THE FUNCTIONAL RELATIONS OF THE NERVOUS SYSTEM.
23. The distinguishable parts of this system are the central axis, the
cranial nerves, and the spinal nerves, with the chain of ganglia and
nerves composing the Sympathetic. Let us briefly set down what is known
of their special offices.
Men very early discovered that the nerves were in some way ministrant
to Sensation and Movement; a divided nerve always being accompanied
by insensibility and immobility in the limb. Galen, observing that
paralysis of movement sometimes occurred without insensibility,
suggested that there were two kinds of nerve; but no one was able to
furnish satisfactory evidence in support of this suggestion until early
in the present century, when the experiments of Charles Bell, perfected
by those of Majendie and Müller, placed the suggestion beyond dispute.
[Illustration: Fig. 12.--_Transverse sections of spinal cord (dorsal
region)._]
24. Fig. 12 is a _diagram_ (not a drawing of the actual aspect, which
would be hardly intelligible to readers unversed in such matters)
representing two transverse sections of the spinal cord just where the
nerve-roots issue. The gray substance is somewhat in the form of a
rude H, in the dorsal region, and of the expanded wings of a butterfly
in the lumbar enlargements (Figs. 4–6); the extremities of this gray
substance are the _anterior and posterior horns_. We have already said
that from the anterior horns of each half issue the roots of the motor
nerves, which pass to the muscles. From the posterior horns issue the
sensory nerves, which, soon after leaving the cord, enter the ganglia
before joining the motor nerves, and then pass to the skin, in the
same sheath with their companions, separating again as they reach the
muscles and surfaces where they are to be distributed. When this mixed
nerve is cut through, or tied, all sensation and movement disappear
from the parts innervated. But if only one of the roots be cut through,
above the ganglion, there will then be only a loss of movement or a
loss of sensation. Thus suppose the section be made at _a_, _b_, A:
we have then divided a sensory nerve, and no pinching or pricking of
the part innervated by that nerve will be felt; but movement will take
place if the under nerve be irritated, or if a sensation _elsewhere_ be
excited. Now reverse the experiment, as at B, _c_, _d_. Then, pricking
of the skin will be felt, but no movement will respond. The nerve which
enters the cord at the upper (posterior) part is therefore a sensory
nerve; that which enters at the under (anterior) part is motor. The
direction is in each case indicated by the arrow. The central end _b_,
if irritated, will produce sensation; whereas the peripheral end _a_
produces neither sensation nor movement. The central end _d_ produces
neither sensation nor movement; the peripheral end _c_ produces
movement.
25. Two facts are proved by these experiments. First, that the
co-operation of the centre is necessary for Sensation, but not for
Movement. Although normally all the muscles of the trunk are moved only
when their centre has been excited, yet any irritation applied directly
to the muscle nerve, even when separated from its centre, produces a
movement. And to this we may add that a slighter stimulus will move the
muscle by direct irritation of the nerve, than by indirect irritation
through the centre; a slighter stimulus also will suffice when applied
to the nerve than when applied to the muscle itself.
26. The second fact proved is known as _Bell’s Law_, that the sensory
and motor channels are respectively the posterior and anterior nerves.
The fact is indisputable, but its theoretic interpretation can no
longer be accepted in its original form. Bell supposed the two nerves
to be different in kind, endowed with different specific energies, the
one sensitive, the other motor. The majority of writers still express
themselves as if they adopted this view. We shall, however, presently
see reason for replacing it by the more consistent interpretation
which assigns one and the same property to both nerves, marking their
distinction by the terms afferent and efferent; the one set being
anatomically so disposed that it conveys stimuli from the surfaces to
the centre, and the other set conveying stimuli from the centre to the
muscles, glands, and other cells.[89]
27. Bell’s discovery was rapidly generalized. The principle of
localization was extended to all nerves, and of course to the posterior
and anterior columns of the spinal cord, which indeed were assumed to
be continuations of the nerves. Bell, who was greater as an anatomist
than as a philosopher, always maintained that anatomical deduction was
superior to experiment. But this was to misunderstand the reach of
deduction, which is only valid to the extent of its premises.[90] In
the present case, the premises assumed that the posterior columns were
continuations of the posterior roots, and carried impressions to the
brain, the anterior columns carrying back from the brain the “mandates
of the will.” Experiment has, however, decisively shown that it is
_not_ through the posterior columns that sensory impressions travel to
the brain, but through the central gray substance.
