SENSIBILITY.
68. The principles laid down in the preceding chapter are equally
applicable to the central system. But here greater difficulties await
us. We cannot expect traditional views to be easily displaced, when
they have taken such hold on the mind, as is the case with regard to
Sensibility. To admit that all nerves have a common property, and that
their functional relations depend on the organs which they innervate,
demands small relinquishment of cherished opinions. But to admit that
all nerve-centres have a common property, and that their functional
relations depend on their anatomical connections, is to sweep away at
once a mass of theoretic interpretations which from long familiarity
have acquired an almost axiomatic force. That the brain, and the brain
only, is the source and seat of Sensibility is the postulate of modern
Physiology.
69. The question is one of extreme complexity, but may be greatly
simplified, if we can manage to reduce it to purely physiological
terms, and consider the phenomena in their objective aspect. In dealing
with nerves and their actions this was comparatively easy; we had for
the most part only physiological processes to unravel. It is otherwise
in dealing with nerve-centres--the subjective or psychological aspect
of the phenomena inevitably thrusts itself on our attention; and all
the mysteries of Feeling and Thought cloud our vision of the neural
process. Do what we will, we cannot altogether divest Sensibility of
its psychological connotations, cannot help interpreting it in terms
of Consciousness; so that even when treating of sensitive phenomena
observed in molluscs and insects, we always imagine these more or less
suffused with Feeling, as this is known in our own conscious states.
70. Feeling is recognized as in some way or other bound up with
neural processes; but Physiology proper has only to concern itself
with the processes; and the question whether these can, and do, go on
unaccompanied by Feeling, is, strictly speaking, one which belongs
to Psychology. It demands as a preliminary that the term Feeling be
defined; and the answer will depend upon that definition, namely,
whether Feeling be interpreted as synonymous with Consciousness in the
restricted sense, or synonymous with the more general term Sentience.
If the former, then since there are unquestionably neural processes of
which we are not conscious, we must specify the particular groups which
subserve Feeling; as we specify the particular groups which subserve
the sensations of Sight, Hearing, Taste, etc.; and localize the
separate functions in separate organs. If the latter, then, since all
neural processes have a common character, we have only to localize the
particular variations of its manifestation, and distinguish sensitive
phenomena as we distinguish motor phenomena.
71. It is absolutely certain that the Feeling we attribute to a
mollusc is different from that which we attribute to a man; if only
because the organisms of the two are so widely different, and have
been under such different conditions of excitation. If every feeling
is the functional result of special organic activities, varying with
the co-operant elements, we can have no more warrant for assuming the
existence of the same _particular_ forms of Feeling in organisms
that are unlike, than for assuming the 47th proposition of Euclid to
be presented by any three straight lines. The lines are the necessary
basis for the construction, but they are not the triangle, except when
in a special configuration. This is not denying that animals _feel_
(in the general sense of that term), it is only asserting that their
feelings must be very _unlike_ our own. Even in our own race we see
marked differences--some modes of feeling being absolutely denied to
individuals only slightly differing from their fellows. If, however,
we admit that different animals must have different modes of Feeling,
we must also admit that the neuro-muscular activities are generically
alike in all, because of the fundamental similarity in the structures.
Whether we shall assign Feeling to the mollusc or not will depend on
the meaning of the term; but, at all events, we require some term
general enough to include the phenomena manifested by the mollusc,
and those manifested by all other animals. Sensibility is the least
objectionable term. Unless we adopt some such general designation,
physiological and psychological interpretations become contradictory
and obscure. The current doctrine which assigns Sensibility to the
brain, denying it to all other centres, is seriously defective,
inasmuch as it implies that tissues similar in kind have utterly
diverse properties; in other words, that the same nerve-tissue which
manifests Sensibility in the brain has no such property in the spinal
cord.
72. How is this tenable? No one acquainted at first hand with the facts
denies that the objective phenomena exhibited by the brainless animal
have the same general character as those of the animal possessing a
brain: the actions of the two are identical in all cases which admit
of comparison. That is to say, the objective appearances are the
same; differing only in so far as the mechanisms are made different
by the presence or absence of certain parts. The brain not being a
necessary part of the mechanical adjustments in swimming, or pushing
aside an irritating object, the brainless frog swims and defends itself
in the same way as the normal frog. But no sooner do we pass from
the objective interpretation, and introduce the subjective element
of Feeling among the series of factors necessary to the product--no
sooner do we ask whether the brainless frog _feels_ the irritation
against which it struggles, or _wills_ the movements by which it
swims--than the question has shifted its ground, and has passed from
Physiology to Psychology. The appeal is no longer made to Observation,
but to Interpretation. Observation tells us here nothing directly
of Feeling. What it does tell us, however, is the identity of the
objective phenomena; and Physiology demands that a common term be
employed to designate the character common to the varied phenomena.
