of the nerves is composed which subserve sensation. You
must conceive that these little threads, being inclosed
in tubes, which are always distended and kept open by
the animal spirits which they contain, neither press
upon nor interfere with one another, and are extended
from the brain to the extremities of all the members
which are sensitive—in such a manner, that the slightest
touch which excites the part of one of the members to
which a thread is attached, gives rise to a motion of
the part of the brain whence it arises, just as by
pulling one of the ends of a stretched cord, the other
end is instantaneously moved.... And we must take care
not to imagine that, in order to feel, the soul needs
to behold certain images sent by the objects of sense
to the brain, as our philosophers commonly suppose;
or, at least, we must conceive these images to be
something quite different from what they suppose them
to be. For, as all they suppose is that these images
ought to resemble the objects which they represent, it
is impossible for them to show how they can be formed by
the objects received by the organs of the external senses
and transmitted to the brain. And they have had no reason
for supposing the existence of these images except this;
seeing that the mind is readily excited by a picture to
conceive the object which is depicted, they have thought
that it must be excited in the same way to conceive those
objects which affect our senses by little pictures of
them formed in the head; instead of which we ought to
recollect that there are many things besides images which
may excite the mind, as, for example, signs and words,
which have not the least resemblance to the objects which
they signify.”[44]
Modern physiology amends Descartes’ conception of the mode of action of
sensory nerves in detail, by showing that their structure is the same
as that of motor nerves; and that the changes which take place in them,
when the sensory organs with which they are connected are excited, are
of just the same nature as those which occur in motor nerves, when the
muscles to which they are distributed are made to contract: there is a
molecular change which, in the case of the sensory nerve, is propagated
towards the brain. But the great fact insisted upon by Descartes, that
no likeness of external things is, or can be, transmitted to the mind
by the sensory organs; on the contrary, that, between the external
cause of a sensation and the sensation, there is interposed a mode
of motion of nervous matter, of which the state of consciousness is
no likeness, but a mere symbol, is of the profoundest importance. It
is the physiological foundation of the doctrine of the relativity
of knowledge, and a more or less complete idealism is a necessary
consequence of it.
For of two alternatives one must be true. Either consciousness is
the function of a something distinct from the brain, which we call
the soul, and a sensation is the mode in which this soul is affected
by the motion of a part of the brain; or there is no soul, and a
sensation is something generated by the mode of motion of a part of
the brain. In the former case, the phenomena of the senses are purely
spiritual affections; in the latter, they are something manufactured
by the mechanism of the body, and as unlike the causes which set that
mechanism in motion, as the sound of a repeater is unlike the pushing
of the spring which gives rise to it.
The nervous system stands between consciousness and the assumed
external world, as an interpreter who can talk with his fingers stands
between a hidden speaker and a man who is stone deaf—and Realism is
equivalent to a belief on the part of the deaf man, that the speaker
must also be talking with his fingers. “Les extrêmes se touchent;” the
shibboleth of materialists that “thought is a secretion of the brain,”
is the Fichtean doctrine that “the phenomenal universe is the creation
of the Ego,” expressed in other language.
IV. _The motion of the matter of a sensory nerve may
be transmitted through the brain to motor nerves, and
thereby give rise to contraction of the muscles to which
these motor nerves are distributed; and this reflection
of motion from a sensory into a motor nerve may take
place without volition, or even contrary to it._
In stating these important truths, Descartes defined that which we now
term “reflex action.” Indeed he almost uses the term itself, as he
talks of the “animal spirits” as “réfléchis,”[45] from the sensory into
the motor nerves. And that this use of the word “reflected” was no mere
accident, but that the importance and appropriateness of the idea it
suggests was fully understood by Descartes’ contemporaries, is apparent
from a passage in Willis’s well-known essay, “De Animâ Brutorum,”
published in 1672, in which, in giving an account of Descartes’ views,
he speaks of the animal spirits being diverted into motor channels,
“velut undulatione reflexâ.”[46]
Nothing can be clearer in statement, or in illustration, than the view
of reflex action which Descartes gives in the “Passions de l’Âme,” Art.
xiii.
