ACTION WITHOUT NERVE-CENTRES.
89. It has long been one of the unquestioned postulates of Physiology
that no nerve-action can take place without the intervention of a
centre; and as a corollary, that all movement has its impulse--reflex
or volitional--from a centre.[115] The postulate rests on the
assumption that nerves derive their “force” from their centre. This
assumption we have seen to be erroneous. Yet, in consequence of its
acceptance, experimenters have failed to notice the many examples of
nerve-action independent of centres. Indeed, except Schiff, Goltz, and
Engelmann, I can name no one who has ventured to suggest that movements
may be excited through nerves without the co-operation of centres;[116]
nor have even they explicitly formulated the conclusion to which their
observations point.
It is true that the majority of muscular movements are determined by a
reflex from centres; and that any break in the triple process of the
ingoing nerve, centre, and outgoing nerve, prevents such movements.
It is true that the more conspicuous and harmoniously co-ordinated
phenomena belong to this class. But it is also demonstrable that many
nerve-actions _may_, and some _do_, take place by direct stimulation
of the nerve, or direct stimulation of the muscle, without the
intervention of a centre, without even the intervention of a ganglion.
This must obviously be the case in animals which have no centres; and
even in some which have well-developed nervous centres, there is every
reason to believe that these centres often act rather in the way of
co-ordinating than of directly stimulating actions.
90. I was first led to doubt the reigning doctrine by a surprising
observation (frequently repeated) after I had removed the whole nervous
centres from a garden snail (_Helix pomatia_). The muscular mass called
“the foot” was thrown into slow but energetic contraction whenever the
skin was pricked with the point of a scalpel, or touched with acid;
nay, even when a glass rod dipped in the acid was brought close to,
without absolutely touching, the skin, the foot curled up, and then
slowly relaxed. The same effect was produced on the “mantle”--where
there was of course no centre. But direct irritation of the muscles
under the skin produced no such contraction; only through the skin
could the stimulation take effect. In one case I observed this strange
phenomenon five hours after removal of the centres. It was a great
puzzle. At first I concluded that there must be minute ganglia in
the skin, serving as reflex-centres. I searched for them in vain;
and although a longer search on better methods might _possibly_ have
detected ganglionic cells, I soon relinquished the search, because I
had other grounds for believing that even the presence of abundant
ganglia would not suffice, until some better proof were afforded that
such ganglia were reflex-centres.
91. That direct stimulation of the nerve suffices to move the muscles,
is familiar to all experimenters. There is no centre, or ganglion,
in the amputated leg of the frog, which nevertheless contracts
whenever the sciatic nerve is stimulated. And after the nerve has been
exhausted, and refuses to respond to any stimulus, the muscle itself
may be directly stimulated. Inasmuch as the movement depends on the
contractility of the muscles, a stimulation through centre, through
motor-nerve, or through muscle, will be followed by contraction. Let
us take a clear case of reflex action. The pupil of the eye contracts
when a beam of light falls on it, and dilates when the beam is shut
off. The path of the neural process is normally this: the light
stimulates the optic nerve, which in turn stimulates the corpora
quadrigemina; (here the nerves which move the eye are experimentally
proved to be stimulated;) and it is through these that the pupil is
caused to contract. If the optic nerve be divided, no such reflex takes
place--proving that the contraction does not, at least normally, come
from the ciliary ganglion.
But now it is matter of observation that the pupil will contract and
dilate under the stimuli of light and darkness, when there is no such
reflex pathway open. Removal of the eye from the body obliterates
this path, cuts the eye off from all connection with the centre.
Brown Séquard removed both eyes from a frog, placed one in a dark
box, and left the other exposed to the light: the pupil of the former
was found dilated, that of the latter contracted. On reversing the
experiment, and placing the eye with contracted pupil in the dark
box, he found it there dilate, while the dilated pupil exposed to
the light contracted.[117] In frogs with very irritable tissues, I
have found not only the pupil contracting, after the whole cranial
cavity has been emptied, but even the eyelid close, on irritating
the conjunctiva[118]--yet this is one of the typical reflex actions!
