_REFLEX ACTION AND THE FUNCTIONS OF THE SPINAL CORD_
I
Until 1820 physiologists believed that all nerves had the same
functions; that is, that all were sensory.
We can scarcely picture the confusion in the mind of anyone studying
the nerves of the face when, besides those destined to the organs of
smell, sight, and hearing, he would notice two other large nerves--the
Trigeminus and Facialis--passing off separately from the brain and
spinal marrow, and which, with a double ramification of filaments,
cover all superficial and underlying parts of the face; and again, when
he saw the three nerves of various origin which go to the tongue, the
four which are distributed in the throat, and finally, in the midst
of this net of nerves, thick bundles of fine filaments and ganglia of
which the origin was untraceable.
It was an English physiologist, Charles Bell, who solved this problem
by showing that the most important nerves of the face, with the
exception of the special sensory nerves, are confined to two. If one
of these nerves, called the trigeminal, be cut through, every trace of
sensibility immediately disappears from the corresponding side of the
face; if the other, the facial nerve, be severed, sensibility remains,
but the face completely loses the power of movement, there is no longer
any contraction of the muscles or change of expression in the face.
I quote Charles Bell’s own words, since these two simple experiments
still form the base of the physiology of the nervous system.
'If we cut the division of the fifth nerve which goes to the lips of an
ass, we deprive the lips of sensibility; so when the animal presses the
lips to the ground, and against the oats lying there, it does not feel
them, and consequently there is no effort made to gather them. If, on
the other hand, we cut the seventh nerve where it goes to the lips, the
animal feels the oats, but it can make no effort to gather them, the
power of muscular motion being cut off by the division of the nerve.’[4]
[4] Ch. Bell: _Anatomy and Physiology of the Human Body_, v. ii., p.
394. London, 1826.
The same takes place in the hand, the legs, and in all other parts of
the body, which, according as the one or the other set of nerves is
injured, feel but cannot move, or move and do not feel.
In the ordinary circumstances of life no one becomes aware of these two
fundamental properties of the nervous system, or at least we do not
reflect that there are two distinct apparatus: the nerves which make
us feel, and the nerves which cause movement. The intimate connection
between these in the nerve centres and at the surface of the body
renders special methods necessary to separate them, and allow them to
act independently of each other.
Claude Bernard, the greatest of French physiologists, and one of
the most agreeable and successful authors who have ever popularised
science, showed how these two elements may be dissociated by
introducing certain poisons into the blood, which kill the finest
ramifications of the nerves in the most inaccessible parts of the
organism.
If one scratches the skin of a dog with a poisoned arrow, like those
used in war by certain savage tribes of America, the animal succumbs in
less than a quarter of an hour. This terrible poison, called _curari_,
destroys the motor nerves, but produces no change in the intelligence,
and the functions of the sensory nerves. The dog scarcely notices the
slight puncture on the skin and continues to walk about the room; but
in a short time the hind-legs become stiff, one can see that they no
longer obey the will; the posterior part of the body sways and falls.
The animal rises and stumbles; then the fore-legs fail and the dog
stands still. If we call him, or pat him, he responds with movements
of the head, the ears, the eyes, and by wagging his tail. Soon however
he cannot lift his head and lies stretched out, breathing quietly, as
though reposing at his ease. On being called, he moves his eyes and
feebly wags his tail, without any manifestation of pain. At last the
respiratory muscles cease to act and life ebbs out without a single
convulsive movement, and for a few moments sensibility and intelligence
are still distinguishable in the fixed and glassy eye. It is like a
corpse that perceives and understands everything going on around it,
without being able to move, retaining sentiment and will but having no
means of manifesting them.
II
In an investigation which I made with Professor E. Guareschi[5] into
the effect of cadaveric venom, we found that all substances which
slowly destroy the organism must produce phenomena analogous to those
of curari, since the motor nerves, according to our researches, have
less vitality than the sensory.
[5] 'Les Ptomaïnes.’ _Archives italiennes de Biologie_, ii. p. 367;
iii. p. 241.