After recapitulating the manner in which sensory impressions
transmitted by the sensory nerves to the brain give rise to sensation,
he proceeds:—
“And in addition to the different feelings excited in the
soul by these different motions of the brain, the animal
spirits, without the intervention of the soul, may take
their course towards certain muscles, rather than towards
others, and thus move the limbs, as I shall prove by
an example. If some one moves his hand rapidly towards
our eyes, as if he were going to strike us, although
we know that he is a friend, that he does it only in
jest, and that he will be very careful to do us no harm,
nevertheless it will be hard to keep from winking. And
this shows, that it is not by the agency of the soul
that the eyes shut, since this action is contrary to
that volition which is the only, or at least the chief,
function of the soul; but it is because the mechanism
of our body is so disposed, that the motion of the hand
towards our eyes excites another movement in our brain,
and this sends the animal spirits into those muscles
which cause the eyelids to close.”
Since Descartes’ time, experiment has eminently enlarged our knowledge
of the details of reflex action. The discovery of Bell has enabled us
to follow the tracks of the sensory and motor impulses, along distinct
bundles of nerve fibres; and the spinal cord, apart from the brain, has
been proved to be a great centre of reflex action; but the fundamental
conception remains as Descartes left it, and it is one of the pillars
of nerve physiology at the present day.
V. _The motion of any given portion of the matter of the
brain excited by the motion of a sensory nerve, leaves
behind a readiness to be moved in the same way, in that
part. Anything which resuscitates the motion gives
rise to the appropriate feeling. This is the physical
mechanism of memory_.
Descartes imagined that the pineal body (a curious appendage to the
upper side of the brain, the function of which, if it have any, is
wholly unknown) was the instrument through which the soul received
impressions from, and communicated them to, the brain. And he thus
endeavours to explain what happens when one tries to recollect
something:—
“Thus when the soul wills to remember anything, this
volition, causing the [pineal] gland to incline itself
in different directions, drives the [animal] spirits
towards different regions of the brain, until they reach
that part in which are the traces, which the object
which it desires to remember has left. These traces are
produced thus: those pores of the brain through which the
[animal] spirits have previously been driven, by reason
of the presence of the object, have thereby acquired a
tendency to be opened by the animal spirits which return
towards them, more easily than other pores, so that the
animal spirits, impinging on these pores, enter them
more readily than others. By this means they excite a
particular movement in the pineal gland, which represents
the object to the soul, and causes it to know what it is
which it desired to recollect.”[47]
That memory is dependent upon some condition of the brain is a fact
established by many considerations—among the most important of which
are the remarkable phenomena of aphasia. And that the condition of the
brain on which memory depends, is largely determined by the repeated
occurrence of that condition of its molecules, which gives rise to the
idea of the thing remembered, is no less certain.
Every boy who learns his lesson by repeating it exemplifies the fact.
Descartes, as we have seen, supposes that the pores of a given part
of the brain are stretched by the animal spirits, on the occurrence
of a sensation, and that the part of the brain thus stretched, being
imperfectly elastic, does not return to exactly its previous condition,
but remains more distensible than it was before. Hartley supposes
that the vibrations, excited by a sensory, or other, impression,
do not die away, but are represented by smaller vibrations or
“vibratiuncules,” the permanency and intensity of which are in relation
with the frequency of repetition of the primary vibrations. Haller has
substantially the same idea, but contents himself with the general term
“mutationes,” to express the cerebral change which is the cause of a
state of consciousness. These “mutationes” persist for a long time
after the cause which gives rise to them has ceased to operate, and
are arranged in the brain according to the order of coexistence and
succession of their causes. And he gives these persistent “mutationes”
the picturesque name of _vestigia rerum_, “quæ non in mente sed in ipso
corpore et in medulla quidem cerebri ineffabili modo incredibiliter
minutis notis et copia infinita, inscriptæ sunt.”[48] I do not know
that any modern theory of the physical conditions of memory differs
essentially from these, which are all children—_mutatis mutandis_—of
the Cartesian doctrine. Physiology is, at present, incompetent to
say anything positively about the matter, or to go farther than the
expression of the high probability, that every molecular change
which gives rise to a state of consciousness, leaves a more or less
persistent structural modification, through which the same molecular
change may be regenerated by other agencies than the cause which first
produced it.