I am disposed to think that even the action of swallowing may be
faintly excited by stimulation of the pharynx of a brainless frog; but
I have not observations sufficiently precise to enable me to speak
confidently. Goltz has, however, shown that after removal of brain
and spinal cord and heart, there is spontaneous and active movement
in œsophagus and stomach.[119] This will no doubt be referred to the
agency of the ganglionic plexus; but similar movements have been
observed by Engelmann in the ureter, and in isolated fragments of the
ureter in which not a ganglionic cell was present.[120]
92. That nerves are stimulated by internal changes has long been
recognized with reference to “subjective sensations.” The divided
nerve, in that portion which remains connected with the centre, will
at times cause great pain. Obscure organic conditions, changes of
temperature, states of the blood, excite the nerves, and the patient
feels as if the surface of the amputated limb were irritated. It is
all very well to call these “subjective sensations”; that does not
alter the fact of the nerve being called into activity by other than
the normal stimuli from the surface; in like manner muscular movements
(which are not to be explained as “subjective movements”) will be
excited by organic stimuli when motor-nerves are separated from their
centres. In each case it has sufficed that the nerve should be excited;
and when excited, no matter by what means, the effect is always similar.
93. Here are a few facts. Stimulation of the nerves which send
filaments to the chromatophores of the skin in reptiles causes the skin
to become paler, and even colorless: the color-specks disappear under
this contractile stimulus. This being known, Goltz deprived a frog of
brain, spinal cord, and heart, thus eliminating all possible influence
from them, slit up the skin of the back, and displayed the nerves which
pass from each side of the spine to the skin; these nerves he then
divided on the right side, and observed the skin on this side slowly
become paler and paler, till finally it was as yellow as wax; the left
side, having its nerves intact, retained its color. Two conclusions
seemed to him warranted by this experiment: First, that even in the
dead frog the nerves separated from their centre were still active;
secondly, that the irritation of the nerves resulting from their
section was the cause of the color-specks disappearing. This second
conclusion was strengthened when he found that the irritation was
increased when he cut the nerves bit by bit.
It is not at present, I believe, clearly made out that the color-specks
of the Cephalopoda are in direct connection with nerves; but it is
tolerably certain that they are in some way under the influence of
nervous stimulation, directly or indirectly. D’Orbigny, indeed, goes
so far as to say they are dependent on the will of the animal.[121]
This seems very lax language; but restricting ourselves to the fact
of nervous influence, the experiments of Goltz receive further
illustration in an observation I have elsewhere recorded.[122] I found
that a strip of skin taken from the dead body of a calamary (_Loligo_)
showed the color-specks expanding and contracting with vigor.
94. The heart is well known to beat after death, if death be not the
result of a gradual decay. Sometimes, indeed, its muscular irritability
is so active that the heart will beat for hours. E. Rousseau
observed it beating in a woman twenty-seven hours after she had been
guillotined.[123] Not only will it beat after death, but in many
animals even after removal from the body: the heart of a young puppy,
or kitten, will beat for three or four hours after its removal; that
of a full-grown dog, or cat, not one hour; whereas the beating of that
of a tortoise, or a frog, will, under proper precautions, be preserved
for days--and even after it has stopped, it may be stimulated to fresh
pulsations.