28. The spinal cord with its central gray substance is at each point
a centre of reflexion. Connected as it is with different organs, we
artificially consider it as a chain of different centres, and try to
detect the functional relations of its parts. The inquiry is important,
but we must bear in mind the cardinal principle that diversity of
Function depends on the organs innervated, and not on a diversity of
Property in the nervous tissue. Although all nerves have a common
structure and common property, yet we distinguish them as sensory and
motor; and the sensory we subdivide into those of Special Sensation
and those of Systemic Sensation. The motor we divide into muscular,
vasomotor, and glandular. The hypothesis of specific energies must be
relinquished (§ 63).
In like manner all centres have a common structure and a common
property, with a great diversity of functional relations. Here also the
hypothesis of specific energies has been generally adopted, owing to
a mistaken conception of the biological principle just mentioned. The
cerebral hemispheres are credited with the properties of sensation,
thought, and volition; the cerebellum with the property of muscular
co-ordination; the spinal cord with the property of reflexion.
29. No attempt to assign the true functional relations of the centres
will be made at the present stage of our exposition. We must learn more
of the processes in Sensation, Thought, and Volition, before we can
unravel the complex physiological web on which they depend. But here,
provisionally, may be set down what observation and experiment have
disclosed respecting the part played by certain centres. We know, for
example, that when the cerebral hemispheres are carefully removed from
a reptile or a bird, all the essentially _vital_ functions go on pretty
much as before, but a great disturbance in some of the _psychical_
functions is observed. The brainless bird eats, drinks, sleeps, moves
its limbs separately and in combination, manifests sensibility to
light, sound, and touch, performs such instinctive actions as preening
its feathers, or thrusting the head under the wing while roosting.
Throw it into the air and it will fly. In its flight it will avoid
obstacles, and will alight upon a ledge, or your shoulder. But it will
not fly unless thrown into the air; it will not escape through the
open door or window; it will avoid objects, but will show no fear of
them,--alighting on your head, for example, without hesitation. It is
sensitive to light, and may in a certain sense be said to _see_; but
it fails to _perceive_ what is _seen_. It will eat and drink, if food
and water be administered, but it will starve near a heap of grain and
never peck it, not even if the beak be thrust into the heap. A grain,
or strip of meat, may be thrust inside the beak; there it will remain
unswallowed, unless it touches the back of the mouth, then swallowing
at once follows the stimulus. The bird _with_ its brain will fly away
if you turn the finger, or stick, on which it is perching; _without_
its brain, it makes no attempt to fly, but flutters its wings, and
balances itself. If you open the mouth of a cat, or rabbit, and drop
in some bitter fluid, the animal closes its mouth firmly, and resists
your efforts to repeat the act; without its brain, the animal shows the
same disgust at the taste, but never resists the preliminaries of the
repetition.
30. These, and analogous facts, have been noted by various
experimenters. They are very far from proving what is usually
concluded; but they prove the important negative position that the
cerebrum is not the centre of innervation for any of the organs on
which the observed actions depend. Thus, the cerebrum is not necessary
to sight: _ergo_ it does not innervate the eye. It is not necessary to
hearing: _ergo_ it does not innervate the ear.[91] It is not necessary
to breathing, swallowing, flying, etc.: _ergo_ it does not innervate
the organs of these functions.
What then is lost? We have only to remember that the cerebrum is
continuous with the thalami and corpora striata, and, through its
crura, with the medulla oblongata and medulla spinalis, to foresee
that its removal must more or less affect the whole neural axis,
and consequently disturb the actions of the whole organism; this
disturbance will often have the appearances which would be due to the
removal of a central apparatus, so that we shall be apt to attribute
the cessation of a function to the loss of its organ, when in fact the
cessation is due simply to an arrest of the organ by irritation. Thus
the cessation of consciousness, or of any particular movements, when
the cerebrum is removed, is no decisive proof that the cerebrum is the
organ of consciousness, or of the movement in question. This point will
be duly considered hereafter. What we have now to consider is the facts
observed after removal of the cerebrum.
First, we observe a loss of that power of combining present states
with past states, present feelings with feelings formerly excited in
conjunction with them, the power which enables the animal to adjust
its actions to certain sensations _now unfelt_ but which _will be_
felt in consequence of the adjustment. Secondly, we observe a loss of
Spontaneity: the bird, naturally mobile and alert, now sits moveless
for hours in a sort of stupor, occasionally preening its feathers, but
rarely quitting its resting-place. All the most conspicuous phenomena
which we assign to Intelligence and Will seem absent. The sensations
are altered and diminished. Many Instincts have disappeared, but some
remain. The sexual feeling is preserved, although the bird has lost
all power of directing its actions so as to gratify the desire. But
these effects are only observed when the whole of both hemispheres have
been removed. If a small portion remain the bird retains most of its
faculties, though with less energy. In frogs and fishes there is little
discernible effect observed when a large portion of the cerebrum is
removed.
