NEURILITY.
41. Observation having found that the activity of a nerve was always
followed by a sensation when the nerve ended in a centre, and by a
movement when the nerve ended in a muscle, Theory was called upon
to disclose the nature of this peculiar property of nerves. That a
peculiar and mysterious power did act in the nerves no one doubted;
the only doubt was as to its nature. The ancient hypothesis of Animal
Spirits seemed all that was needed. The spirits coursed along the
nerves, and obeyed the mandates of the Soul. When this hypothesis fell
into discredit, its place was successively taken by the hypotheses of
Nervous Fluid, Electricity, and Nerve Force. The Fluid, though never
manifested to Sense, was firmly believed in, even so late as the days
of Cuvier;[95] but when the so-called electrical currents were detected
in nerves, and the nervous phenomena were shown to resemble electrical
phenomena, there was a general agreement in adopting the electrical
hypothesis. The brain then took the place of a galvanic battery; the
nerves were its electrodes.
42. Closer comparison of the phenomena detected various irreconcilable
differences, which, if they proved nothing else, proved that
nerve-action took place under conditions so special as to demand a
special designation. Electricity itself is so little understood, that
until its nature is more precisely known, we cannot confidently say
more than that nerve-action resembles electrical-action; meanwhile the
speciality of neural conditions renders all deduction illusory which
is based on electrical-action as observed under _other_ conditions.
In presence of these difficulties, cautious physiologists content
themselves with assigning the observed phenomena to the observed
and inferred conditions, condensing these in the convenient symbol
“nerve-force,” without pretending to any specification of the nature
of that force. It may be a wave of molecular movement dependent on
isometric change or on metamorphic change. It may be the liberation of
molecular tension resembling electricity; it may be electricity itself.
But whatever the nature of the change, it is an activity of the tissue,
and as such comes under the general dynamic conception of Force or
Energy.
43. In this sense the term has nothing equivocal or obscure.
It is a shorthand expression symbolizing certain well-defined
observations. Nevertheless, it is a term which we shall do well to
avoid when possible, and to replace by another having less danger of
misinterpretation; the reason being that Force has become a sort of
shibboleth, and a will-o’-wisp to speculative minds. All that we _know_
of Force is Motion. But this is too meagre for metempirical thinkers,
who disdain the familiar experiences expressed in the term Motion,
and demand a transcendent cause “to account” for what is observed.
They seek an entity to account for the fact. Motion is a very definite
conception, expressing precise experiences; we know what it means, and
know that the laws of moving bodies admit of the nicest calculation.
A similar precision belongs to Force when understood as “mass
acceleration,” or M V². But this does not content those metaphysicians
who understand by Force “the unknown reality behind the phenomena”--the
_cause_ of Motion. This cause they refuse to recognize in some
antecedent motion (what I have termed a “differential pressure”), but
demand for it a physical or metaphysical _agent_: the physical agent
being a subtle fluid of the nature of Ether, or a nerve atmosphere
surrounding the molecules; the metaphysical agent being a Spirit or
aggregate of Soul-atoms. The second alternative we may decline here
to discuss. The first alternative is not only a pure fiction, but one
which is inconsistent with the demonstrable velocity of the neural
process, which is not greater than the pace of a greyhound, whereas
the velocities of light and electricity are enormously beyond this. It
is inconsistent also with the observation that a much feebler current
of electricity is requisite for the stimulation of a muscle through
its nerve than when directly applied to the muscle: a proof that the
nerve does not act solely by transmission of electricity--unless we
gratuitously assume that the nerve is a multiplicator.
When it is said that the living nerve is incessantly liberating Force
which can be communicated to other tissues, the statement is acceptable
only if we reject the metaphysical conceptions it will too generally
suggest--the conceptions of Force as an entity, and of its being
passed from one object to another like an arrow shot from a bow. The
physical interpretation simply says that the molecules of the nerve are
incessantly vibrating, and with varying sweep; these vibrations, when
of a certain energy, will set going vibrations in another substance by
disturbing the tension of its molecules, as the vibrations of heat will
disturb the tension of the gunpowder molecules, and set them sweeping
with greater energy: this is the _communication_ of the force. Just as
we say that a magnet communicates magnetic force to a bit of iron,
though all we mean is that the magnet has so altered the molecular
condition of the iron as to have given it the movements called
magnetism--in short, has excited in the iron the dormant property of
becoming magnetic--so we say the nerve communicates its force to the
muscle, exciting in the muscle its dormant property of contraction.