Thus far, the propositions respecting the physiology of the nervous
system which are stated by Descartes have simply been more clearly
defined, more fully illustrated, and, for the most part, demonstrated,
by modern physiological research. But there remains a doctrine to which
Descartes attached great weight, so that full acceptance of it became
a sort of note of a thorough-going Cartesian, but which, nevertheless,
is so opposed to ordinary prepossessions that it attained more general
notoriety, and gave rise to more discussion, than almost any other
Cartesian hypothesis. It is the doctrine, that brute animals are mere
machines or automata, devoid not only of reason, but of any kind of
consciousness, which is stated briefly in the “Discours de la Méthode,”
and more fully in the “Réponses aux Quatrièmes Objections,” and in the
correspondence with Henry More.[49]
The process of reasoning by which Descartes arrived at this startling
conclusion is well shown in the following passage of the “Réponses:”—
“But as regards the souls of beasts, although this is not
the place for considering them, and though, without a
general exposition of physics, I can say no more on this
subject than I have already said in the fifth part of my
Treatise on Method; yet, I will further state, here, that
it appears to me to be a very remarkable circumstance
that no movement can take place, either in the bodies of
beasts, or even in our own, if these bodies have not in
themselves all the organs and instruments by means of
which the very same movements would be accomplished in a
machine. So that, even in us, the spirit, or the soul,
does not directly move the limbs, but only determines
the course of that very subtle liquid which is called
the animal spirits, which, running continually from the
heart by the brain into the muscles, is the cause of all
the movements of our limbs, and often may cause many
different motions, one as easily as the other.
“And it does not even always exert this determination;
for among the movements which take place in us, there
are many which do not depend on the mind at all, such
as the beating of the heart, the digestion of food, the
nutrition, the respiration, of those who sleep; and,
even in those who are awake, walking, singing, and other
similar actions, when they are performed without the
mind thinking about them. And, when one who falls from
a height throws his hands forwards to save his head, it
is in virtue of no ratiocination that he performs this
action; it does not depend upon his mind, but takes place
merely because his senses being affected by the present
danger, some change arises in his brain which determines
the animal spirits to pass thence into the nerves, in
such a manner as is required to produce this motion,
in the same way as in a machine, and without the mind
being able to hinder it. Now since we observe this in
ourselves, why should we be so much astonished if the
light reflected from the body of a wolf into the eye of
a sheep has the same force to excite in it the motion of
flight?
“After having observed this, if we wish to learn by
reasoning, whether certain movements of beasts are
comparable to those which are effected in us by the
operation of the mind, or, on the contrary, to those
which depend only on the animal spirits and the
disposition of the organs, it is necessary to consider
the difference between the two, which I have explained
in the fifth part of the Discourse on Method (for I do
not think that any others are discoverable), and then
it will easily be seen, that all the actions of beasts
are similar only to those which we perform without the
help of our minds. For which reason we shall be forced
to conclude, that we know of the existence in them of no
other principle of motion than the disposition of their
organs and the continual affluence of animal spirits
produced by the heat of the heart, which attenuates and
subtilises the blood; and, at the same time, we shall
acknowledge that we have had no reason for assuming any
other principle, except that, not having distinguished
these two principles of motion, and seeing that the one,
which depends only on the animal spirits and the organs,
exists in beasts as well as in us, we have hastily
concluded that the other, which depends on mind and on
thought, was also possessed by them.”
