THE FUNCTIONS OF THE BRAIN.
=General Idea of Nervous Function.=--If I begin chopping the foot of a
tree, its branches are unmoved by my act, and its leaves murmur as
peacefully as ever in the wind. If, on the contrary, I do violence to
the foot of a fellow-man, the rest of his body instantly responds to the
aggression by movements of alarm or defence. The reason of this
difference is that the man has a nervous system, whilst the tree has
none; and the function of the nervous system is to bring each part into
harmonious coöperation with every other. The afferent nerves, when
excited by some physical irritant, be this as gross in its mode of
operation as a chopping axe or as subtle as the waves of light, conveys
the excitement to the nervous centres. The commotion set up in the
centres does not stop there, but discharges through the efferent nerves,
exciting movements which vary with the animal and with the irritant
applied. These acts of response have usually the common character of
being of service. They ward off the noxious stimulus and support the
beneficial one; whilst if, in itself indifferent, the stimulus be a sign
of some distant circumstance of practical importance, the animal's acts
are addressed to this circumstance so as to avoid its perils or secure
its benefits, as the case may be. To take a common example, if I hear
the conductor calling 'All aboard!' as I enter the station, my heart
first stops, then palpitates, and my legs respond to the air-waves
falling on my tympanum by quickening their movements. If I stumble as I
run, the sensation of falling provokes a movement of the hands towards
the direction of the fall, the effect of which is to shield the body
from too sudden a shock. If a cinder enter my eye, its lids close
forcibly and a copious flow of tears tends to wash it out.
These three responses to a sensational stimulus differ, however, in many
respects. The closure of the eye and the lachrymation are quite
involuntary, and so is the disturbance of the heart. Such involuntary
responses we know as 'reflex' acts. The motion of the arms to break the
shock of falling may also be called reflex, since it occurs too quickly
to be deliberately intended. It is, at any rate, less automatic than the
previous acts, for a man might by conscious effort learn to perform it
more skilfully, or even to suppress it altogether. Actions of this kind,
into which instinct and volition enter upon equal terms, have been
called 'semi-reflex.' The act of running towards the train, on the other
hand, has no instinctive element about it. It is purely the result of
education, and is preceded by a consciousness of the purpose to be
attained and a distinct mandate of the will. It is a 'voluntary act.'
Thus the animal's reflex and voluntary performances shade into each
other gradually, being connected by acts which may often occur
automatically, but may also be modified by conscious intelligence.
=The Frog's Nerve-centres.=--Let us now look a little more closely at what
goes on.
The best way to enter the subject will be to take a lower creature, like
a frog, and study by the vivisectional method the functions of his
different nerve-centres. The frog's nerve-centres are figured in the
diagram over the page, which needs no further explanation. I shall first
proceed to state what happens when various amounts of the anterior parts
are removed, in different frogs, in the way in which an ordinary student
removes them--that is, with no extreme precautions as to the purity of
the operation.
If, then, we reduce the frog's nervous system to the spinal cord alone,
by making a section behind the base of the skull, between the spinal
cord and the medulla oblongata, thereby cutting off the brain from all
connection with the rest of the body, the frog will still continue to
live, but with a very peculiarly modified activity. It ceases to breathe
or swallow; it lies flat on its belly, and does not, like a normal frog,
sit up on its forepaws, though its hind-legs are kept, as usual, folded
against its body and immediately resume this position if drawn out. If
thrown on its back it lies there quietly, without turning over like a
normal frog. Locomotion and voice seem entirely abolished. If we suspend
it by the nose, and irritate different portions of its skin by acid, it
performs a set of remarkable 'defensive' movements calculated to wipe
away the irritant. Thus, if the breast be touched, both fore-paws will
rub it vigorously; if we touch the outer side of the elbow, the
hind-foot of the same side will rise directly to the spot and wipe it.
The back of the foot will rub the knee if that be attacked, whilst if
the foot be cut away, the stump will make ineffectual movements, and
then, in many frogs, a pause will come, as if for deliberation,
succeeded by a rapid passage of the opposite unmutilated foot to the
acidulated spot.
[Illustration:
FIG. 39.--_C_, _H_, cerebral hemispheres; _O Th_, optic thalami; _O
L_, optic lobes; _Cb_, cerebellum; _M O_, medulla oblongata; _S C_,
spinal cord.
]
The most striking character of all these movements, after their
teleological appropriateness, is their precision. They vary, in
sensitive frogs and with a proper amount of irritation, so little as
almost to resemble in their machine-like regularity the performances of
a jumping-jack, whose legs must twitch whenever you pull the string. The
spinal cord of the frog thus contains arrangements of cells and fibres
fitted to convert skin-irritations into movements of defence. We may
call it the _centre for defensive movements_ in this animal. We may
indeed go farther than this, and by cutting the spinal cord in various
places find that its separate segments are independent mechanisms, for
appropriate activities of the head and of the arms and legs
respectively. The segment governing the arms is especially active, in
male frogs, in the breeding season; and these members alone, with the
breast and back appertaining to them, and everything else cut away, will
actively grasp a finger placed between them and remain hanging to it for
a considerable time.
