When experimental data first began to accumulate, physiologists

drew diagrams and made models of the nervous system in which they

represented it as composed of conducting arcs. The arcs were superposed

to indicate that they were of various grades—spinal for ordinary

reflexes, bulbar for co-ordinated actions, through the grey matter

in the centre of the great brain for “ideo-motor” actions, through

the cortex of the great brain for voluntary acts. They spoke of

authority and responsibility, comparing the nervous system to an

army or a club. It is premature to attempt a theory of the nervous

system compatible with recent discoveries regarding its structure and

mode of working, but it is clear that the diagrams and metaphors to

which we have just referred were misleading. In place of attempting

to disarticulate the machine, we ought to emphasize its structural

unity. The results obtained by uniting heterologous nerves cannot be

explained by reference to a model made of wires and pieces of cork.

They do not fit in with any organization of human units or with any

postal system or telephonic apparatus for transmitting news. Probably

the lines of thought which will prove most fruitful are somewhat as

follows: (1) An efferent discharge occurs as the result of the opening

of a circuit from a muscle back to the muscle. Afferent impulses—call

them sensory, on the understanding that this does not imply that they

appear in consciousness—are ceaselessly flowing from receptors to

effectors in the muscle. A sensation—in the case of skeletal muscles

usually a skin sensation—reinforces them to discharging-point. If

the spinal cord has been severed from the brain, the up-and-down flow

does not reach beyond its grey matter. It is short-circuited. If the

brain is in normal connection with the spinal cord, sensory impulses

travel upwards to its cortex (without, save in exceptional instances,

arousing consciousness, or, as we should prefer to express it in this

connection, without attracting attention) to a degree which varies with

the several classes of receptor and with the animal. A monkey reduced

to the condition of a “spinal animal”—_i.e._, with its spinal cord

severed from its brain—is less competent than a dog, and a man is far

less competent than a monkey. In other words, a man habitually uses

his brain more than does a monkey, and a monkey more than a dog. The

proportion which brain-weight bears to body-weight roughly indicates

the part the brain plays in conducting the traffic of the body. (2)

Communication within the nervous system is almost unrestricted. If,

before the median nerve was divided at the wrist, receptor A usually

initiated a current which passed through the circuit to effector X, and

receptor B to effector Y, and if the new fibres which grew downwards

lost their way so that the one which used to receive messages from A

attached itself to B, and the one which used to transmit commands to

X attached itself to Y, A is not thereby cut off from X, or B from

Y. Such a mechanical association is restricted to our diagrams. It

does not enter into Nature’s plan. The spinal cord is not scored with

unchangeable paths. A messenger from A could always reach either X or

Y. It was not the path, but the struggle with competing messengers,

which directed him to X.

When we endeavour to picture the mechanism of the nervous system, we

find ourselves faced by phenomena which appear irreconcilable. One set

of observations leads to the conception of closed paths; another set

points to an open conductor. The experimental crossing of nerves to

which we have just alluded shows that the nervous system is adaptable,

to a degree which seems extraordinary to anyone who attempts to compare

it with any of Man’s devices for establishing communication. Paths

appear to make themselves. On the other hand, the more important, and

therefore dominant, reflex actions, such as swallowing, breathing,

the maintenance of position, are due to the union of receptors and

effectors by lines which are either reserved for their sole use, or, if

shared by other currents, it is on the understanding that they have a

first and altogether prepotent claim. No competing impulses can divert

them or block their way. All reflexes which in the history of the race

have established their right to dominance not only seize and hold a

route through the nervous system, to the exclusion of all competitors,

but, as we have already shown in the case of the swallowing impulse,

the traffic in neighbouring routes is suspended for their benefit.

At the other end of the scale we find reflexes which may be termed

“occasional,” in that, although of frequent occurrence, they exhibit

illimitable variability in form. Occasional reflexes require, as a

preliminary to their transmission, that the afferent impulses which

give rise to them should secure for a time the exclusive use of the

motor neurones by which they are carried out. The receptors bring the

motor neurones into tune with themselves, and while in tune they will

respond to impulses from no others. But the tuning lasts for a short

time only. Either receptor or neurone, or both, soon tire. There is no

danger of a particular reflex being prolonged to the detriment of the

organism as a whole. As an illustration of an occasional reflex, we may

cite the scratching movement of a dog. Its skin is punctured by a flea.

It scratches the place. A second flea bites it somewhere in the same

neighbourhood. The dog does not shift its hind-foot so as to scratch

midway between the two bites. It finishes out one scratch before paying

attention to its second tormentor. The exact position to which the

hind-foot is raised depends upon the position of the irritant; and

since this may be shifted over a very considerable surface, the form of

the reflex varies equally widely. Each of the very numerous receptors

in the skin tunes a slightly different group of motor neurones; and

since a second irritant may reinforce the first, instead of making an

alteration in the group of neurones which the reflex is discharging, it

is clear that there is no fixed path uniting receptor A with neurones

X, Y, Z and receptor B with neurones W, X, Y. If, however, the second

irritation occurs at a spot lying at a considerable distance from

A, in place of reinforcing the scratching movement which A has set

going, it weakens and shortens it. The receptor C, which is calling

for the discharge of a markedly different set of motor neurones, tends

to inhibit those which are already active. These results are tested

with precision upon a “spinal dog” and with the aid of an electric

needle, the other pole from the battery being a large flat plate

placed in contact with the animal’s body. The conception of definite

paths, to which the contemplation of permanent reflexes gives rise, is

inappropriate to occasional reflexes. The latter show so wide a range

of variability and adaptability as to prove that a given receptor

may bring any of a great variety of groups of motor neurones into

connection with itself; just as a given group of neurones may be played

upon by impulses from a great number of different receptors. We have

called it a tuning of the motor neurones. One metaphor is as good as

another. The physical process which in the brainless frog underlies the

preparation for discharging motor neurones in the spinal cord, on the

same side as the leg on which vinegar is placed, so long as that leg

is free, and on the opposite side, when that leg is fixed, is unknown.

We seem to catch a glimpse of a doubleness of action, receptors in

the muscles combining with receptors in the skin in determining the

paths along which impulses shall be reflected—the efficient muscles

sensitizing their own neurones to the tuning influence of impulses from

sounding cutaneous nerve-endings. But it is impossible to formulate a

working scheme in the present state of knowledge.

=Sense-Organs and Nerve-Centres.=—A vast amount of labour has been

devoted to the study of the external form of the central nervous

system and to unravelling its internal structure; to plotting out its

various groups of nerve-cells, to disentangling its innumerable tracts

of fibres. The surface of the brain and spinal cord has been mapped

and measured. Every millimetre of its substance has been cut into

sections on the micro-tome. Organs which, fifty years ago, appeared

too complicated for investigation have been described in the minutest

detail. An immense accumulation of data is available for purposes of

reference; yet anyone who submits the theory of the nervous system as

it is held at the present day to a general review must allow that the

results of anatomical research enter but little into its construction.

The reason for this is not far to seek. As knowledge has advanced, the

apparent, or rather the expected, complication of the system has given

place to ideas of unity and simplicity. Its external configuration and

the varied arrangement in “nuclei” of its nerve-cells may, without

impropriety, be described as accidental. The form of the body and the

consequent location of the clients of the nervous system determine

the disposition and degree of concentration of its various business

centres. It shows, when followed throughout the whole animal kingdom,

extreme variability of its constituent organs, with absolute uniformity

of plan. Indeed, from the physiological point of view the term “organ”

is scarce admissible. It implies diversity of function in too high a

degree. The several parts into which the central nervous system is

obviously divisible co-operate so intimately as to preclude us from

thinking of them as separate organs.

If the citadel of the central nervous system is to be captured, all

lines of approach must be tried. Its outward form must be studied,

its minute structure examined with the microscope, its modifications

in various animals compared, its development followed, its reactions

to artificial stimuli tested, its pathological deficiencies and

vagaries watched. Yet, of all the means which have been made use of in

attempting to penetrate its secrets, the study of its history, by the

methods of comparative anatomy and embryology, has probably contributed

most to the development of sound ideas regarding the manner of its

working. The first differentiation visible in the blastoderm—the

globe of cells into which the ovum divides and out of which the embryo

is built—has relation to the formation of the nervous system. If

the earliest stages of its growth are followed, and the different

phases through which it passes are compared with the forms which it

assumes permanently in lower animals, the plan or type upon which

it is constructed shows up distinctly. Looking down the line to the

earliest vertebrata, we can discern clearly the form of nervous system

possessed by their prototype. Not that this “ideal ancestor” ever

existed. Experience teaches that it is unlikely that any animal that

ever lived was absolutely regular and symmetrical in all its parts;

nevertheless, the type can be presented in a perfectly regular scheme.