But in truth nothing has passed from magnet to iron, or from nerve to
muscle.
44. Do what we will, however, there is always, in the present condition
of philosophical chaos, the danger of being misunderstood when we
employ the term Nerve-force; and I have proposed the term Neurility as
an escape from the misleading suggestions. It is a symbol expressing
the general property of nerve-tissue. For reasons presently to be
stated, I restrict Neurility to the peripheral system, employing
Sensibility for the central system. The excited muscle manifests its
special property of Contractility; the excited nerve manifests its
special property of Neurility; the excited centre manifests its special
property of Sensibility.[96] The terms are simply descriptive, and
carry with them no hypothesis as to _what_ Neurility is in its hidden
process, nor _how_ Sensibility arises in a nerve-centre, and not
elsewhere. We know that a stimulated muscle contracts, and we express
the fact by assigning to muscular tissue the property of Contractility.
We know that a stimulated nerve translates an impulse from one point to
another, and excites the muscle to contract; and we express the fact
by assigning to nerve-tissue the property of transmitting stimulation,
which is further specified, as unlike other transmissions, by the term
Neurility.
45. What is the meaning attached to the term Property, and how it is
distinguished from Function, has been already expounded in Problem
1, §§ 81–6. There also was laid down the principle of _identity of
structure implying identity of property_. Inasmuch as observation
reveals a fundamental similarity in the structure of the nervous tissue
throughout the animal kingdom, we must conclude the existence of a
fundamental similarity in the property of that tissue: a conclusion
confirmed by observation. There is a corresponding agreement in the
organs and functions; so that, within certain limits, the experiments
performed on an insect may be verified on a mammal. Everywhere
nerve-tissue has certain characters in common, accompanied by
variations in the degree and mode of manifestation corresponding with
variations in structure and connection. Obvious as the fact is, we
must emphasize the great variety which accompanies the underlying
uniformity, for this is recognizable both in the individual organism
and in the animal kingdom at large. Even such seemingly individual
terms as nerve-cell and nerve-fibre are in truth generic; and the
description which accurately represents one cell or fibre needs
modifying for others.
Properties are generalized expressions; they result from the
_composition_, the _structure_, and the _texture_ of a substance. Thus
one bar of iron may differ from another of equal bulk in being more or
less crystalline in structure, though having the same composition and
the same texture. This difference will modify the mode of manifestation
of the iron-properties. Cast-iron pillars, for example, will support,
as a roof, a weight which would break them if suspended; wrought-iron
pillars of similar bulk will bear a weight suspended which would crush
them as a roof. Yet both cast and wrought iron pillars have the same
properties, because they have the _same_ composition and _similar_
structure; the variation of structure only producing a difference in
the modes. Texture may also vary. The bar of iron may be beaten into
a plate, rolled into a cylinder, or split into wire-work, without any
change in its properties, but with marked differences in its modes of
manifestation, and in the _uses_ to which it may be applied. These uses
are of course dependent on the connections established between the iron
and other things. In Physiology, _uses_ are called _functions_.
46. Nerve-tissue must be understood as having everywhere the same
general Property. In one animal and in another, in one part and in
another, Neurility is the same in kind, but not everywhere manifesting
the same degree, nor applied to the same Function. The _composition_
of nerve-tissue varies, but not more than the composition of all other
organized substances; the _structure_ is variable, but only within
a small range; the _texture_ also; while the _connections_ are very
various. Hence, whatever the variations in composition or structure,
the nerve-fibre has everywhere one fundamental property, which in
connection with a muscle has the functional activity of exciting
contraction; in connection with a gland of exciting secretion; and in
connection with a centre of exciting reflexion.[97]
47. Had a clear idea of Function as dependent on connexion been present
to their minds certain physiologists would hardly have raised the
mirage of “Nerve-force,” a mysterious entity endowed with “specific
energies,” and capable of producing vital and psychical phenomena by
an occult process; nor would others have been led to the monstrous
hypothesis of particular nerve-cells being endowed with thought,
instinct, and volition. They would have sought an explanation of
functions in the combined properties of the co-operant organs and
tissues. They would not have endowed one nerve with Sensibility, and
another nerve of identical structure with Motility;[98] one nerve with
a motor property, and another with the opposite property of inhibition.