Descartes’ line of argument is perfectly clear. He starts from reflex
action in man, from the unquestionable fact that, in ourselves,
co-ordinate, purposive, actions may take place, without the
intervention of consciousness or volition, or even contrary to the
latter. As actions of a certain degree of complexity are brought about
by mere mechanism, why may not actions of still greater complexity
be the result of a more refined mechanism? What proof is there that
brutes are other than a superior race of marionettes, which eat without
pleasure, cry without pain, desire nothing, know nothing, and only
simulate intelligence as a bee simulates a mathematician?[50]
The Port Royalists adopted the hypothesis that brutes are machines,
and are said to have carried its practical applications so far, as to
treat domestic animals with neglect, if not with actual cruelty. As
late as the middle of the eighteenth century, the problem was discussed
very fully and ably by Bouillier, in his “Essai philosophique sur l’Âme
des Bêtes,” while Condillac deals with it in his “Traité des Animaux;”
but since then it has received little attention. Nevertheless, modern
research has brought to light a great multitude of facts, which not
only show that Descartes’ view is defensible, but render it far more
defensible than it was in his day.
It must be premised, that it is wholly impossible absolutely to prove
the presence or absence of consciousness in anything but one’s own
brain, though, by analogy, we are justified in assuming its existence
in other men. Now if, by some accident, a man’s spinal cord is divided,
his limbs are paralysed, so far as his volition is concerned, below
the point of injury; and he is incapable of experiencing all those
states of consciousness, which, in his uninjured state, would be
excited by irritation of those nerves which come off below the injury.
If the spinal cord is divided in the middle of the back, for example,
the skin of the feet may be cut, or pinched, or burned, or wetted
with vitriol, without any sensation of touch, or of pain, arising in
consciousness. So far as the man is concerned, therefore, the part of
the central nervous system which lies beyond the injury is cut off
from consciousness. It must indeed be admitted, that, if any one think
fit to maintain that the spinal cord below the injury is conscious, but
that it is cut off from any means of making its consciousness known to
the other consciousness in the brain, there is no means of driving him
from his position by logic. But assuredly there is no way of proving
it, and in the matter of consciousness, if in anything, we may hold by
the rule, “De non apparentibus et de non existentibus eadem est ratio.”
However near the brain the spinal cord is injured, consciousness
remains intact, except that the irritation of parts below the injury
is no longer represented by sensation. On the other hand, pressure
upon the anterior division of the brain, or extensive injuries to it,
abolish consciousness. Hence, it is a highly probable conclusion,
that consciousness in man depends upon the integrity of the anterior
division of the brain, while the middle and hinder divisions of the
brain, and the rest of the nervous centres, have nothing to do with it.
And it is further highly probable, that what is true for man is true
for other vertebrated animals.
We may assume, then, that in a living vertebrated animal, any
segment of the cerebro-spinal axis (or spinal cord and brain)
separated from that anterior division of the brain which is the
organ of consciousness, is as completely incapable of giving rise to
consciousness, as we know it to be incapable of carrying out volitions.
Nevertheless, this separated segment of the spinal cord is not passive
and inert. On the contrary, it is the seat of extremely remarkable
powers. In our imaginary case of injury, the man would, as we have
seen, be devoid of sensation in his legs, and would have not the least
power of moving them. But, if the soles of his feet were tickled, the
legs would be drawn up, just as vigorously as they would have been
before the injury. We know exactly what happens when the soles of the
feet are tickled; a molecular change takes place in the sensory nerves
of the skin, and is propagated along them and through the posterior
roots of the spinal nerves, which are constituted by them, to the grey
matter of the spinal cord. Through that grey matter, the molecular
motion is reflected into the anterior roots of the same nerves,
constituted by the filaments which supply the muscles of the legs, and,
travelling along these motor filaments, reaches the muscles, which at
once contract, and cause the limbs to be drawn up.
In order to move the legs in this way, a definite co-ordination of
muscular contractions is necessary; the muscles must contract in a
certain order and with duly proportioned force; and moreover, as the
feet are drawn away from the source of irritation, it may be said that
the action has a final cause, or is purposive.