The ideal ancestor of the vertebrata was segmented, like a caterpillar

or a worm. Its mouth was not at the anterior extremity of the body, but

two (or more) segments behind it. Every segment bore a sense-organ (at

one period two sense-organs) on either side. Beneath each sense-organ

there was a clump of “grey matter.” Each segment also contained

(although not at the earliest epoch) two clumps of nerve-cells and

neuropil in a more central situation. These “ganglia” were united by

longitudinal and transverse commissures. They received the axons of the

cells which lay in the clumps beneath the sense-organs. They gave axons

to various muscles. Such is the type out of which the modern nervous

system has developed: two separate sense-organs and a complete nervous

system for each segment, the sense-organs connected with the ganglion

of the same side, the ganglia of the two sides bound together across

the middle line, and each row of sense-organs and each row of ganglia

united by longitudinal commissures into a chain. From the nervous

system as we see it now the majority of these segmental sense-organs

have disappeared; but the mode of formation of the cerebro-spinal

ganglia shows that they are the clumps of nerve-cells which lay beneath

the vanished organs. In the nose and the eye the grey matter retains

its original situation in the immediate vicinity of the receiving

epithelial cells—as the olfactory bulb and the deeper (anterior)

layers of the retina. The ganglia of the auditory nerve lie within the

bones of the ear. Spinal ganglia are close to the spinal cord. Auditory

and spinal ganglia contain only the cell-bodies of the first collecting

neurones (sensory nerves) together with certain curious bracketing

cells already referred to (p. 324), all the other constituents of the

peripheral clumps of grey matter which are found in the olfactory bulb

and retina having been withdrawn from the spinal ganglia into the axis

of the brain and spinal cord.

The sense-organs in front of the mouth have had from the beginning

an immense advantage over the others as observing-stations. Whereas

the body-organs collected information regarding the things with which

the animal came in contact, and consequently specialized in touch,

pressure, temperature, and, in the case of fishes, sensitiveness to

the chemical constitution of the medium in which the animal lived,

the head-organs specialized in responsiveness to forces acting from a

distance—particles suspended in air, vibrations of light, pulsations

of sound. Sensitiveness to touch, if it is to be useful, must be

widely distributed. The body-organs therefore broke into scattered

groups of sense-cells. Touch-spots are scattered all over the surface,

although they are set much closer together in the areas of skin which

are usually the first to come into contact with external objects

than they are elsewhere. The efficiency of the sense-organs of the

head—nose, eye, and ear—depended upon their remaining compact.

Progress in animal life, as we understand it—the rise from lower to

higher forms—has depended upon increasing integration of the body and

co-ordination of its functions. The nervous system is the agent which

has accomplished this unification. Each step in advance has depended

upon the provision of more nerve-tissue for the lacing together of

the various parts. We have seen already (p. 329) how intimate is

the union of receptors and effectors of every kind via the spinal

cord and brain. The overwhelming predominance in the direction of

action of the nose, the eye, and the ear has led to the accumulation

in their vicinity of the ever-increasing grey matter. The cerebral

hemispheres, or “great brain,” are pouched outgrowths from the first

pair of ganglia directed towards the olfactory pits. The original eyes

bore a similar relation to the second pair of ganglia—the epithet

“original” implying that the eyes which we now use are not the organs

with which our prevertebrate ancestors saw. First one of the original

eyes disappeared, and then the other. The vestige of the second is

still to be seen in the “pineal body” which is found on the dorsal

side of the brain of every vertebrate animal—in a mammal deeply

hidden in the cleft between the cerebrum and cerebellum. In place of

the pineal eyes two other sense-organs have specialized as eyes. They

are constructed on a different plan, being, to put it shortly, pineal

eyes turned inside out; for whereas in the pineal eyes, as in most of

the eyes of invertebrate animals, the rods and cones, which are the

cells of the retina sensitive to light, are directed forwards towards

the lens, the rods and cones of our permanent eyes are directed away

from the source of light. This change has made it possible to provide

more abundantly for their nutrition, and hence a greater power of

discriminating separate points in space and of distinguishing colours

is conferred upon them. The substitution of other sense-organs for the

original eyes has complicated the pictures which are presented to us by

a brain in its successive stages of growth; but it does not prevent us

from recognizing the general plan. Probably the secondary eyes, like

their predecessors, belonged to a pre-oral segment. The sense-organs

of a segment behind the mouth developed into ears; and the ear was

in its earliest phases, and still is, something more than an organ of

hearing. Its semicircular canals give information of displacements

in space. Knowledge of the position of its body is, to a fish, of

far more importance than its ability to hear breakers on the rocks.

Three looped tunnels, opening at either end into a common chamber, are

hollowed in the bone which contains the ear (_cf._ Fig. 38). Placed

at right angles one to the other, they occupy all three dimensions of

space. Open a notebook until, one of its covers lying horizontally,

the other is vertical, and place a sheet of paper vertically against

the bottom of the pages. A curved line drawn on each of these three

surfaces will represent the three semicircular canals. Arrange another

notebook in the same way, and let the two rest on the table with the

two vertical covers inclining one to the other, anteriorly, at an

angle of 90 degrees. The six surfaces will be in the planes of the six

semicircular canals. Within each bony canal is a membranous tube, to

which nerves are distributed, filled with fluid. When the position of

the head is changed, the fluid within the membranous tubes slides on

their walls. It is left behind at the moment the movement commences. It

overtakes its receptacle when the movement stops. The stimulus received

by the nerve-endings is recognized as indicating an alteration in the

orientation of the head. If the movement of the fluid is violent,

as when one waltzes, the loss of the sense of position disconcerts

the brain to such an extent that giddiness results. For a time the

quiet assurance upon which so much depends, that one knows how the

body stands in relation to its surroundings, gives way to a chaos of

sensations. From the nature of the case, the information which the

semicircular canals afford relates to change. They give no help in

ascertaining the position of the head when it is at rest. This must be

the reason, although the connection is not very clear, for the waning

of the effect in consciousness when stimulation is prolonged, and also

for the very marked after-sensation. At the commencement of a voyage

attention may be unpleasantly attracted to the rolling of the ship.

After a few days it ceases to be noticeable; yet when the voyager,

the night after landing, wakes in the dark, he finds his bed-room as

unsteady as his cabin. Rising hurriedly, the attempt to adjust his

position to the heaving floor (we speak from personal experience)

may result in a heavy fall. Although this phenomenon must be classed

with other “after-sensations,” it is so prolonged as to suggest that

consciousness, having become accustomed to a world which causes a

backward and forward flow of endolymph, misinterprets the absence of

sensation as indicative of change.

Taste is, practically, a special kind of smell. A fish’s olfactory

membrane, taste-buds, and chemical organs “of the lateral line” serve

the same sense, although, no doubt, they are applicable to the analysis

of different forms of matter in solution.

Our ideal prevertebrate has now left its primitive undifferentiated

condition. In front of its mouth it bears organs with which it searches

the world. Close behind the mouth are its auditory and orienting

organs. The rest of the surface of the body is endowed with the

capacity of recognizing “taste,” temperature, and contact. Smell,

sight, and orientation determine the development of the brain.

The cerebrum which has eventually become, as the seat of consciousness,

and hence the apparatus of mind, the dominant factor in the nervous

system, was in the first instance the part of the brain concerned with

the distribution to the muscles of impulses generated in olfactory

organs. There is scarcely any indication in a fish’s brain of the

representation in the cerebral hemispheres of any other sense, even

that of vision.

A bird’s brain presents a striking contrast to the brain of a fish.

With the exception of the apteryx and other ground-birds of New

Zealand, all birds are apparently destitute of the sense of smell.