They would have seen that all nerves have the same _property_, but
different _uses_ when in different connexions.
48. Throughout the animal kingdom we see movement following on
stimulation. Stimulation may be defined the change of molecular
equilibrium. The stimulation of a muscle is produced indirectly through
a change in the nerve, or directly through a change in the muscle
itself. In the simplest organisms there is no trace of nerve-tissue;
but their substance manifests Irritability (or as it is often called
Sensibility); and a stimulus to one part is propagated throughout--the
whole body moves when touched. Even in Polypes, where there is the
beginning of a differentiation, the motion is slowly propagated from
one part to the rest. A single tentacle retracts when touched; but the
movement rarely ends there; it is slowly communicated from one tentacle
to the other, and from them to the whole mass. Touching the body,
however, will not, if the touch be slight, cause the tentacles to move;
so that we see here a beginning of that principle of specialization
which is so manifest in the higher organisms: the tentacles have
become the specially sensitive parts. Ascending higher in the scale of
organisms we find those which habitually move particular parts without
at the same time necessarily moving the rest; and this independence
of parts, accompanying a more perfect consensus, we find to be
developed _pari passu_ with a nervous system. An immense variety of
part-movements, with varying combinations of such movements, is the
physiological expression of the more complex nervous system.
48 _a_. Deferring what has to be said of Sensibility till the next
chapter, we may here touch on its relation to Irritability, which is
often used as its synonym. _Objectively_ it cannot be distinguished
from Irritability, nor indeed from the most general phenomenon of
reaction under stimulation; in this it is an universal property. But
_subjectively_ it is distinguishable as a peculiar mode of reaction,
only known in nerve-tissues. While all tissues are irritable, and react
on being stimulated, each tissue has its special mode of reaction. The
secreting-cell reacts differently from the muscle-cell. The reaction
of the nerve is the innervation of a centre or a muscle; the reaction
of an innervated centre is sensation; of a muscle, contraction. There
are three aspects of neural reaction: excitation, propagation of the
disturbance, and innervation. The first is expressed by irritability,
the second by conductibility, the third by sensibility; but these are
only artificial distinctions in the general phenomenon of transmitted
excitation. The nerve substance is specially distinguished by its
instability of molecular equilibrium; it undergoes chemical change
with a readiness comparable to that of explosive substances. Hence
its facility of propagation of disturbance. There is irritability and
propagation of disturbance in muscular tissue, notably evident in
the continuous tissue of the heart, intestines, and ureter; but the
propagation is slow and diffusive; whereas in the nerve it is rapid,
and restricted along a definite path. By this rapidity and restriction
the force of the impact is increased; and thus a slight stimulus
applied to the nerve is capable of disturbing the state of the muscle.
49. Thus while molecular movement is a fundamental condition of
Vitality, and is incessant throughout organized substance, the massive
movements of the organism, and the movements of particular parts, are
the _directed quantities_ of this molecular agitation. They are due
to stimulation. We distinguish this from mechanical impulsion. It
is a vital process involving molecular change; it is not simply the
_communication_ of motion from without, but the _excitation_ of motion
within. It is not like the blow which merely displaces an object, but
like the blow which disturbs its molecular equilibrium. The effect,
therefore, depends on this molecular condition: the blow which scatters
a heap of gunpowder will explode a fulminating salt, and this, in
exploding, will excite the gunpowder to explode. The stimulus which
is too feeble to excite contraction in a muscle will be powerful
enough to excite the neurility of a nerve, and _that_ will excite the
contractility of the muscle. The nerve-force is simply neural stimulus.
It acts upon the other tissues as the nitrogenous salt upon the
gunpowder.
Although it is now common to speak of nerves as transmitting waves of
molecular motion, and to regard nerves as the passive medium for the
“transference of force,” whereby the force is thus made an abstract
entity, we must always remember that such phrases are metaphors, and
that the truer expression will be not “transference of force,” but the
“propagation of excitation.” I mean that it is not the _force_ of the
impact nor its _energy_ which a nerve transmits, it is the vibratory
change produced in the nerve by the impact, which excites another
change in the organ to which the nerve goes. We know by accurate
measurements that the excitation of a nerve lasts much longer than the
_stimulus_, a momentary impact producing an enduring agitation. We know
also that the excitation of a centre lasts longer than the muscular
contraction it has initiated. We know, moreover, that a nerve may be
totally incapable of conducting an external stimulus, yet quite capable
of conducting a central stimulus; were it a passive conductor like a
wire this would not be so.[99]
50. The nerve is essentially an exciter of change, and thereby a
regulator. A muscle in action does not appreciably determine action
in any other (except in the comparatively rare cases of anastomosing
muscles); a secreting cell does not propagate its excitation to others.