In order to be convinced of this fact, it suffices to take a rabbit and
stop the circulation in its hind-legs. Placed on the ground, after a
few seconds the animal cannot move its hind-legs, but if one presses
them it squeaks and tries to escape with the aid of its fore-legs,
dragging after it the hinder part of its body, which remains paralysed
for a few moments. A sudden anæmia can therefore destroy motility but
leave sensibility uninjured.
When life is slowly ebbing, when the circulation gradually slackens and
the death-agony is prolonged, I believe that there is always a point
of time in which, with the exception of the respiratory and cardiac
muscles, all others are already paralysed, in which all is dead but the
sensory nerves.
The hand, which with a last effort has been laid in blessing on our
head, has sunk back on the coverlet never to be raised again, never to
move the fingers which still feel the pressure of the farewell clasp;
but the fixed eye still sees the shadows of the loved ones bending down
to press tearful kisses on the brow, and when the last breath has fled,
the mother still hears the despairing cry of her children and can no
longer respond even by a look.
III
We have therefore two sorts of nerves: of sensibility and of movement.
Let us now try to form a correct notion of an involuntary or reflex
movement, which I shall illustrate by the following example.
Let us imagine a large house of which the entrance is at some distance
from the street door. A bell is fixed inside, the wire of which,
after passing through various holes, terminates in a handle near the
outer door. When some one comes and pulls the handle the bell rings,
and the maid opens the door by pulling at the cord inside the house.
This series of actions represents what physiologists call a reflex
movement. The maid is a nerve-centre, the bell-wire a sensory nerve,
and the cord which opens the door a motor nerve. In the organism we see
muscles or glands instead of the door, but the mechanism is similar.
Just as the door-bell rings a hundred times a day on all imaginable
occasions without our needing to open the door, and without the maid
coming first to our study to ask what she must do; so we have in our
nervous system two distinct parts: the maid, represented by the spinal
cord, and the master, by the brain.
Let us now see what happens when the master is not at home, or what an
animal does when its head is cut off, and only the spinal cord is left.
We shall see here, too, that the more liberty the master gives to the
maid, the more arrogant she grows, at last lording it over the master
himself.
A decapitated frog does not die immediately; it may move for days, and
if deprived of the brain only remains alive for some time.
We will consider the more usual case, namely, that of a frog of which
the head has been completely cut off with a pair of scissors. The
animal shivers and writhes for a few moments, then it stops, and
would remain motionless if it were kept under a glass cover in a damp
atmosphere, where nothing would irritate the skin. But if we touch
its leg or put a drop of vinegar upon it, the animal tries at once to
escape and to remove the disturbing agent from the surface of its body.
If we put a drop of vinegar on the left leg it tries to wipe it off
with the right, and _vice versa_. But if we cut off one of the legs or
bind it fast, and then put a drop of vinegar on the other leg which is
at liberty, the frog makes use of this same leg to rub away the drop.
At first sight this seems to be an act of intelligence. It may be
maintained that it is done by choice, but we cannot say that this
activity requires the guidance of the intelligence. A dog of which the
spinal cord has been severed and a sleeping man make the same movements.
Neither must it be thought that these movements are only to be found in
frogs and the lower animals. We shall see that in man also they perform
all the most indispensable vital functions without the co-operation of
the brain. Fontana, one of the most celebrated Italian physiologists,
discovered, as early as the middle of the last century, that one could
decapitate rabbits and guinea-pigs without causing their immediate
death. And he also found that if care were taken by previously binding
the most important arteries; so that the animal should not lose too
much blood, and if the respiration were sustained artificially by means
of bellows, it could live for some time, and show itself sensitive to
external stimuli.[6]
[6] Fontana: _Veleno della Vipera_, i. p. 317.