Vision is the sense upon which their activity depends. It has invaded

the cerebrum, converting it into an organ in which sensations of

sight are worked up into “mind-stuff.” The optic lobe connection is

restricted to the production of reflex actions in which vision is

immediately followed by movement.

All the senses are represented in the great brains of mammals.

The cerebrum, which owes its existence to its connection with the

favourably-situated sense-organ of the nose, and grew in importance

when vision invaded it, has now taken in the senses of hearing, taste,

and touch. Only what may be termed in general visceral sense, and the

sense of orientation, are excluded.

Looking back to the starting-point, we see a segmented animal; its

segments of equal value; its nervous reactions unisegmental, although

linked in functional sequence. If it starts to walk, owing to

stimulation of one of its sense-organs, the impulse to walk spreads

from segment to segment. Comparing the latest product of evolution

with the earliest, we find that nervous tissue has concentrated

at the anterior end of the body. The double chain of ganglia, now

condensed into the axis of the brain and the spinal cord, still

contain all the effector neurones by which muscles are called into

action. Sensory nerves still arborize in the axis, providing the

mechanism for actuating motor neurones. But the vast majority of

intermediate or intercalated neurones have been attracted to the two

huge brain-masses—the cerebellum and cerebrum. In the former all

sensations (not conscious) connected with tone, position, orientation

and equilibrium are worked into appropriate impulses for the regulation

of the muscular system. In the latter all sensations which convey

information regarding the relation of the environment, including the

body, to the ego—the _not-me_ to the _me_—are transformed into motor

discharges which set a-going the movements (and the thoughts) by means

of which the purposes of life are fulfilled; for in the cortex of the

great brain alone is the passage of nerve-currents accompanied by

consciousness. Concentration of nerve-tissue allows of the combination

of sensations. It also facilitates the no less important effect

of mutual influence, interference. Sensations are suppressed, and

therefore the multitude of reactions to which they would give rise are

inhibited, in the interests of restricted and sustained movement or

thought.

=The Cerebellum.=—Sharks and other swift-swimming fishes have large,

deeply fissured cerebella, for the cerebellum is the part of the brain

which has gathered into itself most of the grey matter associated

with balancing, attitude, posture. The cerebellum is in birds large

and deeply folded. Developed from the ganglia to which the auditory

nerve distributes impulses from the semicircular canals, it has

established connections with all the other nervous tissues concerned

with sensations of position, strain, or pressure, including the

eyes, which afford information regarding the position of our limbs

relatively to the trunk, and of the whole body relatively to external

objects. Morphologically it is a median growth. The adverb is one of

those qualifying terms, convenient in science, which direct thought

without confining it. As used above, it implies that anyone who passes

before his mind the cerebella of all animals from fishes to Man, and

in all stages of growth, from their earliest appearance in the embryo

to their condition in the adult, sees the organ as a median prominence

surmounting the medulla oblongata. The bulgings of its sides which,

in human anatomy, are termed hemispheres, do not disturb its central,

unpaired plan of structure. It has, it is true, a lateral appendage

on either side (the combined flocculus and paraflocculus of mammalian

anatomy), but this lobe, although of great historic interest, is

so small, as compared with the median growth, as not to affect our

general conception of the form of the organ. By transverse fissures the

cerebellum is divided into a series of lobes.

In appearance the cerebellum varies greatly in the different classes

and orders of Vertebrata. Yet underlying this variety there is

marked unity of plan. A sagittal section of the organ of a shark, of

a bird, of a kangaroo, of a dog, of a whale, of Man, shows that it

is divided, from before backwards, into the same number of lobes in

animals occupying every position from the bottom to the top of the

vertebrate scale. A very little effort to grasp the significance of

this mystic number, nine, convinces one of the hopelessness of any

attempt to correlate the form of the cerebellum with the muscular

development or sensory endowments of vertebrates as a sub-kingdom.

It is the same for animals with limbs and animals without; animals

with well-developed noses or eyes, and animals destitute of one or

other of these sense-organs. This uniformity is extremely significant,

when contrasted with the wide differences exhibited by the cerebral

hemispheres. It shows that, unlike the great brain which mediates

between the several senses and the muscular system, the little brain is

concerned in bringing about adjustments to the environment which are

equally important to all animals, no matter how far they may depart

from the common type. The cerebellum is crossed by deep fissures,

dividing it into narrow convolutions or folia. The folia are grouped

in nine lobes. If the reader has secured as an illustration the brain

of a sheep, he will notice that the lateral regions of the cerebellum

present a complicated appearance owing to the contortion of the folia,

which results from the unequal development on its sides of the several

lobes. In its total size the cerebellum keeps step with the cerebrum,

the right side of one organ being associated with the left side of the

other.

[Illustration: FIG. 23.—VERTICAL SECTION OF THE CORTEX OF THE

CEREBELLUM, CUT PARALLEL WITH THE LONG AXIS OF A FOLIUM.

A shows three cells of Purkinje, their espalier

systems of dendrites being seen in profile. A

“mossy fibre” enters the granular layer from the

white matter. About a dozen of the granules are

shown, each with four or five dendrites and a

single axon. The axon bifurcates in the molecular

layer, its two branches running for a considerable

distance to left and right along the folium. B

shows the other nervous elements which are found in

the cortex: a cell of Golgi with a ramified axon,

a climbing fibre, a basket-cell, of which the axon

divides into four branches, and a small stellate

cell.]

The grey matter which covers the surface of the cerebellum, its

cortex, is singularly regular in microscopic pattern (Fig. 23). It

is divided into three sheets: superficially, the molecular layer in

which the dendrites of the cells of Purkinje branch; beneath this, the

thin layer in which are situate the cell-bodies of these neurones;

thirdly, the layer of small cells, or granules. Cells of Purkinje and

granules have been already described (p. 303). To these must be added

the stellate, bracketing cells of the molecular layer, the axons of

which divide to form baskets about a number of Purkinje-cells, and the

cells of Golgi of the granular layer. These last are comparatively

large cells, which have thornless dendrites, and axons which branch

repeatedly in the granular layer, without passing into the white matter

which underlies the cortex. Two kinds of nerve-fibre bring impulses

to the cortex: (1) “Mossy” fibres, which bear rosettes of filaments

which distribute impulses to the granules; and (2) “climbing” fibres

or “tendril” fibres, which, passing through the granular layer, cling

like ivy to the trunk and principal boughs of the dendritic processes

of Purkinje-cells. The axons of the cells of Purkinje undoubtedly carry

impulses away from the cortex, but their destination is not certainly

known.

The uniformity of structure of the cerebellum suggests that it “acts

as a whole.” Anatomy gives no warrant for the expectation that work of

different kinds is done by its several lobes. Its simplicity leads one

to hope that its mechanism may some day be understood; but at present

there are so many gaps in our knowledge that it is difficult, perhaps

hardly profitable, to attempt to string together the few anatomical

facts of which we are sure.

By means of tracts of afferent fibres the cerebellum has a very

extensive connection with the grey matter of the cerebro-spinal axis

(including the optic thalamus) into which sensory impulses of all kinds

are poured. Experimental results indicate that the organ distributes

impulses to the whole length of the cerebro-spinal axis, from the level

of the neurones which govern the muscles which move the eyes to its far

hinder end. No nerve-roots enter it. Its afferent fibres are the axons

of cell-bodies which lie in the posterior horns of the grey matter of

the spinal cord and in the corresponding grey matter of the axis of the

brain, especially that part related to the nerve from the semicircular

canals. Another set of afferent fibres lies at the periphery of the

spinal cord, forming one of the best defined of the spinal tracts. It

is also one of the oldest, being found in the same situation in all

vertebrate animals. Its fibres, which are exceptionally large, are

the axons of cells which form a very definite column—the “vesicular

column of Clarke”—on the median side of the posterior horn. Further

than this we cannot go. We are ignorant of the nature of the sensory

impressions collected by the cells of Clarke. The cerebellum also

receives through its middle peduncle the axons of cells which lie in

the pons Varolii on the opposite side; which cells are discharged by

impulses descending from the cortex of the great brain. It is not

improbable that it gives to the great brain as many fibres as it

receives from it.