Similarly of the medulla oblongata, optic lobes, and other centres
between the spinal cord and the hemispheres of the frog. Each of them is
proved by experiment to contain a mechanism for the accurate execution,
in response to definite stimuli, of certain special acts. Thus with the
medulla the animal swallows; with the medulla and cerebellum together he
jumps, swims, and turns over from his back; with his optic lobes he
croaks when pinched; etc. _A frog which has lost his cerebral
hemispheres alone is by an unpractised observer indistinguishable from a
normal animal._
Not only is he capable, on proper instigation, of all the acts already
mentioned, but he guides himself by sight, so that if an obstacle be set
up between him and the light, and he be forced to move forward, he
either jumps over it or swerves to one side. He manifests the sexual
instinct at the proper seasons, and discriminates between male and
female individuals of his own species. He is, in short, so similar in
every respect to a normal frog that it would take a person very familiar
with these animals to suspect anything wrong or wanting about him; but
even then such a person would soon remark the almost entire absence of
spontaneous motion--that is, motion unprovoked by any present incitation
of sense. The continued movements of swimming, performed by the creature
in the water, seem to be the fatal result of the contact of that fluid
with its skin. They cease when a stick, for example, touches his hands.
This is a sensible irritant towards which the feet are automatically
drawn by reflex action, and on which the animal remains sitting. He
manifests no hunger, and will suffer a fly to crawl over his nose
unsnapped at. Fear, too, seems to have deserted him. In a word, he is an
extremely complex machine whose actions, so far as they go, tend to
self-preservation; but still a _machine_, in this sense--that it seems
to contain no incalculable element. By applying the right sensory
stimulus to him we are almost as certain of getting a fixed response as
an organist is of hearing a certain tone when he pulls out a certain
stop.
_But now if to the lower centres we add the cerebral hemispheres_, or
if, in other words, we make an intact animal the subject of our
observations, all this is changed. In addition to the previous responses
to present incitements of sense, our frog now goes through long and
complex acts of locomotion _spontaneously_, or as if moved by what in
ourselves we should call an idea. His reactions to outward stimuli vary
their form, too. Instead of making simple defensive movements with his
hind-legs, like a headless frog, if touched; or of giving one or two
leaps and then sitting still like a hemisphereless one, he makes
persistent and varied efforts of escape, as if, not the mere contact of
the physiologist's hand, but the notion of danger suggested by it were
now his spur. Led by the feeling of hunger, too, he goes in search of
insects, fish, or smaller frogs, and varies his procedure with each
species of victim. The physiologist cannot by manipulating him elicit
croaking, crawling up a board, swimming or stopping, at will. His
conduct has become incalculable--we can no longer foretell it exactly.
Effort to escape is his dominant reaction, but he _may_ do anything
else, even swell up and become perfectly passive in our hands.
* * * * *
Such are the phenomena commonly observed, and such the impressions which
one naturally receives. Certain general conclusions follow irresistibly.
First of all the following:
_The acts of all the centres involve the use of the same muscles._ When
a brainless frog's hind-leg wipes the acid, he calls into play all the
leg-muscles which a frog with his full medulla oblongata and cerebellum
uses when he turns from his back to his belly. Their contractions are,
however, _combined_ differently in the two cases, so that the results
vary widely. We must consequently conclude that specific arrangements of
cells and fibres exist in the cord for wiping, in the medulla for
turning over, etc. Similarly they exist in the thalami for jumping over
seen obstacles and for balancing the moved body; in the optic lobes for
creeping backwards, or what not. But in the hemispheres, since the
presence of these organs _brings no new elementary form of movement_
with it, but only _determines differently the occasions_ on which the
movements shall occur, making the usual stimuli less fatal and
machine-like, we need suppose no such machinery _directly_ coördinative
of muscular contractions to exist. We may rather assume, when the
mandate for a wiping-movement is sent forth by the hemispheres, that a
current goes straight to the wiping-arrangement in the spinal cord,
exciting this arrangement as a whole. Similarly, if an intact frog
wishes to jump, all he need do is to excite from the hemispheres the
jumping-centre in the thalami or wherever it may be, and the latter will
provide for the details of the execution. It is like a general ordering
a colonel to make a certain movement, but not telling him how it shall
be done.
_The same muscle, then, is repeatedly represented at different heights_;
and at each it enters into a different combination with other muscles to
coöperate in some special form of concerted movement. At each height the
movement is discharged by some particular form of sensorial stimulus,
whilst the stimuli which discharge the hemispheres would seem not so
much to be elementary sorts of sensation, as groups of sensations
forming determinate _objects_ or _things_.
