functions more or less separate. One of these functions is to excite

sensations and the other is to excite movements, by reflex action,

especially in glands and muscles. In this latter function, namely the

reflexifacient, all that the receptive organs effect is effected by

means of the efferent nerves. They all have to use the efferent,

especially the motor, nerves of the body. So rich is the connexion of

the receptive organs with the efferent nerves that it is not improbable

that, through the central nervous organ, each receptive organ is

connected with every motor nerve of the whole nervous system,--the facts

of strychnine poisoning show that if this is not literally true it is at

least approximately so. Hence one of the goals to which each afferent

fibre from a receptive organ leads is a number of motor nerves. Their

conducting paths must, therefore, converge in passing to the

starting-points of the motor nerves; because these latter are

instruments common to the use of a number of different receptive organs

in so far as they excite reflex actions. On the other hand those of

their conducting paths which are concerned in the genesis of sensation,

instead of converging, diverge, at least as far as the _cortex cerebri_,

or if not divergent, remain separate. These considerations would make it

appear likely that the conducting path from each receptive organ divides

in the central nervous system into two main lines, one of which goes off

to its own particular region of the _cortex cerebri_ whither run

conductors only of similar sensual species to itself, while the other

main line passes with many others to a great motor station where, as at

a telephone exchange, coordinate use of the outgoing lines is assured to

them all. Now there is in fact a portion of the cortex in mammals the

functions of which are so pre-eminently motor, as judged by our present

methods, that it is commonly designated the _motor cortex_ (see fig.

24). This region of the cortex occupies in the Primates, including Man,

the pre-central gyrus. Among the items of evidence which reveal its

motor capabilities are the following.

[Illustration: FIG. 24.--Diagram of the Topography of the Main Groups of

Foci in the Motor Field of Chimpanzee.]

_The Precentral or Motor Region of the Cortex._--The application to it

of electric currents excites movements in the skeletal muscles. The

movements occur in the half of the body of the side crossed from that of

the hemisphere excited. The "motor representation," as it is termed, is

in the cortex better described as a representation of definite actions

than of particular muscles. The actions "represented" in the top part of

the gyrus, namely next the great longitudinal fissure, move the leg;

those in the lowest part of the gyrus belong to the tongue and mouth.

The topical distribution along the length of the gyrus may be described

in a general way as following a sequence resembling that of the motor

representation in the spinal cord, the top of the gyrus being taken as

corresponding with the caudal end of the spinal cord. The sequence as

the gyrus is followed downwards runs: perineum, foot, knee, hip,

abdomen, chest, shoulder, elbow, wrist, hand, eyelids and ear, nose,

mouth and tongue. The nature of the movement is very fairly constant for

separate points of this motor cortex as observed both in the same and in

similar experiments. Thus flexion of the arm will be excitable from one

set of points, and extension of the arm from another set of points;

opening of the jaw from one set and closure from another, and so on.

These various movements if excited strongly tend to have characters like

those of the movements seen in an epileptic convulsion. Strong

stimulation excites in fact a convulsion like that of epilepsy,

beginning with the movement usual for the point stimulated and spreading

so as to assume the proportions of a convulsion affecting the entire

skeletal musculature of one half or even of the whole body. The

resemblance to an epileptic seizure is the closer because the movement

before it subsides becomes clonic (rhythmic) as in epilepsy. The

determination of the exact spots of cortex in which are represented the

various movements of the body has served a useful practical purpose in

indicating the particular places in the cortex which are the seat of

disease. These the physician can localize more exactly by reason of this

knowledge. Hence the surgeon, if the nature of the disease is such as

can be dealt with by surgical means, can without unnecessarily damaging

the skull and brain, proceed directly to the point which is the seat of

the mischief.