The nerve, on the contrary, not only propagates its excitation, and
awakens the activity of the muscle or gland with which it is connected,
but through the centre affects the whole organism--
“Ein Schlag tausend Verbindungen schlägt.”
Thus it is that stimulation which in the simpler organisms was diffused
throughout the protoplasm, has in the complex organisms become the
specialized property of a particular tissue.
51. Two general facts of supreme importance must now be stated: One is
the law of stimulation--_every excitation pursues the path of least
resistance_. The second is the condition of stimulation--_unlike
mechanical impulsion, it acts only at insensible distances_.
52. This means that although a nerve may be excited by any stimulus
external to it which changes its molecular condition, no propagation of
that change (i. e. no stimulation through the nerve) is possible except
through continuity of substance. Mere physical contact suffices to
excite the nerve; but if there be an interruption of continuity in the
nerve itself, no stimulus-wave passes across that line. Cut a nerve,
and bring the divided surfaces once more into close contact, there will
still be such a solution of continuity as to arrest the stimulus-wave,
mere physical contact not sufficing for the propagation. Whereas across
the cut ends of a divided nerve, even visibly separated, the electric
current easily passes. This necessity for the vital continuity of
tissue in the propagation of stimulation must always be borne in mind.
The presence of a membrane, however delicate, or of any tissue having
a different molecular constitution, suffices to arrest or divert the
wave. I conceive, therefore, that it is absolutely indispensable that
a nerve should terminate in and _blend_ with a muscle or a centre,
otherwise no stimulation of muscle or centre will take place through
the nerve.
[Illustration: Fig. 13.]
53. The difference between excitation from contact and stimulation
from continuity may be thus illustrated. In Fig. 13 we see the legs
of a frog attached to the spine by the lumbar nerves (_l_), and lying
on the muscles (_m_) of one leg is the nerve (_c_) of another frog’s
leg. Applying the electrodes to (_l_), the muscles (_m_) are violently
contracted; not only so, but their contraction excites the other nerve
(_c_), and the leg attached to this nerve is thereby thrown into
contraction. This “secondary contraction,” as Dubois Reymond calls it,
might be supposed to be due to a diffusion of the electrical current;
but that it is due to a change in the muscles (_m_) is proved by
delicate experiments showing that the movements in the detached leg are
of precisely the same kind as those in the legs directly stimulated.
If there is only a muscular shock in the one case, there is only a
muscular shock in the other; if there is tetanus in the one, there is
tetanus in the other; if the muscles of the first leg are fatigued
and respond slowly and feebly, the response of the second is slow and
feeble. Moreover, the secondary contraction may be produced by chemical
or mechanical stimulus, as well as by the electrical.
54. Although the contraction of a muscle is thus seen to be capable
of exciting a nerve in contact with it, the reverse is not true: we
can produce no contraction in a muscle by exciting a nerve simply in
contact with the muscle, and not penetrating its tissue and terminating
there. Accordingly we always find a nerve when about to enter a muscle
or a centre losing its _protecting_ envelopes; it gradually becomes
identified as a protoplasmic thread with the protoplasm of the muscle
or the centre.
55. Neurility, then, is the propagation of molecular change. Two
offices are subserved by the nervous system, which may respectively be
called Excitation--the disturbance of molecular _tension_ in tissues,
and consequent liberation of their energies; and Co-ordination--the
direction of these several energies into combined actions. Thus, when
the muscle is in a given state of molecular tension, the stimulation of
its nerve will change that state, causing it to contract if it be in
repose. But this stimulation, which will thus cause a contraction, will
be arrested, if at the same time a more powerful stimulation reaches
the antagonist muscle, or some distant centre: then the muscle only
_tends_ to contract.
ORIGIN OF NERVE-FORCE.