If we had no experimental evidence as to the part which the cerebellum

plays in the harmonious working of the whole nervous system, we

should infer from its structure and connections that it is somewhat

mechanical, a co-ordinator of the activities of other parts rather

than in itself a functionally independent organ. Pathological and

physiological observations very definitely justify this conclusion.

They show that the cerebellum is not essential to life. It may be

completely destroyed by disease or removed by operation without robbing

the individual of any single function or capacity. Disease of the

cerebellum does not diminish the patient’s sensitiveness to every kind

of stimulus, nor does it deprive him of the use of any single muscle;

but it reduces him to the condition of a person who in gait, but not

in mind, is habitually drunk. When he walks he staggers from side to

side; when he stretches out his hand it trembles. His movements are

jerky; his head shakes, his eyes oscillate; he suffers from a feeling

of giddiness; his speech comes haltingly. Cerebellar ataxia, which is a

rare disease, resembles in many respects the much commoner “locomotor

ataxia” produced by disease of the spinal ganglia and the parts of

the cord connected with posterior roots; but careful analysis of the

symptoms shows that they are due, not to want of the sensations which

guide movements, but to inability to regulate the force of muscular

contractions. A man suffering from locomotor ataxia falls when he

closes his eyes, because, not being able to feel with his feet, he is

dependent upon vision for information as to his attitude. When the

cerebellum is diseased, the patient is no less unsteady with his eyes

open than he is with them closed.

The results of cerebellar disease or injury bring home to us the

fact that a nice adjustment of movements is needed to maintain

equilibrium. A dog from which the cerebellum has been removed retains

all its natural enterprise, all its instincts, all its emotions; but

every action which requires it to maintain its centre of gravity in

an unstable position gives it trouble. Placed in water so that its

body is supported, it swims almost as well as a normal dog. It is,

however, easy to lay too much stress upon the balancing function of the

cerebellum. The disturbance of this function attracts our attention;

yet it is probably but the indirect result of the suppression of

activities of a more widespread character. No animal ventures such

liberties with its centre of gravity as the biped Man accomplishes,

without thinking, every time that he descends a flight of stairs. Yet

the cerebellum of the limbless whale, that lives in a medium which

decentralizes its gravity, so to speak, bears the same proportional

relation to the rest of the nervous system as that of Man. Strangely

enough, it is the only cerebellum in the animal kingdom which so

closely resembles Man’s that it might be passed off as belonging to

a human giant; another reminder of the difficulty of deducing the

functions of the several parts of the organ from a study of their

relative development. What have a man and a whale in common which

determines the identity in form of their cerebella? How has it come

about that two cerebra as widely unlike as a man’s and a whale’s should

be associated with a common form of cerebellum?

If we apply to grey matter the distinction between sensory and motor

nerve-tissue—having no exact terminology, it is difficult to avoid

these metaphorical expressions—the cerebellum is essentially a sensory

development. It grows from the very margin of the infolding groove,

which, when closed, becomes the central canal of the brain and spinal

cord, its elements being marshalled in intimate association with

sensory root-fibres. Its millions of loops formed by the axons of

granules and the collecting processes of Purkinje-cells, are by-paths

which tap the conductors of sensory impulses. From some—those,

for example, which originate in the muscles and tendons, and in

the semicircular canals—more of the impulse is diverted to the

cerebellum, from others less. The organ has no motor functions. It

does not discharge neurones which control skeletal muscles, or plain

muscle, or glands. Yet it influences the passage of impulses through

sensori-motor chains, and apparently its influence is universal. It

regulates tone, reflex action, voluntary action. There is no part of

the nervous system over which its control is not felt. By its action

on the apparatus which binds the infinity of receptors which the body

contains to its muscle-fibres and other effectors, it unifies the

body. The cerebrum, as we shall see, is the organ which unifies the

personality. In the progress of evolution two functions which were

originally combined have, for convenience of concentration, been

divorced. The great brain has been set free from the more mechanical

part of the work. That it can perform the functions of the cerebellum

as well as its own is proved in cases of congenital deficiency of

that organ. In several instances malformation, amounting to a very

considerable reduction in the size of the cerebellum, was not detected

until after death, there being no symptoms of a sufficiently pronounced

character to call attention to it during life.

=The Cerebrum.=—All observations made on the great brain prior to

1870 showed it as absolutely inexcitable. Surgeons and physiologists

agreed that cutting, burning, passing electric currents through its

substance, neither yielded evidence of sensation nor movement of any

part of the body. Concerning its structure little was known beyond the

fact that whereas the grey matter, or cortex, which covers its surface

contains nerve-cells, only fibres are to be found in the white matter

which constitutes the greater part of its bulk. It seemed a hopeless

task to attempt to make anything out of a mass of tissue so uniform

in constitution and so irresponsive to experiment. Removing portions

of it appeared to cause a general dulling of the intellect without

loss of any particular mental quality. Physiologists, therefore, spoke

of the cerebrum as “functioning as a whole.” Phrenologists, having

classified the various phases of mental activity as “faculties,”

discovered “bumps” on the surface of the skull which they correlated

with the possession of the several faculties in a marked degree. They

parcelled out the brain in organs concerned with different kinds

of thought; but their localization of function was anatomically as

baseless as their classification of the various aspects of mind,

viewed as a system of philosophy, was absurd. In 1870 it was announced

that electrical stimulation of certain areas of the cortex of the

cerebrum of an animal under the influence of an anæsthetic, and

therefore incapable of voluntary action, induces definite movements.

Although the surgical applications of this discovery have proved

immensely important, its physiological value, as affording a method of

investigating the functions of the brain, is extremely small. Yet the

discovery gave an impetus to the further study of the cortex, which

has been rewarded with many exact results. By the discovery of its

excitability to electric currents it was proved that the whole cortex

has not exactly the same work to do, or—perhaps this is the safer form

of statement—does not do its work in exactly the same way. As soon as

it was known that it is divisible into areas differing in function,

many methods by which the delimitation of the areas might be attempted

were devised. The converging efforts made during the past forty years

by comparative anatomists, histologists, physiologists, pathologists,

and physicians, have resulted in the acquisition of an accurate, if

very restricted, understanding of the construction and mode of working

of the apparatus of thought. Of some of the new data the psychologist

is able to make use; but so far as the physiologist is concerned, it is

the vehicle of mind which is the subject of study, not its contents.

A new subject has been created since 1870. There is therefore nothing

to be gained, so far as our present purpose is concerned, from the

consideration of views which were current before that date; and since,

as must always occur when a science is rapidly advancing, observations

which logically should have been the first to be made were not thought

of until it became necessary to devise methods of checking results

obtained in other ways, we will consider the various sources of our

information without regard to the chronological order in which they

were opened up.

The cerebral hemisphere contains two large central masses of grey

matter, the nucleus caudatus and the nucleus lenticularis, often

described as a single structure under the name “corpus striatum.” Their

functions are unknown. The nerve-fibres which connect the cerebral

hemispheres with the rest of the central nervous system form two thick

limbs or crura on the under side of the brain. Each crus turns upwards

into its hemisphere, between the nucleus caudatus and optic thalamus

(the latter belongs to the “between-brain”) on the inner side, and

the nucleus lenticularis on the outer. In this passage the compact

crus, which is somewhat flattened, is termed the “internal capsule.”

Immediately above the three grey masses the internal capsule disperses

as a fountain of fibres which go to all parts of the cortex. Mingled

with these radiating fibres are vast numbers of others, proper to the

hemispheres, which run tangentially. Some, crossing the median plane,

as the corpus callosum, bind the two hemispheres together. Others form

tracts which can be followed from one end or pole of the hemisphere to

the other. Groups of fibres, dipping but little below the cortex, unite

nearly adjacent spots or neighbouring convolutions.

The folding of the cortex beneath fissures is due to the necessity

of disposing of a certain bulk of grey matter without increasing its

thickness beyond the proper limit. Since the superficial area of a

sphere varies as the square of its radius, whereas its capacity varies

as the cube, it is possible for a fixed relation to be maintained

between the amount of cortex and the amount of white matter in the

brain, only by the folds increasing in depth as the size of the brain

increases. Fissuring is a response to a mechanical need. This does not

imply, however, that the lines along which it takes place are devoid of

morphological meaning. The similarity in pattern of the convolutions

and fissures in various animals, and the regular progress of their

development in each individual, prove the contrary. If they are not

absolutely trustworthy as boundaries of areas of separate function—and

further evidence will be needed before a decision can be pronounced

upon this disputed question—they are in the main satisfactory as

landmarks.