=The Pigeon's Lower Centres.=--The results are just the same if, instead
of a frog, we take a pigeon, cut out his hemispheres carefully and wait
till he recovers from the operation. There is not a movement natural to
him which this brainless bird cannot execute; he seems, too, after some
days to execute movements from some inner irritation, for he moves
spontaneously. But his emotions and instincts exist no longer. In
Schrader's striking words:
"The hemisphereless animal moves in a world of bodies which ... are all
of equal value for him.... He is, to use Goltz's apt expression,
_impersonal_.... Every object is for him only a space-occupying mass, he
turns out of his path for an ordinary pigeon no otherwise than for a
stone. He may try to climb over both. All authors agree that they never
found any difference, whether it was an inanimate body, a cat, a dog, or
a bird of prey which came in their pigeon's way. The creature knows
neither friends nor enemies, in the thickest company it lives like a
hermit. The languishing cooing of the male awakens no more impression
than the rattling of the peas, or the call-whistle which in the days
before the injury used to make the birds hasten to be fed. Quite as
little as the earlier observers have I seen hemisphereless she-birds
answer the courting of the male. A hemisphereless male will coo all day
long and show distinct signs of sexual excitement, but his activity is
without any object, it is entirely indifferent to him whether the
she-bird be there or not. If one is placed near him, he leaves her
unnoticed.... As the male pays no attention to the female, so she pays
none to her young. The brood may follow the mother ceaselessly calling
for food, but they might as well ask it from a stone.... The
hemisphereless pigeon is in the highest degree tame, and fears man as
little as cat or bird of prey."
=General Notion of Hemispheres.=--All these facts lead us, when we try to
formulate them broadly, to some such conception as this: _The lower
centres act from present sensational stimuli alone; the hemispheres act
from considerations_, the sensations which they may receive serving only
as suggesters of these. But what are considerations but expectations, in
the fancy, of sensations which will be felt one way or another according
as action takes this course or that? If I step aside on seeing a
rattlesnake, from considering how dangerous an animal he is, the mental
materials which constitute my prudential reflection are images more or
less vivid of the movement of his head, of a sudden pain in my leg, of a
state of terror, a swelling of the limb, a chill, delirium, death, etc.,
etc., and the ruin of my hopes. But all these images are constructed out
of my past experiences. They are _reproductions_ of what I have felt or
witnessed. They are, in short, _remote_ sensations; and the main
difference between the hemisphereless animal and the whole one may be
concisely expressed by saying that _the one obeys absent, the other only
present, objects_.
_The hemispheres would then seem to be the chief seat of memory._
Vestiges of past experience must in some way be stored up in them, and
must, when aroused by present stimuli, first appear as representations
of distant goods and evils; and then must discharge into the appropriate
motor channels for warding off the evil and securing the benefits of the
good. If we liken the nervous currents to electric currents, we can
compare the nervous system, _C_, below the hemispheres to a direct
circuit from sense-organ to muscle along the line _S ...C ...M_ of Fig.
40. The hemisphere, _H_, adds the long circuit or loop-line through
which the current may pass when for any reason the direct line is not
used.
[Illustration: FIG. 40.]
Thus, a tired wayfarer on a hot day throws himself on the damp earth
beneath a maple-tree. The sensations of delicious rest and coolness
pouring themselves through the direct line would naturally discharge
into the muscles of complete extension: he would abandon himself to the
dangerous repose. But the loop-line being open, part of the current is
drafted along it, and awakens rheumatic or catarrhal reminiscences,
which prevail over the instigations of sense, and make the man arise and
pursue his way to where he may enjoy his rest more safely. Presently we
shall examine the manner in which the hemispheric loop-line may be
supposed to serve as a reservoir for such reminiscences as these.
Meanwhile I will ask the reader to notice some corollaries of its being
such a reservoir.
First, no animal without it can deliberate, pause, postpone, nicely
weigh one motive against another, or compare. Prudence, in a word, is
for such a creature an impossible virtue. Accordingly we see that nature
removes those functions in the exercise of which prudence is a virtue
from the lower centres and hands them over to the cerebrum. Wherever a
creature has to deal with complex features of the environment, prudence
is a virtue. The higher animals have so to deal; and the more complex
the features, the higher we call the animals. The fewer of his acts,
then, can _such_ an animal perform without the help of the organs in
question. In the frog many acts devolve wholly on the lower centres; in
the bird fewer; in the rodent fewer still; in the dog very few indeed;
and in apes and men hardly any at all.
The advantages of this are obvious. Take the prehension of food as an
example and suppose it to be a reflex performance of the lower centres.
The animal will be condemned fatally and irresistibly to snap at it
whenever presented, no matter what the circumstances may be; he can no
more disobey this prompting than water can refuse to boil when a fire is
kindled under the pot. His life will again and again pay the forfeit of
his gluttony. Exposure to retaliation, to other enemies, to traps, to
poisons, to the dangers of repletion, must be regular parts of his
existence. His lack of all thought by which to weigh the danger against
the attractiveness of the bait, and of all volition to remain hungry a
little while longer, is the direct measure of his lowness in the mental
scale. And those fishes which, like our cunners and sculpins, are no
sooner thrown back from the hook into the water than they automatically
seize the hook again, would soon expiate the degradation of their
intelligence by the extinction of their type, did not their
extraordinary fecundity atone for their imprudence. Appetite and the
acts it prompts have consequently become in all higher vertebrates
functions of the cerebrum. They disappear when the physiologist's knife
has left the subordinate centres alone in place. The brainless pigeon
will starve though left on a corn-heap.
Take again the sexual function. In birds this devolves exclusively upon
the hemispheres. When these are shorn away the pigeon pays no attention
to the billings and cooings of its mate. It is the same, according to
Goltz, with male dogs who have suffered large losses of cerebral tissue.