31. Now take away from this mutilated bird its cerebellum: all the
functions continue as before except that _some_ combined movements can
no longer be effected; flight is impossible; walking is a mere stagger.
Remove only the lateral lobes, and though flight is still possible
great _incoherence_ of the wings is observed, whereas walking is not
much affected. If only the cerebellum be removed, the cerebrum being
intact, the phenomena are very different. All the perceptions and
almost all the emotions, all the spontaneity and vivacity are retained;
but the sexual instinct, which was manifested when the cerebrum was
removed, is now quite gone. What we call Intelligence seems unaffected.
The bird hears, and understands the meaning of the sounds, sees and
perceives, sees and fears, sees and adjusts its movements with a mental
vision of unseen consequences.[92]
32. Are we from these facts to conclude that the cerebrum is the “organ
of the mind”; that it is “the seat” of sensation, thought, emotion,
volition; and that the cerebellum is the “seat” of the sexual instinct,
and muscular co-ordination? Such conclusions have found acceptance,
even from physiologists who would have been startled had any one
ventured to affirm that the medulla oblongata was the “organ” of
Respiration, because Respiration ceases when this centre is destroyed.
I shall have to combat this notion at various stages of my exposition.
Here let me simply say that it is irreconcilable with any clear
conception of organ and function; and is plainly irreconcilable with
any survey of psychical phenomena in animals in whom the cerebrum does
not exist, and in animals from whom it has been removed.
What the facts indisputably prove is that the cerebrum has an important
part in the mechanism by which the most complex psychical combinations
are effected, and that the cerebellum has an important part in the
mechanism by which the most complex muscular combinations are
effected. The supreme importance of the cerebrum may be inferred from
its dominating all the other centres, and from its preponderance in
size. In man it stands to all the other cranial centres together in
the relation of 11 to 3. It is about five times as heavy as the spinal
cord--that is to say from 1,100 to 1,400 grammes, compared with 27 to
30 grammes. The quantity of blood circulating through it is immense.
Haller estimated the cranial circulation as one fifth of the whole
circulation. If, therefore, the Nervous Centres are agents in the
production of Sensation and Intelligence, by far the largest share must
be allotted to the cranial centres, and of these the largest to the
Cerebrum.
33. It is, however, one thing to recognize the Cerebrum as having
an important part in the production of psychical phenomena, another
thing to localize all the phenomena in it as their organ and seat--a
localization which soon becomes even more absurd, when of all the
cerebral structure the multipolar cells alone are admitted as the
active agents!
As was said just now, we recognize in the Medulla Oblongata the nervous
centre of Respiration, but we do not suppose that Respiration has its
_seat_ there, nor that this centre is absolutely indispensable for
the essential part of the process. We respire by our skin, as well as
by our lungs; many animals respire who have nothing like a medulla
oblongata; as many animals feel, and manifest will, who have nothing
like a cerebrum. The destruction of centres is of course a disturbance
of the mechanisms which they regulate. But even the observed results
of a destruction require very close examination, and are liable to
erroneous interpretations. The disappearance of a function following
the destruction, or disease of a particular part, is not to be accepted
as a proof that this part is the organ of the lost function; because
precisely the same phenomena may often be observed following the
destruction of a totally different part.[93] But one result may always
be relied on, and that is the _persistence_ of a function after removal
of a particular part. Thus there is a certain spot of the cerebral
convolutions from which movements of the limbs are excited when the
electrodes are applied to it; removal of the substance is immediately
followed by paralysis of the limbs. Are we to conclude that this spot
is the organ of the function? It is true that the function is called
into action by a stimulus applied to this spot: true that the function
suddenly vanishes when the substance of this spot is destroyed.
Nevertheless, what seems a loss of function is only a disturbance. In
two or three days the paralysis begins to disappear, and at the end
of a week the limbs are moved nearly in the normal manner. And the
same is true when the spot in question is destroyed on both sides. The
recovery of the function shows that the absent part was not its organ.
There is a paradoxical experiment recorded by M. Paul Bert which may be
cited here. He removed the right cerebral hemisphere from a chameleon,
and found that the limbs on the left side were paralyzed; but on his
then removing the left cerebral hemisphere the limbs of the left side
recovered their activity. A similar result was obtained by Lussana and
Lemoigne by extirpation of the thalami. When we find combined movements
persisting after the cerebellum has been destroyed, we may be sure that
the cerebellum is not the organ by which such combinations take place;
and when we find sensation and volition manifested after the cerebrum
has been removed, we may be sure that the cerebrum is not the organ for
these sensations and volitions.