56. After this brief account of Neurility we may pass to the
consideration of its origin. Are we to understand that this property
belongs to the nerves themselves in the sense in which Contractility
belongs to the muscles? or are we to accept the teaching which assigns
the origin of “nerve-force” to the ganglia, and regards the nerves
simply as passive conductors of a force developed in the cells?
57. It is now many years since I ventured to criticise the reigning
doctrine, and to urge the necessity Of consistently carrying out the
distinction between Property and Function. I called attention to the
positive evidence which contradicted the idea of passive conduction;
and pointed out the illusory nature of the favorite analogy, in which
ganglia were likened to batteries, and nerves to the conducting wires.
But the old image still exerts its empire; and writers are still
found speaking of the brain as a telegraphic bureau, the ganglia as
stations, and the nerves as wires. In the cells of the gray substance
they place a constantly renewing reservoir of nerve-force. There the
force is elaborated, stored up, and from thence directed along the
nerves. The sensory nerve “transmits an impression to the brain”--as
the wire transmits a message to the bureau. The motor nerve, in turn,
“transmits the mandates of the will”--and all is clear! Clear, until
we come to translate metaphors into visible facts, or try to conjure
up some mental image of the process. For myself, I can only conceive
nerve-force as the activity of the nerve, and not of something
_else_. This becomes still more evident when I find that the activity
is equally manifest after its imaginary source has been removed.
Transmitting impressions, or messages, implies as a preliminary that
there should be an impressible agent, or a _message-sender_, at the
periphery. No one supposes that simply touching one end of a wire
would send an “impression” or a “message” to the battery; or that
without the battery this touch would evolve a current. The battery is
indispensable; in _it_ is evolved the current which the wire transmits.
Not so the ganglion, or brain. Remove the wire from its connection with
the battery, and it is a bit of wire, nothing more. But remove a nerve
from its connection with a ganglion, and it is still active as nerve,
still displays its Neurility when excited, still moves the muscle as
before. The amputated limb will move when its nerves are stimulated,
just as when a reflex from its centre moved it. Every one knew the
fact; it was staring them in the face, yet they disregarded it. Even
the old anatomist, Willis, had recorded experiments which ought to have
opened their eyes. He tied the phrenic nerve, and found that, when
he irritated it below the ligature, the diaphragm moved; but when he
irritated it above the ligature, no movement followed. Since his days,
thousands of experiments have shown that the presence of a ganglion is
not necessary to the action of a nerve.[100]
58. Of course an explanation was ready. The nerve was said to have been
“endowed with force” from its ganglion during their vital connection;
and this force, stored up in the nerve, was disposable for some time
after separation from the ganglion. We need not pause to criticise this
misty conception of one part “endowing” another with force; the plain
facts afford the best answer. There seemed, indeed, a confirmation
of the hypothesis in the fact that although the nerve separated from
its ganglion was capable of excitation, yet after a few excitations
it was exhausted, and ceased to stimulate the muscle. It seemed like
the piece of magnetized iron which would act as a temporary magnet,
though quickly losing this borrowed power. But the whole fabric
fell--or ought to have fallen--when extended observation discovered
that this exhausted nerve would, if left in repose, _recover_ its
lost power. A nerve preserves its excitability as long as it preserves
its structural integrity, and recovers its power in recovering that
integrity. The length of time varies.[101] Gratiolet found the muscles
in the leg of a tortoise, which had been amputated a week before,
contract when the nerves were irritated; and Schiff found the divided
nerve of a winter frog excitable at the end of three weeks. Even
after all excitability has disappeared, it will reappear if arterial
blood be injected; just as muscles which have already begun to assume
cadaveric rigidity recover their contractility after transfusion. Nor
is this all. The separated nerve finally degenerates, and loses all its
structural characters and physiological properties; yet under favorable
conditions it will regenerate--recover its structures and properties;
and this even apart from a centre, as Vulpian showed. Very noticeable
is the fact that the force said to be produced in the centre, and only
“conveyed” by the nerve, vanishes gradually from the centre to the
periphery, and recovers from the periphery to the centre--the part of
the nerve which is farthest from the centre being excitable when the
part nearest the centre is still inexcitable. Again, when a nerve is
pinched, contraction in the muscle follows; but the pinch has for a
time so disturbed the structural integrity of the nerve (at that spot)
that no irritant applied to the spot, or _between_ it and the centre,
will be followed by contraction, whereas _below_ the spot an irritation