As the nervous system grows, the axons of its neurones acquire

their fatty (myelin) sheaths in the order in which they come into

functional activity. The passage through them of impulses is the

stimulus which leads to the deposition of fat. The study of the

progress of myelination enabled the anatomist Flechsig to ascertain

the situation within the brain of the tracts of fibres related to

the several senses, and hence the traffic of the areas of the cortex

to which they go. Glistening white streaks appear successively in

the pulpy yellowish-pink substance of the interior of the brain. At

the time of birth all the fibres which enter or leave the cerebral

hemispheres have acquired their myelin sheaths. In the baby’s brain the

sense-organs have established all their connections with the cortex. No

new fibres will appear in the nerves of the eye, the ear, or the other

sense-organs, nor will their end-stations in the cortex be further

multiplied. (The use of the expression “end-stations” is legitimate

so far as sensations are concerned; notwithstanding that all sensory

impulses are retransmitted by neurones in the cerebro-spinal axis.) But

the cortex is very far from having finished its growth. It contains

a large amount of embryonic tissue, which gradually spreads outwards

from the developed areas into the surrounding unoccupied zones. The

taking up of new territory, and the consequent increase in the size

of the brain, is continued into adult life. The study of progressive

myelination enabled Flechsig to divide the cortex into “sensory

centres,” and intervening “association-zones”; although, doubtless, the

difference in function between the portions which receive sensations

direct and the portions in which the products of sensation are worked

up is one of degree, and not of kind.

[Illustration: =Fig. 24.=—VERTICAL SECTIONS OF THE CORTEX OF THE

CEREBRUM—A, OF THE VISUAL SENSORY; B, OF THE VISUAL ASSOCIATION AREA.

Between the two sections are shown the principal

types of cell, at the levels at which they

are severally found: _a_, small pyramid; _b_,

medium-sized pyramid; _c_, large pyramid. The size

of a pyramid is an indication of the distance to

which its axon extends before branching; the longer

its traject, the more widespread, it would seem, is

its terminal arborization. The axon of _c_, one of

the very large pyramids found in this association

area, passes to the front of the cerebrum, where

it breaks up in an association area of the tactual

sense of the hand, or of sensations concerned

with the regulation of gait, or in a centre for

movements of the eyeball. _d_, a tangential cell

of the surface; _e_, a Golgi cell with ramified

axon; _f_, a polymorph cell, with its axon directed

towards the surface. In sensory areas, tangential

fibres and granules are more numerous; in

association areas, small and medium-sized pyramids.]

The structure of the cortex is not quite the same in sensory and

association areas; but it is everywhere so far from showing the

diagrammatic simplicity which characterizes the cortex of the

cerebellum as to make it difficult to summarize the modifications which

distinguish its various regions. To a considerable extent its elements

shade one into the other, differing in size and in orientation rather

than in form. Commonly it is described as divisible into five layers:

(1) A thin superficial layer, containing cells of various forms and

fibres derived from the cells of the deeper strata. Some of the cells

are pluripolar, possessing several axons which run parallel with the

surface. Their destination is unknown. They do not appear to form

baskets like the cells of the molecular layer of the cerebellum. The

dendrites of pyramidal cells extend into this layer. (2) The layer

of small pyramids; cells with a branching apical process, root-like

dendrites from the basal angles of the pyramid, and an axon which sinks

into the white matter. (3) Granules. Carmine or other nuclear stains

show that small cells are present in very large numbers, especially

in the sensory areas; but since they are not, like the granules of

the cerebellum, coloured by the chrome-silver method, their form

and the disposition of their axons are unknown. (4) Large pyramids

exactly similar in form to the small ones. Their apical processes

are very thorny. Their axons give off several collaterals. Pyramids

are the most conspicuous elements in the cortex. Properly speaking,

they do not occur in layers, but are scattered throughout its whole

thickness, although their cell-bodies are not seen in either its most

superficial or its deepest strata. The largest are those of which the

axons either descend into the spinal cord or pass to a very distant

region of the cortex. They are found singly or in small clusters in the

deeper levels. (5) Polymorphous cells, some of them pyramids lying on

their sides, or even directing their axons towards the surface; some

fusiform or irregular cells; some Golgi-cells (p. 340). The axons of

pyramids enter the white matter, and many fibres from the white matter

radiate towards the surface between the pyramids; but the way in which

afferent, sensory fibres are connected with the collecting processes,

dendrites, of the pyramids is not known. We have already referred to

thorns, and to the possible nerve-net (p. 301). Sheets of tangential

fibres also occur in the cortex. A particularly distinct sheet divides

the granules in the visual cortex into two strata. In sections of this

region the sheet of fibres appears as a white line, distinctly visible

without a lens.

The limits of the several areas can be determined by examining the

structure of the cortex; but the individual peculiarities of the

various regions are not so marked as to indicate that they have

different kinds of work to do; if by kinds of work we wish to imply

that one part is “sensory,” another “motor,” a third concerned with

“intellectual processes.” On the contrary, its relative uniformity

shows unmistakably that all parts are engaged in the same work.

Nevertheless, certain broad conclusions can be drawn with regard to

the form of the neurones more immediately concerned with sensation,

with motion—that is to say, with the discharge to the grey matter

of the cerebro-spinal axis of the impulses which call its neurones

into activity—and with the secondary processes, called collectively

“association,” which occur within the cortex. Granules, as everywhere

throughout the nervous system, are receivers and distributors of

sensory impulses; although a study of the cerebral cortex does

not justify the conclusion that they are necessary links in its

sensori-motor arcs. Large pyramids are occupied with the nutrition

of fibres which have a long traject through the system. Hence they

are “motor.” They constitute a marked feature of the area which

is susceptible to stimulation. They occur also in the visual area

and elsewhere. Small pyramids are associational; that is to say,

their axons do not leave the cerebral hemispheres. They distribute

impulses from sensory areas to association-zones, and from one part

of an association-zone to another. The layer of polymorphous cells

is relatively thicker in animals in which the cortex of the brain

exercises less control over action than in animals in which the cortex

is supreme—in a rabbit thicker than in a monkey; in a monkey thicker

than in Man. This layer is therefore said to be concerned with the

lower functions of the cortex, whatever this expression may mean. Since

the relative abundance of small pyramids is a test of the supremacy

of the cortex, we may speak of them vaguely as concerned with its

higher functions. But a surer test of the capacity of the cortex for

the elaboration of the raw materials of thought which sensory nerves

deliver to it is the relative abundance of the tissue which intervenes

between its cells. The number of cell-bodies to be counted in a square

millimetre of a section of a given thickness is smaller in Man than in

a monkey, in a monkey than in a dog, and in a dog than in a rabbit.

A comparison of the brains of various mammals in which particular

sense-organs are either deficient or exceptionally well developed

affords the clearest proof of the localization of sensory areas. This,

if it were possible to make satisfactory measurements, would be by

far the best class of evidence as to the part played by the several

senses in an animal’s mental life. Unfortunately, measurement appears

to be out of the question; but a glance at a rabbit’s brain, placed by

the side of a mole’s, shows that vision is localized in the occipital

region. All marine mammals are destitute of the sense of smell; the

brain of a dog, compared with that of a porpoise or a whale, shows

that the sphenoidal region (_cf._ Fig. 25) is associated with this

sense. The brain of an otter exhibits very clearly the area into which

impulses arising in the nerve-endings of the sensory bristles of the

cheek are poured.

“Nihil est in intellectu quod non prius in sensu fuerit.” The organ

of the intellect is the cortex of the great brain, a sheet of grey

matter which has developed in connection with the various sense-organs.

The cerebral hemisphere of an infant is merely an extension of the

nerve-tissue associated with its sense-organs. Such it remains in a

microcephalous idiot. In the lower animals its capacity of growth after

birth is very small. But in a normal child the inflow of impressions

through sense-organs, the experience acquired regarding itself and its

surroundings, education, whether accidental or directed, causes the

extension of nerve-tissue from the sensory areas into the expansible

intervening zones.