IV
If we could hear the soliloquies of the man who is writing a book,
many, I think, would renounce for ever the pleasure of setting the
printing press in motion. It would be a curious experience, if one
could read between the lines the tale of discouragement, uncertainty,
trouble, and know the repeated struggles by which some difficulty was
overcome, a passage was composed, a clause or a sentence written. In
scientific works it would be seen that the most frequent interruptions
and exclamations arise always from doubt, and the anxiety which
torments an author of not making his meaning clear.
There is no remedy. He who wishes to explain a scientific subject in
a clear and simple way must stop from time to time; he must come out
of himself and take his reader’s place, forget all he knows in order
to listen impartially to his own voice, and to judge if what he has
said may be easily understood. And this I shall do, but the reader
must not be repulsed by the first difficulties: our first steps cost
the greatest effort. In order to comprehend the physical nature of
man, and to know how this exquisite machinery of ours works, we must
first examine attentively some of the most important organs which are
constantly at work in our nervous system. It is in science as in the
study of languages, one must first learn the meaning of the most
indispensable words in order to understand what is said to us in the
foreign tongue.
Till the beginning of this century very confused notions prevailed
as to the activity of the brain and spinal cord. Luigi Rolando, the
celebrated physiologist of the University of Turin, was the first who
clearly showed that the medulla oblongata (that part of the spinal
cord which lies nearest the brain) must be regarded as the centre of
the whole nervous system. No one in his time knew the structure of the
nerve-centres better than he, and it was he who proved that the medulla
oblongata 'is the first rudiment of the nervous system, the seat of
physical sensibility, of instinct, the director of voluntary movements,
the centre of life, and the wonderful cause of most surprising
phenomena known under the names of sympathies and consents.’[7]
[7] L. Rolando, _Saggio sopra la vera struttura del cervello e sopra le
funzioni del sistema nervoso_, Sec. III. p. 140. Turin, 1828.
If one cuts the head of a duck off at a blow, it does not remain
motionless but moves, flaps its wings and flutters along, as though it
meant to make its escape. It is said that the Emperor Commodus caused
the heads of the ostriches in the circus to be shot off with curved
arrows, and that the birds still ran on till they reached the goal.
If we cut the head of a dog off with a hatchet, we see that the trunk
wags the tail. There is a curious irony in the fact, but it need not
shock us, for the animal no longer feels. If an irritant is applied to
the skin, it draws its tail between its legs as though it were afraid,
although it is headless.
V
A difficult question confronts us here. There are some physiologists
who maintain that the maid is blind, and that she performs her work
without knowing what she does; that she pulls the cord when the bell
rings, heats the stoves, cooks, cleans the utensils, sweeps the house,
gives the rubbish to the dustman, and so on--but all this without power
of discernment, acting like an automaton, unable to make the slightest
change in what she does merely from habit. Others, again, maintain that
she does possess a few fragments of intelligence, that at certain times
she reasons too, and that the soul of the house does not dwell in the
master alone.
It is a very difficult question; because, if it can be proved that the
maid is blind and does everything from habit, one may also say that the
master--poor man!--does not see much either, and that he has certainly
not been able to teach the maid anything.
I say the question is difficult also because the names of the greatest
living physiologists are connected with it. Goltz and Foster took a
frog, destroyed its brain, and then plunged it into a vessel full of
water. If the frog were then touched it might be seen, like other
frogs in similar circumstances, to respond by swimming about and
even jumping out of the vessel. The water was then warmed up to 40°.
The frog remained motionless, nor did it feel that the water was
growing hot; it did not try to leap out, and thus allowed the heat to
increase until it was boiled without making any movement which might
indicate sensation. Therefore the spinal cord alone cannot think. The
frog moves like a machine whenever it feels those stimuli to which it
is accustomed (like an automaton of which one must press a certain
knob in order to produce a particular movement); it is indifferent
to everything else, allowing itself to be burnt and boiled and never
moving, because no pain is felt.
My friend Tiegel, professor of physiology in Japan, made another
experiment. He took a snake and severed the head at a blow. While the
trunk was writhing on the ground he touched it with a red-hot iron bar,
and the snake wound itself round it and did not desist, although its
flesh was burnt and skin charred. And so, in this case too, the spinal
cord producing these movements is unreasoning.