34. And this we do find. Physiologists, indeed, for the most part,
deny it; or rather, while they admit the observed facts, they refuse
to admit the only consistent interpretation, biassed as they are by
the traditional conception of the brain. After having for many years
persistently denied Sensibility to any centre except the cerebrum, they
are now generally agreed in including the medulla oblongata within the
privileged region; but they still exclude the medulla spinalis.
35. If all the cranial centres as far as the medulla oblongata are
removed from young rabbits, dogs, or cats, there are unmistakable
evidences of Sensibility in their _cries_ when their tails are pinched,
their _moving jaws_ (as in mastication) when bitters are placed in
their mouths, and their _raised paws rubbing their noses_, when
irritating vapors are applied. It is said indeed that the cries are no
signs of _pain_; and this is probable; but they are assuredly signs of
Sensibility.
35. The frog thus mutilated has lost indeed all its special senses,
except Touch, but it still breathes, struggles when grasped, thrusts
aside the pincers which irritate it, or wipes away acid dropped on its
skin. If the eye be lightly touched, the eyelid closes; if the touch
be repeated three or four times, the foreleg is raised to push the
irritant away; if still repeated, the head is turned aside; but however
prolonged the irritation, the frog neither hops, nor crawls away, as he
does when the cerebellum remains. Place the brainless frog on his back,
and if the medulla oblongata remains he will at once regain the normal
position; but if that part is absent he will lie helpless on his back.
The power of preserving equilibrium in difficult positions--which
of course implies a nice co-ordination of muscles--resides in the
so-called _optic lobes_ of the frog (what in mammals are called the
_corpora quadrigemina_).
37. With the destruction of each part of the central mass there will
necessarily be some disturbance of the mechanism; but difficult as
may be the task of detecting by experiment what is the normal action
of any one part, there ought to be no hesitation in recognizing the
persistence of functions after certain parts are destroyed. The
spinal cord is anatomically known to be the centre from which the
limbs, trunk, and genito-urinary organs are innervated. So long as
the mechanism of the actions involving such organs is intact, no
removal of other parts will prevent this mechanism from exhibiting its
normal action. There may indeed arise, and there has arisen, the doubt
whether Sensibility is involved in the action of any nerve centre below
the medulla oblongata. But this doubt is founded on the traditional
hypothesis respecting the seat of Sensation, and is flagrantly at
variance with the logical conclusions of Anatomy and Experiment.
38. Anatomy shows that the structure of the spinal cord is in all
essential characters the same as that of the medulla oblongata; and
indeed that the whole central axis has one continuous tissue, somewhat
variously arranged, and in relation with various organs.
Abundant Experiment has shown that the spinal cord, apart from the
encephalon, is capable of acting as a sensorial and volitional centre.
The striking facts advanced by Pflüger, Auerbach, and myself, have
not been impugned;[94] but their interpretation has been generally
rejected. We showed that a brainless frog responded to stimulation in
actions which bore so close a resemblance to actions admitted to be
sensorial and volitional--showed the frog _adapting_ itself to new
conditions, and _acquiring_ dexterity in executing actions which at
first were impossible or difficult, _devising_ combinations to effect
a purpose which never by any possibility could have formed part of its
habits--manifesting, in a word, such signs of Sensibility, that no one
witnessing the experiments could hesitate as to the interpretation, had
he not been biassed by the traditions of the schools.
39. Our opponents argued that in spite of all appearances there
were profound differences between the actions of the normal and the
brainless animal, and that the latter were due simply to Reflex Action.
I also insist on profound differences; but underlying these there
are fundamental identities. As to the Reflex Action, two points will
hereafter be brought forward: 1°, that _all_ central action is reflex,
the cerebral no less than the spinal; 2°, that the hypothesis of Reflex
Action being purely _mechanical_, and distinguished from Voluntary
Action in not involving Sensibility, is an hypothesis which must be
relinquished.
40. Postponing, however, all discussion of these points, let me here
say that the doctrine maintained in these pages is that the whole
cerebro-spinal axis is a centre of Reflexion, its various segments
taking part in the performance of different kinds of combined action.
It has one common property, Sensibility; and different parts of it
minister to different functions--the optic centre being different from
the auditory, the cerebral from the spinal; and so on. To make this
intelligible, however, we must first learn what is known respecting the
properties of nerve-tissue.