There is still some uncertainty as to the nature of the sensations

received in the excitable area. They may be termed “kinæsthetic”

(sensations connected with movement) without more exact definition.

Some physiologists consider that tactile sensations, as well as the

obscure sensations, originated in the nerve-endings in muscles, around

tendons, or on joint-surfaces, are distributed to the areas, which,

when stimulated, are shown to represent fingers, hand, arm, and other

parts of the body. Others have sought, though with doubtful success,

for a tactile area, independent of the kinæsthetic centres. When first

discovered, these centres were termed “motor,” and still this term

may be retained, on the understanding that it does not imply that the

exchanges which occur in the kinæsthetic centres are of a different

nature to those which take place elsewhere. The region which they

occupy has become the motor area of the cortex because voluntary

movement is possible only under the guidance of sensations of movement.

A sound or a retinal image may prompt the movement; but the part of the

temporal region, or of the occipital region in which the sound-movement

exchange or sight-movement exchange occurs must act through the motor

area by opening kinæsthetic-movement arcs. Destruction of a part of

the kinæsthetic cortex causes in Man and the higher apes permanent

paralysis for the movements directed by the spot destroyed. In lower

animals the definition of the movement centres is vague, and their

removal produces only temporary results. Their mastery over the muscles

is less complete than in the higher apes and Man.

Practically nothing is known with regard to localization of function in

the association-zones, with the exception of the localization of the

centres for words; but this exception is so remarkable as to suggest

that if there were any other faculties, interference with which caused

defects as distinct as those which characterize disorders of speech,

it would be found that the association-zones are made up of definite

centres. As the evidence stands with regard to the broadest continental

divisions, we can merely state that it points, although not very

clearly, to the connection of the frontal zone, the region in front of

the kinæsthetic area, with ideas of personality, of other zones with

ideas of environment. Injury to the frontal region has in certain cases

resulted in the victim’s losing his knowledge of himself, his name, and

his relation to his family. On the other hand, gunshot wounds and other

definite injuries have in a large number of cases destroyed portions

of the cortex behind the forehead without causing any recognizable

intellectual change. It is quite certain that this part of the brain

performs no functions which are of a different, or, as it is often

called, higher order than those of other association-zones. It has been

stated that disease of the zone which intervenes between the visual

and auditory areas is more likely to cause hallucinations, disease of

the frontal zone delusions. A patient fancies in the one case that

he sees things that are not there, or hears voices when no one is

speaking; in the other case he imagines himself a king; but evidence

connecting localized disease with mental derangement is very scanty.

The functional disturbance which causes lunacy is usually of a general

character; or, if local to begin with, it becomes general before the

death of the patient makes possible the examination of his brain.

[Illustration: FIG. 25.—THE SURFACE OF THE LEFT CEREBRAL HEMISPHERE,

CEREBELLUM, AND MEDULLA OBLONGATA.

Sensory areas are enclosed by broken lines; certain

centres in the association-zones are marked by

dots. The sensory area of smell is on the inner

aspect of the brain; so also is the area of vision

which borders the calcarine and retrocalcarine

fissures, and only rarely extends on to the

external surface, as shown in the diagram. The

sensory area of hearing is largely hidden within

the fossa of Sylvius, the opening into which

is indicated by the dark line above it. The

kinæsthetic-sensory areas for the various muscles

of the body occupy the territory between the

dotted line in front and the bottom of the fissure

of Rolando behind. They do not extend on to the

posterior wall of this fissure. It is impossible

at present to define the boundaries of any of the

centres in the association-zones.]

Derangements of =speech= throw a flood of light upon the organization

and manner of working of the association-zones; and, owing to the

accident of the continuation of the line of the carotid artery by

the middle cerebral artery, which supplies the speech centre, there

is no other spot in the cortex so likely to be thrown out of gear. A

little plasma coagulates on one of the cardiac valves, or about an

atheromatous spot in the aorta. Detached by the blood-stream, it is

shot into one of the branches of the middle cerebral artery, which

it plugs, causing apoplexy. A larger or smaller number of muscles on

the opposite side of the body are paralysed. If the plugging occurs

on the left side of the brain, it is accompanied by aphasia; but only

if it occurs on the left side, owing to the fact—perhaps the most

remarkable in connection with the localization of speech—that only on

the left side is the cortex trained to utter words. In course of time

the patient may recover the power of speaking, but not until he has,

with almost as much labour as in childhood, educated the right side to

do the work. There are four speech-centres, quite distinct one from

the other. Near the visual area is the centre for seeing words, or

rather the centre for seeing the meaning of words. If this centre be

diseased, a written word is merely a crooked line. Behind the auditory

area is the centre for recognizing the meaning of words heard. If it

is interfered with, the most endearing or commanding phrases produce

no more impression on the hearer than a bird’s song. In front of the

hand-area—its localization is less certain than that of the other

three—is the centre for writing. In it are associated words heard

or seen, with the movements necessary for the making of letters. In

the centre first referred to, as being the one most often thrown out

of gear, which lies in front of the area for the mouth and throat,

words heard or seen are translated into movements of the parts which

give them sound. No other actions illustrate so clearly the “law of

neural habit.” In the infant’s brain sounds of words are distinguished

from other sounds. They are associated with the objects which they

name. Movements of the mouth and throat, made at first ineffectively,

blunderingly, succeed after a time in securing the thing of which

they sound the name to the child’s satisfaction. Thus, two centres

are gradually established in his mind. Sounds and ideas of things are

associated in the one; words and ideas of the movements necessary to

their pronunciation in the other. Either of the four speech-centres

may be placed out of action without the others suffering. A man may

be able to write without being able to read what he has written. He

may read aloud, although apparently deaf to speech. He may be unable

to write or unable to speak, although understanding what he reads or

hears. Aphasia, when partial, illustrates still further the law of

neural habit. The ability to remember nouns, especially proper names,

is most easily lost. Few are the people who, as age advances, do not

suffer from this failing. Even the names which are most familiar elude

the memory. From one point of view this is strange. Nouns-substantive

are the words first learned. Of all words they have the most definite

objective association. But it is just their definiteness which makes

them difficult of approach when the apparatus of mind is working badly.

There are so few paths by which they can be reached. Their mental

associations are limited. A patient who is recovering from the effects

of a lesion which has rendered him partially aphasic may be able to

recall adjectives when he cannot recall nouns. He may say, “Give me

the black,” when he wants ink, and “Give me the white,” when he needs

paper. Or he may retain control of verbs. “Where is the—— what I put

on—what I think with?” may be the circumlocution for hat.

Psychologists explain the voluntary production of a movement as the

setting flowing of a sensori-motor current. Everyone agrees that it

is impossible to think of the impulses which produce movement as

originating without sensory antecedents. Hence psychologists picture

the nerve-current as originating on the sensory side. Kinæsthetic

images of the sensations which will result from the movement are

described as being called up in the mind by the agitation of the

part of the brain which, by association, is linked with the neurones

which discharge impulses to the appropriate spots in the grey

matter of the spinal cord. The idea of movement flows over to the

muscles. But this conception of the relation of mind to body assumes

too much. It postulates an existent mind in which the images of

movement-sensations—the memories, that is to say, of the sensations

which previously accompanied movement—are stored. The study of the

apparatus of mind does not warrant this assumption of an existent

mind. It finds nothing in the nervous system but apparatus. There is

no mind existent in the brain during sleep. It would appear to be

sufficient to describe the origination of a voluntary movement as

the opening of the channels which convey the afferent impulses which

are ceaselessly pouring into grey matter from nerve-endings in and

about muscles into efferent channels. Our conception of the number

of sensations which reach the realm of consciousness is ludicrously

restricted by our inability to pay attention to more than one sensation

at a time—a restriction, it is needless to remark, which is imperative

in the interests of consistency of behaviour. Two personalities paying

attention to different sequences of sensations would give incompatible

orders. One would command the muscles to cause the body to recline;

the other would direct them to make it stand up. From myriads of

sense-organs impulses are continuously rippling through the cortex

of the brain. The term “impulse” is too heavily weighted by its

association with the idea of currents which are strong enough to prove

effective without the intervention of consciousness; but no other is

available. They ring the bell of consciousness, however little may be

the attention which their summons secures. Attention cannot be directed

to two things simultaneously. It moves, as it were, on a succession of

points. On some it rests longer than on others. They make an impression

which can be recalled; the rest being passed by so rapidly that

they are not remembered, it is as if they had never been perceived.