Thus it follows, that the grey matter of the segment of the man’s
spinal cord, though it is devoid of consciousness, nevertheless
responds to a simple stimulus by giving rise to a complex set of
muscular contractions, co-ordinated towards a definite end, and serving
an obvious purpose.
If the spinal cord of a frog is cut across, so as to provide us with a
segment separated from the brain, we shall have a subject parallel to
the injured man, on which experiments can be made without remorse; as
we have a right to conclude that a frog’s spinal cord is not likely to
be conscious, when a man’s is not.
Now the frog behaves just as the man did. The legs are utterly
paralysed, so far as voluntary movement is concerned; but they are
vigorously drawn up to the body when any irritant is applied to the
foot. But let us study our frog a little farther. Touch the skin of
the side of the body with a little acetic acid, which gives rise to
all the signs of great pain in an uninjured frog. In this case, there
can be no pain, because the application is made to a part of the skin
supplied with nerves which come off from the cord below the point of
section; nevertheless, the frog lifts up the limb of the same side,
and applies the foot to rub off the acetic acid; and, what is still
more remarkable, if the limb be held so that the frog cannot use it, it
will, by-and-by, move the limb of the other side, turn it across the
body, and use it for the same rubbing process. It is impossible that
the frog, if it were in its entirety and could reason, should perform
actions more purposive than these: and yet we have most complete
assurance that, in this case, the frog is not acting from purpose, has
no consciousness, and is a mere insensible machine.
But now suppose that, instead of making a section of the cord in the
middle of the body, it had been made in such a manner as to separate
the hindermost division of the brain from the rest of the organ, and
suppose the foremost two-thirds of the brain entirely taken away. The
frog is then absolutely devoid of any spontaneity; it sits upright in
the attitude which a frog habitually assumes; and it will not stir
unless it is touched; but it differs from the frog which I have just
described in this, that, if it be thrown into the water, it begins to
swim, and swims just as well as the perfect frog does. But swimming
requires the combination and successive co-ordination of a great number
of muscular actions. And we are forced to conclude, that the impression
made upon the sensory nerves of the skin of the frog by the contact
with the water into which it is thrown, causes the transmission to the
central nervous apparatus of an impulse, which sets going a certain
machinery by which all the muscles of swimming are brought into play
in due co-ordination. If the frog be stimulated by some irritating
body, it jumps or walks as well as the complete frog can do. The simple
sensory impression, acting through the machinery of the cord, gives
rise to these complex combined movements.
It is possible to go a step farther. Suppose that only the anterior
division of the brain—so much of it as lies in front of the “optic
lobes”—is removed. If that operation is performed quickly and
skilfully, the frog may be kept in a state of full bodily vigour for
months, or it may be for years; but it will sit unmoved. It sees
nothing; it hears nothing. It will starve sooner than feed itself,
although food put into its mouth is swallowed. On irritation, it jumps
or walks; if thrown into the water it swims. If it be put on the hand,
it sits there, crouched, perfectly quiet, and would sit there for ever.
If the hand be inclined very gently and slowly, so that the frog would
naturally tend to slip off, the creature’s fore paws are shifted on to
the edge of the hand, until he can just prevent himself from falling.
If the turning of the hand be slowly continued, he mounts up with great
care and deliberation, putting first one leg forward and then another,
until he balances himself with perfect precision upon the edge; and, if
the turning of the hand is continued, over he goes through the needful
set of muscular operations, until he comes to be seated in security,
upon the back of the hand. The doing of all this requires a delicacy of
co-ordination, and a precision of adjustment of the muscular apparatus
of the body, which are only comparable to those of a rope-dancer. To
the ordinary influences of light, the frog, deprived of its cerebral
hemispheres, appears to be blind. Nevertheless, if the animal be put
upon a table, with a book at some little distance between it and the
light, and the skin of the hinder part of its body is then irritated,
it will jump forward, avoiding the book by passing to the right or left
of it. Therefore, although the frog appears to have no sensation of
light, visible objects act through its brain upon the motor mechanism
of its body.[51]