Those who have read Darwin's Descent of Man will recollect what an
importance this author ascribes to the agency of sexual selection in the
amelioration of the breeds of birds. The females are naturally coy, and
their coyness must be overcome by the exhibition of the gorgeous
plumage, and various accomplishments in the way of strutting and
fighting, of the males. In frogs and toads, on the other hand, where (as
we saw on page 94) the sexual instinct devolves upon the lower centres,
we find a machine-like obedience to the present incitements of sense,
and an almost total exclusion of the power of choice. The consequence is
that every spring an immense waste of batrachian life, involving numbers
of adult animals and innumerable eggs, takes place from no other cause
than the blind character of the sexual impulse in these creatures.
No one need be told how dependent all human social elevation is upon the
prevalence of chastity. Hardly any factor measures more than this the
difference between civilization and barbarism. Physiologically
interpreted, chastity means nothing more than the fact that present
solicitations of sense are overpowered by suggestions of æsthetic and
moral fitness which the circumstances awaken in the cerebrum; and that
upon the inhibitory or permissive influence of these alone action
directly depends.
Within the psychic life due to the cerebrum itself the same general
distinction obtains, between considerations of the more immediate and
considerations of the more remote. In all ages the man whose
determinations are swayed by reference to the most distant ends has been
held to possess the highest intelligence. The tramp who lives from hour
to hour; the bohemian whose engagements are from day to day; the
bachelor who builds but for a single life; the father who acts for
another generation; the patriot who thinks of a whole community and many
generations; and, finally, the philosopher and saint whose cares are for
humanity and for eternity,--these range themselves in an unbroken
hierarchy, wherein each successive grade results from an increased
manifestation of the special form of action by which the cerebral
centres are distinguished from all below them.
=The Automaton-Theory.=--In the 'loop-line' along which the memories and
ideas of the distant are supposed to lie, the action, so far as it is a
physical process, must be interpreted after the type of the action in
the lower centres. If regarded here as a reflex process, it must be
reflex there as well. The current in both places runs out into the
muscles only after it has first run in; but whilst the path by which it
runs out is determined in the lower centres by reflections few and fixed
amongst the cell-arrangements, in the hemispheres the reflections are
many and instable. This, it will be seen, is only a difference of degree
and not of kind, and does not change the reflex type. The conception of
_all_ action as conforming to this type is the fundamental conception of
modern nerve-physiology. This conception, now, has led to two quite
opposite theories about the relation to consciousness of the nervous
functions. Some authors, finding that the higher voluntary functions
seem to require the guidance of feeling, conclude that over the lowest
reflexes some such feeling also presides, though it may be a feeling
connected with the spinal cord, of which the higher conscious self
connected with the hemispheres remains unconscious. Others, finding
that reflex and semi-automatic acts may, notwithstanding their
appropriateness, take place with an unconsciousness apparently complete,
fly to the opposite extreme and maintain that the appropriateness even
of the higher voluntary actions connected with the hemispheres owes
nothing to the fact that consciousness attends them. They are, according
to these writers, results of physiological mechanism pure and simple.
To comprehend completely this latter doctrine one should apply it to
examples. The movements of our tongues and pens, the flashings of our
eyes in conversation, are of course events of a physiological order, and
as such their causal antecedents may be exclusively mechanical. If we
knew thoroughly the nervous system of Shakespeare, and as thoroughly all
his environing conditions, we should be able, according to the theory of
automatism, to show why at a given period of his life his hand came to
trace on certain sheets of paper those crabbed little black marks which
we for shortness' sake call the manuscript of Hamlet. We should
understand the rationale of every erasure and alteration therein, and we
should understand all this without in the slightest degree acknowledging
the existence of the thoughts in Shakespeare's mind. The words and
sentences would be taken, not as signs of anything beyond themselves,
but as little outward facts, pure and simple. In like manner, the
automaton-theory affirms, we might exhaustively write the biography of
those two hundred pounds, more or less, of warmish albuminoid matter
called Martin Luther, without ever implying that it felt.
But, on the other hand, nothing in all this could prevent us from giving
an equally complete account of either Luther's or Shakespeare's
spiritual history, an account in which every gleam of thought and
emotion should find its place. The mind-history would run alongside of
the body-history of each man, and each point in the one would correspond
to, but not react upon, a point in the other. So the melody floats from
the harp-string, but neither checks nor quickens its vibrations; so the
shadow runs alongside the pedestrian, but in no way influences his
steps.