They blend, as a succession of moving lights blend, in producing a

background to consciousness. Not recognizing their separateness, we

interpret them as fused. A good deal of misleading metaphor has been

used, as it seems to the writer, in accounting for the effect upon

the mind of impressions which make but a weak demand upon attention.

They are spoken of as “marginal” perceptions, from the analogy of

the ineffectiveness of impulses generated at the periphery of the

retina, as compared with those which give rise to direct vision. A

“subconscious,” or even “unconscious,” self is evoked. The self cannot

be less than conscious. Self is the passage of attention from sensation

to sensation. Its relation to the not-self is temporal, not spatial.

Every sensation which is called up into consciousness, though it

occupy attention for the shortest possible time, tends to give rise to

movement—is, indeed, in its very nature an impulse flowing through

a sensori-motor arc. The circuit for the voluntary execution of a

movement is represented as flowing through kinæsthetic-movement arcs.

This may be necessary for volitional actions, but it is not essential

for reflex actions. A spinal frog will remove an irritant from its back

with its hind-leg, after the roots of all the afferent nerves of the

hind-leg have been cut. In this case the reflex is direct, from injured

skin to muscles of the leg. It is not double—muscular sensations from

the leg, liberated into efferent leg-muscle-nerves by skin-sensations

originated simultaneously in a part anterior to the segments in which

the roots have been cut.

The unit of sensation to which attention can be directed has yet to

be defined. Like sensations—sensations which are correlated in

experience, that is to say—seem to fuse in consciousness. A sequence

of similar sensations appeals to attention. Unlike sensations interfere

one with another. The apparent fusion is not a composite neural effect

which consciousness views as a single unit. Not even identical images

simultaneously formed on the two retinæ produce a superimposed effect

upon a particular spot in the brain. Different brain-spots receive the

two separate images which the mind views as one. This raises a doubt

as to whether perceptions are, properly speaking, fused. It suggests

that they are separate points upon which attention rests in rapid

succession; but such a hypothesis does not preclude the conception

of the production of a composite sensation by impulses coming

simultaneously from the same sense-organ—_e.g._, a unified neural

effect as the result of several musical tones.

Every neural agitation which attracts attention has an effect upon

the growth of the nerve-strands in which it occurs. =Memory= is not

an existent. It is the repassage of the same strands. There is no

such _thing_ as memory. It is the neural apparatus which responds in

a similar way to a similar agitation. It is difficult to speak of

association and neural habit, the phenomena upon which not only all

mental life, but all co-ordinated activities, are based, without using

such expressions as “the broadening of the path” or “the thickening

of the conductor” by the impulses which pass through it. Apparently

these analogies may with safety be pressed curiously far. Chaotic

response to stimulation is unknown. Thanks to the nervous system,

action exhibits an ordered relation to stimulation. This relation is

determined by education, giving the term a connotation wide enough

to cover all experience. Nerve-tissue adjusts itself to experience;

and since the nerve-matter which takes the pattern is not labile,

the process of organization is consecutive and the result permanent.

One pattern is not destroyed as another is impressed. Hence temporal

associations are formed. What has been thought once will be thought

again, if the circumstances in which it was thought recur. What has

been done once will be done again under the influence of a similar

sequence of stimuli. The conductors are widened every time that they

are used. But, so far as concerns the mind, a reversed influence comes

into play. The wider the conductor, the less appeal to attention is

made by the impulses which pass through it. It is as if currents which

have to overcome resistance in a narrow path acquire a higher potential

than those which find an open road. And since the making of the road

depends upon attention, the limit of broadening is reached when a

volitional act becomes a habit. The first time that a piece of music is

played consciousness is alert. Marks on the page and movements of the

fingers are felt intensely. With each repetition the need for attention

subsides.

A skilled movement is impossible in the absence of guiding sensations.

I decide to button my coat. Sensation-paths from the muscles of the

forearm are opened into motor paths extending from the large pyramids

in the arm-centres of the kinæsthetic cortex. But it is not sufficient

that the action be started: it must be guided by the sensations which

movement produces. If my fingers are numb with cold, I cannot button

the coat. The muscles which move the fingers are warm enough beneath

the sleeve, but my attempts to will them to move are as futile as they

would be if the muscles belonged to some other person. The will has

no power over the muscles. It is essential that the sensations which

accompany the act of buttoning the coat flow through the same paths as

hitherto in the cortex of the brain. Flowing through the same paths,

they produce the same effect in consciousness, the same perceptions. In

ordinary parlance, one cannot perform any act unless one can remember

what it felt like to perform it on a previous occasion. It is almost

as sound physiology to describe the voluntary action of fastening a

button as commencing in the skin of the fingers as to describe is as

commencing in the brain. The act is due to the direction of attention

to impulses which flow from muscle to muscle, and from skin to muscle.

All skill in the use of muscles is acquired by the method of trial and

error. Familiar movements are tried, combined, modified with a view to

the production of a new result. A man accustomed to striking with the

right hand forwards endeavours to swing a golf-club with the left hand

backwards. For a long time the result is anything but a success. At

length the head of the club takes the right curve. It not only hits the

ball with its centre, but it carries it through in the right line. The

ball travels 120 yards or so towards the green. In golfing terminology,

a successful drive is always “an awful fluke”; but the fluke once

accomplished, nothing is easier for the golfer than to drive equally

well on all succeeding occasions. He need merely remember exactly what

it felt like to give the club a perfect swing, and exclude all other

sensations while he is passing these memories through his sensori-motor

arcs!

The fact that we can deliberately improve an action, fitting it to

the attainment of the object of desire, by suppressing wrong and

emphasizing right sensations, shows how large a part consciousness

plays in the affairs of the nervous system. This brings us to

the frontier of physiology. At this boundary the authority of

the physiologist ends. He cannot define consciousness; he cannot

investigate it. Yet he naturally asks whether the machine which he is

investigating is a machine and nothing more. When the possibilities of

reflex action were first recognized, thought tended to dethrone feeling

and Will in favour of automatism. If the actions of a spinal frog

exhibit so distinct a purposive character, why, it was asked, should

we assume that the frog with a brain is anything more than a reflex

machine? Light, heat, sound are playing upon its sense-organs; surely

these stimuli suffice to set going all the sensori-motor currents which

lead to the various movements which in their totality constitute the

frog’s behaviour! And why assign to a mammal a self-directing authority

which we deny to a frog? The increased complexity of its behaviour is

more than accounted for by the greater variety of its nervous arcs.

All animals, it was argued, including Man, are reflex machines. Their

thoughts and actions are the effects of the play upon their nervous

systems of forces from the outer world. Each inherits a nervous system

of a certain pattern. Its individual development is conditioned by

the sensations which pass through it. The sensations are impressed by

the environment. Therefore the individual is a puppet, his activities

the dance of circumstance. Consciousness is an “epiphenomenon.” Few

physiologists or students of animal behaviour take this material view

of life at the present day. The fact that it leads inevitably to the

conclusion that consciousness is an “epiphenomenon” (Huxley’s term) is

its _reductio ad absurdum_. It is not in harmony with the economy of

Nature that an animal should be endowed with the capacity of feeling

pain and pleasure, if such endowment is useless to it. It can be useful

only by directing activity towards the attainment of pleasure and

the avoidance of pain. This admitted, the mechanical theory falls to

the ground. There is an “It” which feels, selects feelings, chooses

those which have a pleasant tone, wills to perform the acts by which

they are attained. It follows that the value of consciousness lies in

the prerogative which it confers of adapting action, within certain

limits, to circumstance. An animal succeeds in life in proportion as

the nervous system which it inherits reacts satisfactorily to its

environment. A chick which, after being hatched in an incubator, has

been isolated for twelve hours without food, seizes a grain of corn the

instant that it sees it. Its brain contains ready-made sensori-motor

arcs connecting the spot in its cortex in which the visual impression

of the grain is perceived and the motor neurones which control the

pecking muscles. A sheep-dog is quickly broken to sheep, because its

ancestors have been selected by mankind from amongst dogs that readily

adapted themselves to this work. The breeder has selected a pattern of

brain with the same success with which, when appearance is the only

desideratum, he selects a pattern of coat. Beavers set to work at

constructing a dam at the only spot in a valley at which it is possible

to create an artificial lake, because for countless ages Nature has

ruled out the animals which constructed their dams in unsuitable

places. Man also inherits a brain-pattern; but, not being required to

shift for himself soon after birth, he goes through a long period of

infancy and tutelage, during which, by force of circumstance and his

own Will, the pattern is elaborated. His supreme success is due to his

capacity for adapting means to ends. He inherits very few instincts.