But how to explain all the other apparently reasoning acts?
The structure of the nerve-centres can itself give an appearance of
intelligence to results which are purely mechanical. Let us assume
that the nerve-paths passing to the various muscles from one side or
the other transmit more or less easily the stimuli given off from the
spinal cord. A drop of vinegar having been put on the leg of a frog,
as before mentioned, certain muscles will at once move--that is, those
of which the nerves oppose the least resistance to the stimuli produced
in the centre. But if the animal cannot remove the cause of the
irritation, the latter accumulates in the spinal cord, so increasing
in force that the nervous tension makes a way for itself along more
resisting paths, thus giving rise to other less usual movements.
VI
During my medical career I had more than once an opportunity of seeing
the human spinal cord injured or severed. The most interesting case
was that of a peasant, who, in falling from a tree, had severed the
spinal cord in the dorsal region a little below the shoulder-blades,
with a pruning hook. He moved his arms, spoke, but did not feel the
lower part of his body any longer, nor the pain which a wound he had on
the shin-bone would otherwise have caused him, although the leg moved
whenever we touched the sore in order to treat it.
Marshall Hall proved that all generative acts are dependent on the
lower part of the spinal cord, and Brachet tells of a soldier who
became the father of two children although the lower half of his body
was paralysed and quite without feeling. The only thing we do not
find in an animal with the spinal cord severed are those irregular
movements of the part separated from the brain, corresponding by their
spontaneity to those we call voluntary.
Frogs and other animals of which one has cut the spinal cord are in
general motionless and paralysed in the parts separated from the
brain; we must touch them in order to make them move. If one pinches
or slightly presses the hind-paw of a dog with the spinal cord severed
in the dorsal region, he moves it or draws it away, but does it
unconsciously, as we do if we are touched while asleep. If the stimulus
is strong, he moves the other leg and his tail; if stronger still, he
moves his whole body and trembles.
Even when the brain is wanting, slight stimuli produce a wagging of
the tail; strong stimuli the drawing of the tail between the legs.
This proves that certain characteristic phenomena of fear are produced
without any participation of the will or consciousness.
The liveliness and restlessness so characteristic of youth arise from
the greater excitability of the nervous system, which one always
notices in young animals. The age, race, and bodily condition render
very dissimilar the reflex movements by which animals deprived of
their brain respond, even when they are excited in the same manner.
The differences observable in character correspond to anatomical and
functional differences of the nerve-centres.
As it is impossible to find two men having all parts of their brain
or spinal cord exactly alike, we infer that these differences in the
structure of the nerve apparatus materially influence other functional
differences which seem to depend on causes of a higher order known
under the generic name of _will_. What many call free-will is only
a fatal necessity, an indissoluble chain of causes and effects, of
physical and mechanical actions, of automatic and unconscious reactions
in the living machine.
VII
In order to understand certain phenomena of fear, we must first study
a few peculiarities presented by the excitable portions of the nervous
system. If one stimulates the nerve running through the frog’s leg
by very slight electric currents which are incapable of producing a
contraction of the muscles, the force of the current may be slowly
and evenly increased without the leg moving or in any way responding.
This experiment shows us that the motor nerves do not respond to
the stimulus as such, because the latter may be very strong without
producing any visible effect, but that it is the rapid variations and
changes which cause the convulsive movements.
Any pain or fear assailing us unexpectedly causes a great disturbance
in the organism, but have a less serious effect when slowly developed.
There is always a more energetic response during the first moments of
a sensation. This fact is true of all phenomena of the nervous system,
and it is therefore unnecessary to give examples of what everyone
knows from experience. This depends also upon the fact that the nervous
system discharges a part of its energy at every reaction, so that when
the animal is very weak it responds no more after the first two or
three times.
We now understand why slight, unexpected emotions produce such intense
perturbations in the organism, while very serious events for which we
are prepared have in proportion much less effect.