As a mere _conception_, and so long as we confine our view to the
nervous centres themselves, few things are more seductive than this
radically mechanical theory of their action. And yet our consciousness
_is there_, and has in all probability been evolved, like all other
functions, for a use--it is to the highest degree improbable _a priori_
that it should have no use. Its use _seems_ to be that of _selection_;
but to select, it must be efficacious. States of consciousness which
feel right are held fast to; those which feel wrong are checked. If the
'holding' and the 'checking' of the conscious states severally mean also
the efficacious reinforcing or inhibiting of the correlated neural
processes, then it would seem as if the presence of the states of mind
might help to steer the nervous system and keep it in the path which to
the consciousness seemed best. Now on the average what seems best to
consciousness is really best for the creature. It is a well-known fact
that pleasures are generally associated with beneficial, pains with
detrimental, experiences. All the fundamental vital processes illustrate
this law. Starvation; suffocation; privation of food, drink, and sleep;
work when exhausted; burns, wounds, inflammation; the effects of poison,
are as disagreeable as filling the hungry stomach, enjoying rest and
sleep after fatigue, exercise after rest, and a sound skin and unbroken
bones at all times, are pleasant. Mr. Spencer and others have suggested
that these coincidences are due, not to any preëstablished harmony, but
to the mere action of natural selection, which would certainly kill off
in the long-run any breed of creatures to whom the fundamentally noxious
experience seemed enjoyable. An animal that should take pleasure in a
feeling of suffocation would, if that pleasure were efficacious enough
to make him keep his head under water, enjoy a longevity of four or five
minutes. But if conscious pleasure does not reinforce, and conscious
pain does not inhibit, anything, one does not see (without some such _a
priori_ rational harmony as would be scouted by the 'scientific'
champions of the automaton-theory) why the most noxious acts, such as
burning, might not with perfect impunity give thrills of delight, and
the most necessary ones, such as breathing, cause agony. The only
considerable attempt that has been made to explain the _distribution_ of
our feelings is that of Mr. Grant Allen in his suggestive little work,
_Physiological Æsthetics_; and his reasoning is based exclusively on
that causal efficacy of pleasures and pains which the partisans of pure
automatism so strenuously deny.
Probability and circumstantial evidence thus run dead against the theory
that our actions are _purely_ mechanical in their causation. From the
point of view of descriptive Psychology (even though we be bound to
assume, as on p. 6, that all our feelings have brain-processes for their
condition of existence, and can be remotely traced in every instance to
currents coming from the outer world) we have no clear reason to doubt
that the feelings may react so as to further or to dampen the processes
to which they are due. I shall therefore not hesitate in the course of
this book to use the language of common-sense. I shall talk as if
consciousness kept actively pressing the nerve-centres in the direction
of its own ends, and was no mere impotent and paralytic spectator of
life's game.
=The Localization of Functions in the Hemispheres.=--The hemispheres, we
lately said, must be the organ of memory, and in some way retain
vestiges of former currents, by means of which mental considerations
drawn from the past may be aroused before action takes place. The
vivisections of physiologists and the observations of physicians have of
late years given a concrete confirmation to this notion which the first
rough appearances suggest. The various convolutions have had special
functions assigned to them in relation to this and that sense-organ, as
well as to this or that portion of the muscular system. This book is no
place for going over the evidence in detail, so I will simply indicate
the conclusions which are most probable at the date of writing.
=Mental and Cerebral Elements.=--In the first place, there is a very neat
parallelism between the analysis of brain-functions by the physiologists
and that of mental functions by the 'analytic' psychologists.
The phrenological brain-doctrine divided the brain into 'organs,' each
of which stood for the man in a certain partial attitude. The organ of
'Philoprogenitiveness,' with its concomitant consciousness, is an entire
man so far as he loves children, that of 'Reverence' is an entire man
worshipping, etc. The spiritualistic psychology, in turn, divided the
Mind into 'faculties,' which were also entire mental men in certain
limited attitudes. But 'faculties' are not mental _elements_ any more
than 'organs' are brain-elements. Analysis breaks both into more
elementary constituents.
Brain and mind alike consist of simple elements, sensory and motor. "All
nervous centres," says Dr. Hughlings Jackson, "from the lowest to the
very highest (the substrata of consciousness), are made up of nothing
else than nervous arrangements, representing impressions and
movements.... I do not see of what other materials the brain _can_ be
made." Meynert represents the matter similarly when he calls the cortex
of the hemispheres the surface of projection for every muscle and every
sensitive point of the body. The muscles and the sensitive points are
_represented_ each by a cortical point, and the Brain is little more
than the sum of all these cortical points, to which, on the mental side,
as many sensations and _ideas_ correspond. The sensations and ideas of
sensation and of motion are, in turn, the elements out of which the Mind
is built according to the analytic school of psychology. The relations
between objects are explained by 'associations' between the ideas; and
the emotional and instinctive tendencies, by associations between ideas
and movements. The same diagram can symbolize both the inner and the
outer world; dots or circles standing indifferently for cells or ideas,
and lines joining them, for fibres or associations. The associationist
doctrine of 'ideas' may be doubted to be a literal expression of the
truth, but it probably will always retain a didactic usefulness. At all
events, it is interesting to see how well physiological analysis plays
into its hands. To proceed to details.
[Illustration: FIG. 41.--Left hemisphere of monkey's brain. Outer
surface.]
=The Motor Region.=--The one thing which is _perfectly_ well established
is this, that the 'central' convolutions, on either side of the fissure
of Rolando, and (at least in the monkey) the calloso-marginal
convolution (which is continuous with them on the mesial surface where
one hemisphere is applied against the other), form the region by which
all the motor incitations which leave the cortex pass out, on their way
to those executive centres in the region of the pons, medulla, and
spinal cord from which the muscular contractions are discharged in the
last resort. The existence of this so-called 'motor zone' is established
by anatomical as well as vivisectional and pathological evidence.