Physiologists explain this spontaneous movement of the heart as due to
the ganglia in its substance. This explanation, which is founded on
what I cannot but regard as a purely imaginary view of the functions
of ganglionic cells, must stand or fall with that hypothesis. A long
and arduous investigation has led me to doubt whether in _any_ case
the heart’s movements are primarily due to its ganglia; at all events,
the same spontaneous movements are observed in the hearts of molluscs
and crustaceans, which are without even a trace of ganglia; and in the
hearts of mammalian embryos long before ganglia or nerve-fibres make
their appearance. Not less certain is it that movements of contraction
and dilatation are produced in the blood-vessels independently of all
central influence. This has been decisively proved by the Italian
physiologist, Mosso, when experimenting on an organ isolated from the
organism; and although the vessels have their nerve cells and fibres,
he justly doubts whether it is to these that the stimulation is due,
because the phenomena are observed after the nervous vitality has
disappeared. Goltz severed all the tissues in the leg of a rabbit,
so that the only connection of the leg with the rest of the body was
through the crural vein and artery, which kept up the circulation; yet
although the nerves of the skin were thus separated from their centre,
so that no sensation could be produced by stimulating the skin of the
leg, consequently no reflex from the centre on the vessels, Goltz found
that a marked reddening of the skin from congestion of the capillaries
followed the application of mustard to the skin. Physiologists who
believe that the constriction and dilatation of blood-vessels are due
to the action of the ganglionic cells distributed over the walls of the
vessels will explain Goltz’s observation as a case of reflex action;
but those who agree with me that such an hypothesis respecting the part
played by the cells is untenable, will class the observation among
other cases of direct stimulation.
95. But passing from these perhaps questionable cases, let us glance
at other cases. The mobile iris of the bird displays movements after
the nerves have been divided. Even the voluntary striped muscles are
not altogether motionless. Schiff divided the hypoglossus on one side,
and found, of course, the tongue paralyzed on that side; but he also
found that on the third day after the operation some of the muscles of
that side were quivering: the agitation spread to others, till by the
end of the fourth day all the fibres were _rhythmically_ contracting.
From this time onwards, the contractions were incessant; though they
were never able to move the tongue, because the fibres did not contract
simultaneously.
Schiff also observed that the hairs over the eyes and the “whiskers” of
cats, rabbits, and guinea pigs were for months after section of their
nerves in incessant rhythmical vibration. This was observed when the
animals were asleep as when awake. Valentin records the spontaneous
movements in the diaphragm of animals just killed; and this even after
section of the phrenic nerve. The same movements may be seen in the
operculum of fishes. Henle observed the spontaneous contractions of the
intercostal muscles; which Schiff confirms, adding that the movements
observed by him in cats and birds were not simply contractions of some
fibres, but of all the muscles, so that three or four excised ribs
rhythmically contracted and expanded.
I have performed a great many experiments with a view of determining
this question, but the phenomena were so variable that I refrain
from adducing any,[124] and merely state the general result as one
in harmony with the foregoing examples. The great variability of the
phenomena depends upon the variable conditions of muscular irritability
and anatomical relations. When the heart of one woman is found beating
twenty-seven hours after death, while in most men and women it ceases
after a few minutes, we must be prepared to find different, and even
contradictory phenomena under varying unknown conditions. There
is, however, a general agreement among experimenters that muscular
irritability increases after separation from nerve-centres, and then
quickly decreases again.
96. Although the stimulation of muscles usually comes _through_ a
nerve-centre, yet since the muscles do not derive their Contractility
from nerve-centres any stimulation will suffice. Now since we have
abundant proof that sensory nerves are stimulated by certain organic
changes, by poisons in the blood, excess of carbonic acid, etc., we
are justified in concluding that motor nerves will be stimulated in
like manner, and thus muscular movement be produced occasionally
without the intervention of a centre. Pressure on a motor nerve, or the
irritation which results from inflammation, will determine contraction,
or secretion directly. Recently, Erb and Westphal have disclosed
the fact that the leg will be suddenly jerked out if the patella be
gently tapped; and they prove this not to be a reflex action, because
it follows with the same certainty after the skin has been made
insensible.[125]
There are doubtless many other phenomena which, though commonly
assigned to reflex stimulation, are really due to direct stimulation.
Research might profitably be turned towards the elucidation of this
point. Since there is demonstrable evidence that a nerve when no
longer in connection with its centre, or with ganglionic cells, may
be excited by electricity, pressure, thermal and chemical stimuli, we
must conclude that even when it is in connection with its centre, any
local irritation from pressure, changes in the circulation, etc., will
also excite it. But as such local excitations will have only local and
isolated effects, they will rarely be conspicuous.