The motor representation of certain parts of the body is much more

liberal than is that of others. There is little correspondence between

the mere mass of musculature involved and the area of the cortex devoted

to its representation. Variety of movement rather than force or energy

of movement seems to demand extent of cortex. The cortical area for the

thumb is larger than those for the whole abdomen and chest combined. The

cortical area for the tongue is larger than that for the neck. Different

movements of one and the same part are very unequally represented in the

cortex. Thus, flexion of the leg is more extensively represented than is

extension, opening of the jaw has a much larger cortical area than has

closure of the jaws. It is interesting that certain agents, for instance

strychnine, and the poison of the bacilli which cause the disease known

as tetanus or lock-jaw, upset this normal topography, and replace in the

cortex flexion of the limb by extension of the limb, and opening of the

jaw by closure of the jaw. There is, however, no evidence that they do

this by changing in any way the cortical mechanisms themselves. It is

more likely that their action is confined to the lower centres, bulbar

and spinal, upon which the discharge excited from the cortex plays. The

change thus induced in the movement excited by the cortex does, however,

show that the point of cortex which causes for instance opening of the

mouth is connected with the motor nerves to the closing muscles as well

as with those of the opening muscles. This is an item of evidence that

the "centres" of the cortex are connected with the motor nerves of

antagonistic muscles in such a way that when the "centre" excites one

set of the muscles to contract, it simultaneously under normal

circumstances causes inhibition of the motor neurones of the opposed set

of muscles (reciprocal innervation). In the great majority of movements

excited from the motor cortex of a single hemisphere of the cerebrum,

the movement evoked is confined to one side of the body, namely to that

opposite to the hemisphere stimulated. There are, however, important

exceptions to this. Thus, adduction of both vocal cords is excited from

the cortex of either hemisphere. The movement of closure of the eyelids

is usually bilateral, unless the stimulation be very weak; then the

movement is of the eyelids of the opposite side only. The same holds

true for the movements of the jaw. It, therefore, seems clear that with

many movements which are usually bilaterally performed in ordinary life,

such as opening of the jaw, blinking, &c., the symmetrical areas of the

motor regions of both hemispheres are simultaneously in action.

In regard to all these movements elicitable by artificial stimuli from

the motor cortex it is obvious that were there clearer evidence that the

pallial region from which they are elicitable is fairly directly

connected with corticopetal paths subserving cutaneous sensation or

"muscular sense," the movements might be regarded as falling into the

category of higher reflexes connected with the organs of touch, muscular

sense, &c., just as the movements of the eyeball excitable from the

visual cortex may be regarded as higher reflexes connected with vision.

The evidence of the connexion of the reactions of the motor cortex with

cutaneous and muscular senses appears, however, scarcely sufficient to

countenance at present this otherwise plausible view, which has on

general grounds much to commend it.

It is remarkable that movements of the eyeball itself, i.e. apart from

movement of the lids, are not in the category of movements elicitable

from the precentral gyrus, the "motor" cortex. They are found

represented in a region farther forward, namely in front of the

precentral gyrus altogether, and occupying a scattered set of points in

the direction frontal from the areas for movements of arm and face. This

frontal area yields on excitation conjugate movements of both eyeballs

extremely like if not exactly similar to those yielded by excitation of

the occipital (visual) region of the cortex. It is supposed by some that

this frontal area yielding eye-movements has its function in this

respect based upon afferent conductors from other parts of the eyeball

than the retina, for instance upon kinaesthetic (Bastian) impressions or

upon sensual impressions derived from the cornea and the coats of the

eyeball including the ciliary and iris muscles. The ocular muscles are

certainly a source of centripetal impulses, but their connexion with the

cortex is not clear as to either their nature or their seat. The

question seems for the present to allow no clearer answer. It is

certain, however, that the frontal area of eye movements has

corticofugal paths descending from it to the lower motor centres of the

eyeballs quite independent of those descending from the occipital

(visual) area of eye-movements. Further, it seems clear that in many

animals there is another cortical region, a third region, the region

which we saw above might be considered auditory, where movements of the

eyeball similar to those elicitable in the occipital and frontal cortex

can be provoked. A. Tschermak is inclined to give the eyeball movements

of the frontal region the significance of reflex movements which carry

the visual field in various directions in answer to demands made by

sensory data derived from touch, &c., as for instance from the hand. The

movements of the eyeballs elicitable from the occipital region of the

cortex he regards as probably concerned with directing the gaze toward

something seen, for instance, in the peripheral field of vision. The

occipital movement would, therefore, be excited through the retina, and

would result in bringing the yellow spot region of the retinae of both

eyes to bear upon the object. This view has much to justify it. The

movements of the eyeballs excited from the cortex of the auditory region

would in a similar way be explicable as bringing the gaze to bear upon a

direction in which a sound had been located, auditory initiation

replacing the visual and tactual of the occipital and the frontal

regions respectively.