The accompanying figures (Figs. 41 and 42), from Schaefer and Horsley,
show the topographical arrangement of the monkey's motor zone more
clearly than any description.
[Illustration: FIG. 42.--Left hemisphere of monkey's brain. Mesial
surface.]
Fig. 43, after Starr, shows how the fibres run downwards. All sensory
currents entering the hemispheres run out from the Rolandic region,
which may thus be regarded as a sort of funnel of escape, which narrows
still more as it plunges beneath the surface, traversing the inner
capsule, pons, and parts below. The dark ellipses on the left half of
the diagram stand for hemorrhages or tumors, and the reader can easily
trace, by following the course of the fibres, what the effect of them in
interrupting motor currents may be.
[Illustration:
FIG. 43.--Schematic transverse section of the human brain, through
the rolandic region. _S_, fissure of Sylvius; _N.C._, _nucleus
candatus_, and _N.L._, _nucleus lenticularis_, of the corpus
striatum; _O.T._, thalamus; _C_, crus; _M_, medulla oblongata;
_VII_, the facial nerves passing out from their nucleus in the
region of the _pons_. The fibres passing between _O.T._ and _N.L._
constitute the so-called internal capsule.
]
One of the most instructive proofs of motor localization in the cortex
is that furnished by the disease now called aphemia, or _motor aphasia_.
Motor aphasia is neither loss of voice nor paralysis of the tongue or
lips. The patient's voice is as strong as ever, and all the innervations
of his hypoglossal and facial nerves, except those necessary for
speaking, may go on perfectly well. He can laugh and cry, and even sing;
but he either is unable to utter any words at all; or a few meaningless
stock phrases form his only speech; or else he speaks incoherently and
confusedly,
[Illustration: FIG. 44.--Schematic profile of left hemisphere, with the
parts shaded whose destruction causes motor ('Broca') and sensory
('Wernicke') aphasia.]
mispronouncing, misplacing, and misusing his words in various degrees.
Sometimes his speech is a mere broth of unintelligible syllables. In
cases of pure motor aphasia the patient recognizes his mistakes and
suffers acutely from them. Now whenever a patient dies in such a
condition as this, and an examination of his brain is permitted, it is
found that the lowest frontal gyrus (see Fig. 44) is the seat of injury.
Broca first noticed this fact in 1861, and since then the gyrus has gone
by the name of Broca's convolution. The injury in right-handed people is
found on the left hemisphere, and in left-handed people on the right
hemisphere. Most people, in fact, are left-brained, that is, all their
delicate and specialized movements are handed over to the charge of the
left hemisphere. The ordinary right-handedness for such movements is
only a consequence of that fact, a consequence which shows outwardly on
account of that extensive crossing of the fibres from the left
hemisphere to the right half of the body only, which is shown in Fig.
41, below the letter M. But the left-brainedness might exist and _not_
show outwardly. This would happen wherever organs on _both_ sides of the
body could be governed by the left hemisphere; and just such a case
seems offered by the vocal organs, in that highly delicate and special
motor service which we call speech. Either hemisphere _can_ innervate
them bilaterally, just as either seems able to innervate bilaterally the
muscles of the trunk, ribs, and diaphragm. Of the special movements of
speech, however, it would appear (from these very facts of aphasia) that
the left hemisphere in most persons habitually takes exclusive charge.
With that hemisphere thrown out of gear, speech is undone; even though
the opposite hemisphere still be there for the performance of less
specialized acts, such as the various movements required in eating.
=The visual centre= is in the _occipital lobes_. This also is proved by
all the three kinds of possible evidence. It seems that the fibres from
the _left_ halves of _both_ retinæ go to the _left_ hemisphere, those
from the right half to the right hemisphere. The consequence is that
when the right occipital lobe, for example, is injured, 'hemianopsia'
results in both eyes, that is, both retinæ grow blind as to their right
halves, and the patient loses the leftward half of his field of view.
The diagram on p. 111 will make this matter clear (see Fig. 45).
Quite recently, both Schaefer and Munk, in studying the movements of the
eyeball produced by galvanizing the visual cortex in monkeys and dogs,
have found reason to plot out an analogous correspondence between the
upper and lower portions of the retinæ and certain parts of the visual
cortex. If both occipital lobes were destroyed, we should have double
hemiopia, or, in other words, total blindness. In human hemiopic
blindness there is insensibility to light on one half of the field of
view, but
[Illustration:
FIG. 45.--Scheme of the mechanism of vision, after Seguin. The
_cuneus_ convolution (_Cu_) of the right occipital lobe is supposed
to be injured, and all the parts which lead to it are darkly shaded
to show that they fail to exert their function. _F.O._ are the
intra-hemispheric optical fibres. _P.O.C._ is the region of the
lower optic centres (corpora geniculata and quadrigemina). _T.O.D._
is the right optic tract; _C_, the chiasma; _F.L.D._ are the fibres
going to the lateral or temporal half _T_ of the right retina, and
_F.C.S._ are those going to the central or nasal half of the left
retina. _O.D._ is the right, and _O.S._ the left, eyeball. The
rightward half of each is therefore blind; in other words, the
right nasal field, _R.N.F._, and the left temporal field, _L.T.F._,
have become invisible to the subject with the lesion at _Cu_.