takes effect. This is another form of the experiment of Willis. Even in
its normal state, the nerve has different degrees of excitability in
different parts of its course,--a fact discovered by Pflüger which is
quite irreconcilable with the hypothesis of passive conduction. Doubts
have been thrown on Pflüger’s interpretation,[102] namely, that there
is an avalanche-like accumulation of energy proportionate to the length
of the stimulated portion; but the fact remains, that one and the
same irritant applied successively to two different points of a nerve
does not irritate the muscle in the same degree. Munk also finds the
velocity of transmission in a motor nerve increases as it approaches
its termination in the muscle.[103]
59. Nothing can be more unlike the conduction of an electric current
than this excitation of Neurility; nothing more accordant with the
idea of it as a vital property of the tissue. The notion of its being
derived from a centre is on a par with the notion first successfully
combated by Haller,[104] that the muscle derived its Contractility
from the nerves; or the analogous notion that the electric organ
in fishes derived its property from the brain. Indeed, it was in
support of the hypothesis that the brain was a battery, and nerves
the conductors, that the phenomena observed in electrical fishes were
frequently cited. The electric organ was seen to be connected with
the brain; its discharges were under the control of the animal, and
were destroyed on one side when the brain on the corresponding side
was destroyed. But Charles Robin long ago suggested, what indeed ought
never to have been doubted, that the brain was not the source of the
electricity; but that the tissue of the electric organ itself had this
special property, which the nerve merely called into activity. The
suggestion has been experimentally verified by M. Moreau, who divided
all the nerves supplying the electric organ on one side, and, having
thus cut off all communication with the brain, produced electrical
discharges by irritating the nerves; precisely as the muscles are made
to contract when the divided nerves are irritated. Had the experiment
ceased here, it might have been interpreted on the old hypothesis: the
electric organ might be supposed to have a certain amount of electric
force condensed in it, stored up there, as it is said to be in the
nerves, and discharged when the organ is irritated. But experiment has
decided this point also. Electric fishes notoriously exhaust their
power by a few discharges, and recover it after repose. When M. Moreau
had exhausted his mutilated fishes, he replaced them in the water,
and allowed them repose. On again irritating the divided nerves, the
discharges were again produced.[105]
60. On all sides the idea of nerves deriving their power from another
source than their own substance is seen to be untenable. _A priori_
this might have been concluded. Neurility is the vital property of
nerve-tissue. “Nerve-force” is nerve-action--molecular changes in the
nerve itself, not in some remote substance. That nerve and centre are
vitally connected is true; and what their physiological relations are
will hereafter be examined; but we must dismiss the idea of nerves
having the relation to centres that electrodes have to batteries.
61. In proposing the term Neurility, I not only wished to get rid of
the ambiguities which hovered round “nerve-force” and “nerve-current,”
but to recall the physiological principle that properties are
dependent on structures; and therefore that the special property of
nerve-tissue is conditioned by its structure. Neurility is, of course,
an abstraction; but so is _the_ nerve an abstraction. The concrete
manifestations are the several nerve-actions. These we classify and
specify. One class we call sensory, another class motor; not because
the nerve-action itself is different, but because it is in each class
in a different functional relation to other parts. In classing men
as governors and governed, employers and employed, we do not suppose
anthropological distinctions, but only differences in their social
functions.
62. This is the modification of the Law of Bell to which reference was
made in § 26. It replaces the idea of two different kinds of nerve,
sensory and motor, by that of two different anatomical connections. I
need not reproduce here the argument with which I formerly criticised
the supposed distinction between sensory and motor nerves; because
the old idea is rapidly falling into discredit, and physiologists so
eminent as Vulpian and Wundt have explicitly announced their adhesion
to the principle of identity,--a principle which, as Vulpian truly
remarks, dominates the whole physiology of the nervous system.[106]
THE HYPOTHESIS OF SPECIFIC ENERGIES.
63. One development of the theory of Bell, respecting the different
kinds of nerve, has been the still accredited hypothesis that each
nerve has a “specific energy,” or quality, in virtue of which it
acts and reacts only in one way. The optic nerve, no matter how
stimulated, only responds by a sensation of color, the auditory nerve
only by a sensation of sound; and so on. This hypothesis, which (as
I learn from a correspondent)[107] was originally propounded by Bell
himself, was developed and made an European doctrine by Johannes
Müller, first in his remarkable treatise, _Über die phantastischen
Gesichtserscheinungen_ (1826), and afterwards in his _Physiology_.