Turning from these still speculative matters to others less suggestive

but of actual ascertainment, we find that the motor nature of the

precentral cortex as ascertained by electric stimuli is further

certified by the occurrence of disturbance and impairment of motor power

and adjustment following destruction of that region of the cortex. The

movements which such a part as a limb executes are of course manifold in

purpose. The hind limb of a dog is used for standing, for stepping, for

scratching, for squatting, and, where a dog, for instance, has been

trained to stand or walk on its hind legs alone, for skilled acts

requiring a special training for their acquisition. It is found that

when the motor area of the brain has been destroyed, the limb is at

first paralysed for all these movements, but after a time the limb

recovers the ability to execute some of them, though not all. The

scratching movement suffers little, and rapid improvement after cerebral

injury soon effaces the impairment, at first somewhat pronounced, in the

use of the limb for walking, running, &c., and ordinary movements of

progression. Even when both hemispheres have been destroyed the dog can

still stand and walk and run. Destruction of the motor region of the

cortex renders the fore limbs of the dog unable to execute such skilled

movements as the steadying of a bone for gnawing or the trained act of

offering the paw in answer to the command of the master. Skilled acts of

the limb, apart from conjoined movements in which it, together with all

the other limbs, takes part, assume of course a larger share of the

office of the limb in the Primates than in the dog; and this is

especially true for the fore limb. It is when the fore-foot becomes a

hand that opportunity is given for its more skilled individual use and

for its training in movements as a tool, or for the handling of tools

and weapons. It is these movements which suffer most heavily and for the

longest period after injury of the motor region of the cortex. Hence the

disablement ensuing upon injury to the cortex would be expected to be

most apparent in the Primates; and it is so, and most of all in Man.

Further, in Man there ensues a condition called "contracture," which is

not so apparent or frequent a result in other animals,--indeed, does not

occur at all in other animals except the monkey. In contracture the

muscles of the paretic limb are not flaccid, as they are usually in

paralysis, but they are tense and the limb is more or less rigidly fixed

by them in a certain position, usually one of flexion at elbow and

wrist. This condition does not occur at first, but gradually supervenes

in the course of a number of weeks. In Man the destruction of the motor

area of the cortex cripples the limb even for the part it should play in

the combined limb movements of walking, &c., and cripples it to an

extent markedly contrasting with the slight disturbances seen in the

lower mammals, e.g. the dog.

As regards the recovery of motor power after lesions of the motor

cortex, two processes seem at work which are termed respectively

_restitution_ and _compensation_. By the former is understood the

recovery obtained when a part of a "centre" is destroyed, and the rest

of the centre, although thrown out of function at first, recovers and

supplements the deficiency later. An example of restitution would be the

recovery from temporary hemianopia caused by a small injury in one

occipital lobe. By compensation is understood the improvement of an

impaired nervous function, traceable to other centres different from

those destroyed supplying means to compass the reaction originally

dependent on the centres subsequently destroyed. Instances of such

compensation are the recovery of taxis for equilibrium subsequent to

destruction of the labyrinth of the ear, where the recovery is traceable

to assistance obtained through the eye. It will be noted that these

instances of recovery by restitution and by compensation respectively

are taken, from cases of injury inflicted on receptive rather than on

motor centres. It is doubtful how far they really apply to the undoubted

improvement that does within certain limits progress and succeed in

partially effacing the paresis immediately consequent on lesions of the

motor area. It has to be remembered that in all cases of traumatic

injury to the nervous system, especially where the trauma implicates the

central nervous organ, the first effects and impairment of function

resulting are due to a mixed cause, namely on the one hand the

mechanical rupture of conducting paths actually broken by solution of

their continuity, and on the other hand the temporary interruption of

conducting paths by "shock." Shock effects are not permanent: they pass

off. They are supposed to be due to a change at the synapses connecting

neurone with neurone in the grey matter. They amount in effect to a

long-lasting and gradually subsiding inhibition.

For diseases of the brain see NEUROPATHOLOGY, INSANITY, SKULL

(_Surgery_), &c. (C. S. S.)