]
mental images of visible things remain. In _double_ hemiopia there is
every reason to believe that not only the sensation of light must go,
but that all memories and images of a visual order must be annihilated
also. The man loses his visual 'ideas.' Only 'cortical' blindness can
produce this effect on the ideas. Destruction of the retinæ or of the
visual tracts anywhere between the cortex and the eyes impairs the
retinal sensibility to light, but not the power of visual imagination.
[Illustration: FIG. 46.--Fibres associating the cortical centres
together. (Schematic, after Starr.)]
=Mental Blindness.=--A most interesting effect of cortical disorder is
_mental blindness_. This consists not so much in insensibility to
optical impressions, as in _inability to understand them_.
Psychologically it is interpretable as _loss of associations_ between
optical sensations and what they signify; and any interruption of the
paths between the optic centres and the centres for other ideas ought to
bring it about. Thus, printed letters of the alphabet, or words, signify
both certain sounds and certain articulatory movements. But the
connection between the articulating or auditory centres and those for
sight being ruptured, we ought _a priori_ to expect that the sight of
words would fail to awaken the idea of their sound, or of the movement
for pronouncing them. We ought, in short, to have _alexia_, or inability
to read: and this is just what we do have as a complication of _aphasic_
disease in many cases of extensive injury about the fronto-temporal
regions.
Where an object fails to be recognized by sight, it often happens that
the patient will recognize and name it as soon as he touches it with his
hand. This shows in an interesting way how numerous are the incoming
paths which all end by running out of the brain through the channel of
speech. The hand-path is open, though the eye-path be closed. When
mental blindness is most complete, neither sight, touch, nor sound
avails to steer the patient, and a sort of dementia which has been
called _asymbolia_ or _apraxia_ is the result. The commonest articles
are not understood. The patient will put his breeches on one shoulder
and his hat upon the other, will bite into the soap and lay his shoes on
the table, or take his food into his hand and throw it down again, not
knowing what to do with it, etc. Such disorder can only come from
extensive brain-injury.
=The centre for hearing= is situated in man in the upper convolution of
the temporal lobe (see the part marked 'Wernicke' in Fig. 44). The
phenomena of aphasia show this. We studied motor aphasia a few pages
back; we must now consider _sensory aphasia_. Our knowledge of aphasia
has had three stages: we may talk of the period of Broca, the period of
Wernicke, and the period of Charcot. What Broca's discovery was we have
seen. Wernicke was the first to discriminate those cases in which the
patient can _not even understand_ speech from those in which he can
understand, only not talk; and to ascribe the former condition to lesion
of the temporal lobe. The condition in question is _word-deafness_, and
the disease is _auditory aphasia_. The latest statistical survey of the
subject is that by Dr. Allen Starr. In the seven cases of _pure_
word-deafness which he has collected (cases in which the patient could
read, talk, and write, but not understand what was said to him), the
lesion was limited to the first and second temporal convolutions in
their posterior two thirds. The lesion (in right-handed, i.e.
left-brained, persons) is always on the left side, like the lesion in
motor aphasia. Crude hearing would not be abolished, even were the left
centre for it utterly destroyed; the right centre would still provide
for that. But the _linguistic use_ of hearing appears bound up with the
integrity of the left centre more or less exclusively. Here it must be
that words heard enter into association with the things which they
represent, on the one hand, and with the movements necessary for
pronouncing them, on the other. In most of us (as Wernicke said) speech
must go on from auditory cues; that is, our visual, tactile, and other
ideas probably do not innervate our motor centres directly, but only
after first arousing the mental sound of the words. This is the
immediate stimulus to articulation; and where the possibility of this is
abolished by the destruction of its usual channel in the left temporal
lobe, the articulation must suffer. In the few cases in which the
channel is abolished with no bad effect on speech we must suppose an
idiosyncrasy. The patient must innervate his speech-organs either from
the corresponding portion of the other hemisphere or directly from the
centres of vision, touch, etc., without leaning on the auditory region.
It is the minuter analysis of such individual differences as these which
constitutes Charcot's contribution towards clearing up the subject.
Every namable thing has numerous properties, qualities, or aspects. In
our minds the properties together with the name form an associated
group. If different parts of the brain are severally concerned with the
several properties, and a farther part with the hearing, and still
another with the uttering, of the name, there must inevitably be brought
about (through the law of association which we shall later study) such a
connection amongst all these brain-parts that the activity of any one of
them will be likely to awaken the activity of all the rest. When we are
talking whilst we think, the _ultimate_ process is utterance. If the
brain-part for _that_ be injured, speech is impossible or disorderly,
even though all the other brain-parts be intact: and this is just the
condition of things which, on p. 109, we found to be brought about by
lesion of the convolution of Broca. But back of that last act various
orders of succession are possible in the associations of a talking man's
ideas. The more usual order is, as aforesaid, from the tactile, visual,
or other properties of the things thought-about to the sound of their
names, and then to the latter's utterance. But if in a certain
individual's mind the _look_ of an object or the _look_ of its name be
what habitually precedes articulation, then the loss of the _hearing_
centre will _pro tanto_ not affect that individual's speech or reading.