Like all good hypotheses, it has been fruitful; and Helmholtz
still holds it to be of extraordinary importance for the theory of
perception. Although combated by a few physiologists, it has kept its
place firm in the general acceptance; no doubt because it forms a ready
explanation of the facts. But, as I often have to remark, _explanation_
is not _demonstration_.[108]
64. The first criticism to be made on the hypothesis is that it
commits the error of confounding function with property, assigning as
a specific quality of the nerve the reaction of the organ innervated.
Thus Müller speaks of the specific energy as “the essential condition
of the nerves in virtue of which they see light and hear sound.”
But the optic nerve no more _sees_, than the liver-nerve _secretes_
bile. That the optic nerve is one element in the mechanism on which
vision depends, is all that we can say, Müller declares that it is
not sufficient to assume each nerve to be so constituted that it
has a susceptibility to certain stimuli rather than to others; but
that “with Aristotle we must ascribe to each a peculiar energy as
its vital quality. Sensation,” he adds, “consists in the sensorium
receiving through the medium of the nerves a _knowledge of certain
qualities_,--a condition, not of the external bodies, but of the
nerves themselves,”--and these qualities are different in different
nerves. In other words, he assumes a special substance for each special
energy. The sensation of color depends on the special Visual substance
(_Sehsinnsubstanz_); the sensation of sound on the Auditory substance
(_Hörsinnsubstanz_); and so on.
65. We have here an hypothesis analogous to that of Innate Ideas, or
_a priori_ Forms of Thought. It is, in fact, only a reproduction of
that conception carried into the sphere of Sense. No one thinks of
assigning specific energies to the several muscles, yet a movement of
prehension is as different from a movement of extension, a peristaltic
movement is as different from a movement of occlusion, as a sensation
of sound is from a sensation of color. If movement is common to both
of the one class, feeling is common to both of the other: the forms
and mechanism are different and specific. Muscles have the common
property of contracting under stimulation; whatever be the nature of
the stimulus, each muscle has its own particular response, or mode of
reaction: the flexor always bending, never extending the limb; the
sphincter always closing, never opening the orifice. The movements
of the heart are not the same as those of the eye; both are unlike
the movements of the intestine. There are muscles which respond to
some stimuli, and not to others. Those of the eye, or of the vocal
chords, respond to impulses which would leave the masseter or biceps
unstirred. According to Marey, the hyoglossus of a frog will become
tetanic under a stimulus of only ten pulses in a second; whereas the
gastrocnemius of that same frog resists a stimulus of less than twenty
in a second. We find the retina responding to ethereal pulses which
leave the auditorius unaffected; we find the muscles of a gnat’s wing
so exquisitely susceptible that the wing beats eight thousand times in
a second,--a delicacy in comparison with which even our muscles of the
eye are coarse.
66. The facts which the hypothesis of specific energies is called
on to explain are more consistently interpreted on the admission of
a common property in nerve-tissue, manifesting different degrees of
excitability, and entering into different mechanisms, so that the
functional results differ. A nerve which may be stimulated from the
skin will not respond at all, or not in the same way, if the stimulus
be applied under the skin. Are we to suppose that the specific energy
resides in one part of the nerve, and not in another?[109] That the
optic nerve responds to stimuli which will not sensibly excite a
motor nerve, depends on the terminal structures through which the
stimulation is excited; for the optic nerve itself, apart from the
retinal expansion, is as insensible to light as the motor nerve is. And
the specific sensation, or movement, which _results_ from stimulation
of a nerve depends not on the nerve, but on the mechanism of which the
nerve is one element. Sensations of touch, temperature, and pain are
assuredly specific; they are as unlike each other as a sensation of
taste is unlike a sensation of smell. Yet the same nerves, variously
stimulated, produce all three sensations.
67. We conclude, therefore, that the phrase “specific energy” is an
elliptical expression for the particular office of a nerve. In this
meaning there is no obscurity. The optic nerve is not a vasomotor
nerve, the skin nerve is not a muscle nerve; the auditory nerve is a
nerve of special sensation, the vagus is a nerve of systemic sensation;
and so on. Neither movement nor sensation belongs to the nerves
themselves.