Except as regards organic functions, his spinal cord is subservient in

almost all respects to his brain. Most of the actions of an animal are

instinctive—a word which has been sadly misapplied. Its connotation

is negative rather than positive. Owing to the marked pattern of its

brain, an animal finds it difficult to avoid acting in a particular

way. As the nights grow longer and its hours for feeding are curtailed,

a swallow is impelled by its instinct to go South. It makes the same

use of its sensations during its migration, and is as completely

dependent upon them for its guidance as a man would be. The lower we

descend the scale, the more inevitable do an animal’s movements become;

but there can be no doubt but that consciousness is of value to an

animal, as to Man, in that it gives to its individuality the capacity,

within such limits as Nature has selected, of resisting or modifying

its ancestral instincts when they are not absolutely appropriate to the

occasion.

Sentience implies personality. “No system of philosophy can extrude

the ego.” The difference between the performance of the animal machine

as a physiologist studies it, and its behaviour when under the control

of its own driver, is the difference between reflex action and choice.

The ego interacts with physical forces. It does not come within the

province of the physiologist to explain the source of the force which

interferes with force. He finds no trace of it on either credit or

debit side when making up the body’s accounts. He is unable to enter,

“Item, to the development of consciousness ... so much.” He can form

no conception of this immaterial manifestent which hovers over the

infinitely numerous sensori-motor exchanges which are always occurring

in the cortex of the brain, giving to a particular group of agitations,

now here, now there, a special quality; but the manifestent is needed

to account for the potency of the reinforced agitations which enables

them to take possession of the nerve-paths by which muscles are reached.

It is for the psychologist to define the application of the terms

“consciousness,” “attention,” “will.” He cannot define the attributes

of the ego which these terms connote. The moralist must show the way

in which they determine, or should determine, conduct. Yet within the

plain limits of physiology, attention, using the word in its every-day

sense, modifies the responses of the nervous system in a degree which

cannot escape observation. It is astonishing to anyone accustomed to

hospital surgery (although even in this field singular exceptions are

met with) to see the grave operations which a veterinary surgeon may

perform, without the animal showing any evidence of pain, provided its

apprehension has not been aroused and its attention directed to what

is being done. A horse standing in front of a crib of oats, untied,

will hardly whisk its tail while the surgeon is making a great wound

in its flesh, and sawing off a bony excrescence. The knife does not

come within the experience of a horse. It has no anticipations, and

its skin, intensely sensitive to the tickling of a fly or the smart

of a whip, is relatively insensitive to a cut. An eminent surgeon of

the last generation (the writer, as a student, “dressed” for him in

his old age) was in the habit, having arranged that his patient could

not see what he was doing, of performing operations of a very painful

nature whilst assuring his patient, “I am merely making a thorough

examination, in order that I may be perfectly certain of the cuts that

I shall have to make to-morrow in the operating-theatre when you are

under chloroform.” We are not concerned with the ethics of his method;

but the assurance, “Now that’s all over; you will never need to have

that operation performed again,” saved many a sufferer from a night of

apprehension and a miserable “coming round.”

It was stated, during the South African War, that at Ladysmith the

bearer of a critical despatch, who was struck in the palm of the hand

by a bullet which traversed the whole length of his forearm, did

not discover that he was wounded until he saw the dripping blood,

after his errand was successfully accomplished. To deliberately cut

oneself with a razor is most painful, yet shaving in the morning, with

thoughts concentrated on the doings of the day, it is often the sight

of blood which directs attention to the fact that the skin is severed.

Of all evidences of self, the power of paying attention is the most

noteworthy. We can direct attention to certain sensations, which then

become perceptions, and we can deliberately ignore others, within

certain restricted limits.

The control of the nervous apparatus by the self is a truth which no

student of the physiology of human beings can ignore. Isolated from its

relation to all other scientific truths, it has been made the basis of

a nescience which, although positively merely foolish, is, negatively,

harmful—yet a form of folly which answers well to the needs of persons

of a certain category.

It may be objected that the picture of the relation of mind to brain

which is here presented—the one, activity, motion, the other a

labyrinth of conducting paths—makes all mental phenomena entirely

dependent upon current sensations. No results could happen if the

sensations were not there. It affords no ground for the explanation of

=mental images=, =hallucinations=, =dreams=. A few lines may be spared

to show that this objection does not hold. We cannot attempt to explain

the conscious control of thought. It is a part of the impenetrable

mystery to which we have just referred. But, granted that it obtains,

the direction by the ego of afferent nerve-currents through the same

strands which formerly vibrated to sensations which drew a picture, and

hence the revival of its image, is no more incomprehensible than the

liberation of afferent impulses from muscles into efferent channels.

Brain-chains are composed of many links. Their interconnection is

illimitable. When I recall the appearance of the house in which I lived

as a child, I throw into the chain impulses (from somewhere) which

traverse the final links, where passage implies consciousness. At the

edge of the lace-work of linked threads the impulses light up a pattern

which childhood’s experience worked into the apparatus of thought.

If we were to admire the perfection of any special aspect of the

brain’s functioning, the rarity of hallucinations might give us cause

for wonder. That impulses so seldom leave their own paths is more

astonishing than that occasionally, when the brain is excited and its

nutritive conditions deranged, the impulses which the ego can direct

into channels where they revive an image should sometimes, and with

far greater force, make their own way down well-worn paths, lighting

up a picture which deceives the ego. Dreams, by contrast, throw up

in a strong light the part played by attention in intelligent life.

The capacity for alertness is due to the favouring of one set of

impulses by suppressing others. The favoured impulses hold the road.

Concentration of attention is keeping thought to one line by resisting

all temptation to wander into by-paths. The waking condition is the

state in which all nerve-ways are closed, with the exception of those

which consciousness is using. The more severe the closure, the more

vivid is consciousness. In sleep all paths are open. In none is the

potential acquired by impulses in the process of overcoming resistance

high enough to evoke consciousness. A burst of impulses ascends from

the stomach, set a-flowing by undigested fragments of salmon and

cucumber, or mounts from the arm on which the sleeper has been lying

until its circulation has been arrested. They reverberate through the

open corridors of the brain. If they are sufficiently noisy to awaken

the sleeper, he, detecting them in this path and in that, supposes

them to be on the same errands as the impulses which commonly pass

thus. If dreams are analysed, it will be found that, although the

combinations of impressions may be uncommon and extremely bizarre, the

impressions are selected from the most familiar. The images of which

the dream is compounded, which may have lost all normal relations and

may have assumed impossible proportions, are those which the mind

most frequently conjures up. In the large majority of instances some

happening of the day preceding can be recognized as the prompting

cause. A remembered dream is the photograph taken by consciousness

of the sensations which have bombarded it into activity. Especially

if due to impulses originated by visceral discomfort, the dream may

have an unpleasant tone. This may take various forms, but the emotion

most commonly aroused is fear. The objects visualized may have

preposterous dimensions, or they may be not sufficiently distinct for

recognition—elusive imps; but most commonly distress is caused by

the want of harmony of sensations, due to the absence of kinæsthetic

elements. A man is lying on the railway-line; a train is approaching

with increasing speed; he cannot get up. He is in the pulpit, but

cannot speak. Dreams thus confirm the view set forth above as to the

cause of volitional action. Ability to perform an act depends upon the

flow through the kinæsthetic centres of the brain of impulses generated

in the muscles by which the act is to be, or is being, performed.

Kinæsthetic sensations do not under any circumstances play the same

part in mental life as sensations from the skin, the eye, or the ear;

when the body is passive in bed they are not flowing into the cortex.

The dream-photograph shows elements demanding movement, but affords no

evidence that movement is in progress.