He will be mentally deaf, i.e. his _understanding_ of the human voice
will suffer, but he will not be aphasic. In this way it is possible to
explain the seven cases of word-deafness without motor aphasia which
figure in Dr. Starr's table.
If this order of association be ingrained and habitual in that
individual, injury to his _visual_ centres will make him not only
word-blind, but aphasic as well. His speech will become confused in
consequence of an occipital lesion. Naunyn, consequently, plotting out
on a diagram of the hemisphere the 71 irreproachably reported cases of
aphasia which he was able to collect, finds that the lesions concentrate
themselves in three places: first, on Broca's centre; second, on
Wernicke's; third, on the supra-marginal and angular convolutions under
which those fibres pass which connect the visual centres with the rest
of the brain (see Fig. 47, p. 116). With this result Dr. Starr's
analysis of purely sensory cases agrees.
In the chapter on Imagination we shall return to these differences in
the sensory spheres of different individuals. Meanwhile few things show
more beautifully than the history of our knowledge of aphasia how the
sagacity and patience of many banded workers are in time certain to
analyze the darkest confusion into an orderly display. There is no
'organ' of Speech in the brain any more than there is a 'faculty' of
Speech in the mind. The entire mind and the entire brain are more or
less at work in a man who uses language. The subjoined diagram, from
Ross, shows the four parts most vitally concerned, and, in the light of
our text, needs no farther explanation (see Fig. 48, p. 117).
[Illustration: FIG. 47.]
=Centres for Smell, Taste, and Touch.=--The other sensory centres are less
definitely made out. Of smell and taste I will say nothing; and of
muscular and cutaneous feeling only this, that it seems most probably
seated in the motor zone, and possibly in the convolutions immediately
backwards and midwards thereof. The incoming tactile currents must enter
the cells of this region by one set of fibres, and the discharges leave
them by another, but of these
[Illustration: FIG. 48.--_A_ is the auditory centre, _V_ the visual, _W_
the writing, and _E_ that for speech.]
=Conclusion.=--We thus see the postulate of Meynert and Jackson, with
which we started on p. 105, to be on the whole most satisfactorily
corroborated by objective research. _The highest centres do probably
contain nothing but arrangements for representing impressions and
movements, and other arrangements for coupling the activity of these
arrangements together._ Currents pouring in from the sense-organs first
excite some arrangements, which in turn excite others, until at last a
discharge downwards of some sort occurs. When this is once clearly
grasped there remains little ground for asking whether the motor zone is
exclusively motor, or sensitive as well. The whole cortex, inasmuch as
currents run through it, is both. All the currents probably have
feelings going with them, and sooner or later bring movements about. In
one aspect, then, every centre is afferent, in another efferent, even
the motor cells of the spinal cord having these two aspects inseparably
conjoined. Marique, and Exner and Paneth have shown that by cutting
_round_ a 'motor' centre and so separating it from the influence of the
rest of the cortex, the same disorders are produced as by cutting it
out, so that it is really just what I called it, only the funnel through
which the stream of innervation, starting from elsewhere, escapes;
_consciousness accompanying the stream, and being mainly of things seen
if the stream is strongest occipitally, of things heard if it is
strongest temporally, of things felt, etc., if the stream occupies most
intensely the 'motor zone.'_ It seems to me that some broad and vague
formulation like this is as much as we can safely venture on in the
present state of science--so much at least is not likely to be
overturned. But it is obvious how little this tells us of the detail of
what goes on in the brain when a certain thought is before the mind. The
general forms of relation perceived between things, as their identities,
likenesses, or contrasts; the forms of the consciousness itself, as
effortless or perplexed, attentive or inattentive, pleasant or
disagreeable; the phenomena of interest and selection, etc., etc., are
all lumped together as effects correlated with the currents that connect
one centre with another. Nothing can be more vague than such a formula.
Moreover certain portions of the brain, as the lower frontal lobes,
escape formulational together. Their destruction gives rise to no local
trouble of either motion or sensibility in dogs, and in monkeys neither
stimulation nor excision of these lobes produces any symptoms whatever.
One monkey of Horsley and Schaefer's was as tame, and did certain tricks
as well, after as before the operation.
It is in short obvious that our knowledge of our mental states
infinitely exceeds our knowledge of their concomitant cerebral
conditions. Without introspective analysis of the mental elements of
speech, the doctrine of Aphasia, for instance, which is the most
brilliant jewel in Physiology, would have been utterly impossible. Our
assumption, therefore (p. 5), that mind-states are absolutely dependent
on brain-conditions, must still be understood as a mere postulate. We
may have a general faith that it must be true, but any exact insight as
to _how_ it is true lags wofully behind.
Before taking up the study of conscious states properly so called, I
will in a separate chapter speak of two or three aspects of
brain-function which have a general importance and which coöperate in
the production of all our mental states.