VIII
Pliny, in speaking of fear making one close the eyes, relates that
amongst twenty gladiators scarcely two were found who did not wink when
suddenly menaced.[8]
[8] Plinius: _Historia naturalis_, lib. xi., p. 480.
It is striking that such slight causes produce movements so pronounced
that we are not capable of suppressing them. We know that our friend
will certainly not poke his finger into our eye, but the conviction
that it is a joke does not suffice. Even if a thick pane of glass were
between us and the approaching hand, with all the force of reason and
will, many would be unable to avoid shutting the eyes, as though there
were in us two natures: one, animal and unreasoning which commands, and
the other human and intelligent which succumbs.
Again, when a gnat or a grain of dust gets into our eye, the eye
closes irresistibly by an automatic mechanism quite independent of
our will. Sometimes there is not only one contraction, but a somewhat
complicated series of movements excited in parts distant from the
stimulus.
As a convincing example, I shall communicate what I observed in an
investigation of deglutition. This act, performed unceasingly during
eating, is by no means voluntary, for if we try to repeat it a few
times in succession we notice at once that, as soon as we have no
saliva in the mouth, every effort to swallow is in vain. In order to
swallow it is necessary that a morsel of food or some fluid should
touch the mucous membrane of the posterior portion of the mouth. The
sensory nerves stimulated in this way communicate to the spinal cord
that a body is at the entrance of the œsophagus which must be sent to
the stomach. Immediately a succession of orders is issued, one after
the other, by the spinal cord, so that first the upper part of the
œsophagus contracts and propels the morsel a short way down, then a
further order causes a contraction of the next part, then comes another
order whereupon a part still lower down contracts, and so all the
successive portions of the œsophagus transmit the morsel one to another
by means of various separate orders until it reaches the stomach.
We have, therefore, in our nervous system mechanisms which work
automatically, and produce a series of contractions directed to one
object, which may, at first sight, appear voluntary, but is, in
reality, mechanical and unconscious. Some of these mechanisms we
bring into the world with us. If one puts a finger into the mouth of
a new-born child it begins to suck. It is a machine working without
discernment, as if one had touched the spring of an automatic doll; no
one teaches the child, he need not learn it at all in fact, for the
fœtus in the womb makes exactly the same movement. So it is with the
chicken, which pecks when just escaped from the shell. In this case,
what gives rise to the movement is no longer immediate contact as that
of the finger in the mouth, but the impression of light and sight,
which indeed is nothing else but contact with distant things by means
of the rays of light. Scarcely has the image of the grain formed itself
in the eye of the chicken but it pecks at it.
It suffices to observe our movements with a little attention, in order
to be convinced that a greater number of them are automatic than one
thinks. When we step out of bed in winter and thrust our naked feet
into our slippers, the foot has scarcely touched the cold leather but
it withdraws, and an effort is necessary in order to resist. We notice,
too, that when the shoemaker measures us for a pair of shoes, it is
somewhat difficult to keep our foot still even though he does not
tickle us. When one touches iron, a cup, or any other object which is
very hot, the hand lets go at once. This is a very useful circumstance,
because we often let go of a thing which might injure us before we
have even become aware that it burns or pricks. And when we have lost
consciousness through illness or any accident, the body takes care of
itself, as during sleep, by automatically removing itself from anything
which pricks, burns, chills, stings, or presses, and so on. If the pain
produced by burning is faint, only the one side of the body moves, if
it spreads and grows stronger, it affects the opposite side also, and
in the highest degrees, the whole body.
This law, which was established by Pflüger, holds good for normal and
uninjured animals as well as for those from which the brain has been
removed and which are unconscious. It shows us that the postures and
movements of the body, so characteristic of a man responding to sudden
pain, do not depend on his will. All that is most characteristic in
the phenomena of fear: the palpitation, shortness of breath, pallor,
screams, flight, trembling, are reflex movements. The more physiology
advances, the more the domain of free-will is restricted, and the
greater the increase of involuntary movements.