Sensibility is such a term. But most modern physiologists, under the
bias of tradition, refuse to extend Sensibility to the spinal cord,
in spite of the evidences of the spinal cord possessing that property
in common with the brain. They prefer to invoke a new property; they
assign spinal action to a Reflex Mechanism which has nothing of the
character of Sensibility, because they have identified Sensibility with
Consciousness, and have restricted Consciousness to a special group of
sensitive phenomena.
73. Nor is it to be denied that on this ground they have a firm basis.
Every one could testify to the fact that many processes normally
go on without being accompanied by consciousness, in the special
meaning of the term. Reflex actions,--such as winking, breathing,
swallowing,--notoriously produced by stimulation of sensitive surfaces,
take place without our “feeling” them, or being “conscious” of them.
Hence it is concluded that the Reflex mechanism suffices without the
intervention of Sensibility. I altogether dispute the conclusion; and
in a future Problem will endeavor to show that Sensibility is necessary
to Reflex Action. But without awaiting that exposition we may at once
confront the evidence, by adducing the familiar fact that “unconscious”
processes go on in the brain as well as in the spinal cord; and this
not simply in the sphere of Volition, but also in the sphere of
Thought.[110] We act and think “automatically” at times, and are quite
“unconscious” of what we are doing, or of the data we are logically
grouping. We often think as unconsciously as we breathe; although from
time to time we become conscious of both processes. Yet who will assert
that these unconscious processes were independent of Sensibility?
Who will maintain that because cerebral processes are sometimes
unaccompanied by that peculiar state named Consciousness, therefore all
its processes are unaccompanied by Feeling? And if here we admit that
the Reflex mechanism in the brain is a _sensitive_ mechanism, surely we
must equally admit that the similar Reflex mechanism in the spinal cord
is sensitive?
74. Let it be understood that Sensibility is the common property of
nerve-centres, and physiological interpretations will become clear
and consistent. Consciousness, as understood by psychologists, is not
a property of tissue, it is a function of the organism, dependent
indeed on Sensibility, but not convertible with it. There is a greater
distinction between the two than between Sensation, the reaction of
a sensory organ, and Perception, the combined result of sensory and
cerebral reactions; or than that between Contractility, the property of
the muscles, and Flying, the function of a particular group of muscles.
It is not possible to have Consciousness without Sensibility; but
perfectly possible to have Sensations without Consciousness. This will
perhaps seem as inconceivable to the reader as it seemed to Schröder
van der Kolk.[111]
75. Let us illustrate it by the analogy of Pain. There is a vast amount
of sensation normally excited which is totally unaccompanied by the
feelings classed as painful. The action of the special senses may be
exaggerated to an intolerable degree, but the exaggeration never passes
into pain: the retina may be blinded with excess of light, and the ear
stunned with sound--the optic nerve may be pricked or cut--but no pain
results. The systemic sensations also are habitually painless, though
they pass into pain in abnormal states. Clearly, then, Pain is not the
necessary consequence of Sensibility; and this is true not only of
certain sensitive parts, but of all; as is proved in the well-known
facts of Analgesia, in which complete insensibility of the skin as
regards Pain co-exists with vivid sensibility as regards Touch and
Temperature. Hence the majority of physiologists refuse to acknowledge
that the struggles and cries of an animal, after removal of the brain,
are evidences of pain; maintaining that they are “simply reflex
actions.” This is probable; the more so as we know the struggles and
cries which tickling will produce, yet no pain accompanies tickling.
But if the struggles and cries are not evidence of pain, they are
surely evidence of Sensibility.
76. Now for the term Pain in the foregoing paragraph substitute the
term Consciousness, and you will perhaps allow that while it may be
justifiable to interpret the actions of a brainless animal as due to a
mechanism which is unaccompanied by the _specially conditioned forms_
of Sensibility classed under Consciousness--just as it is unaccompanied
by the specially conditioned forms of Perception and Emotion--there
is no justification for assuming the mechanism not to have been a
_sensitive_ mechanism. The wingless bird cannot manifest any Of the
phenomena of flight; but we do not therefore deny that its _other_
movements depend on Contractility.
77. Difficult as it must be to keep the physiological question apart
from the psychological when treating of Sensibility, we shall never
succeed in our analysis unless the two questions are separately
treated. The physiologist considers organisms and their actions
from their objective side, and tries to detect the mechanism of the
observed phenomena. These he has to interpret in terms of Matter and
Motion. The psychologist interprets them in terms of Feeling. The
actions which we _see_ in others we cannot _feel_, except as visual
sensations; the changes which we feel in ourselves we cannot see in
others, except as bodily movements. The reaction of a sensory organ is
by the physiologist called a sensation,--borrowing the term from the
psychologist; he explains it as due to the stimulus which changes the
molecular condition of the organ; and this changed condition, besides
being seen to be followed by a muscular movement, is _inferred_ to
be accompanied by a change of Feeling. The psychologist has direct
knowledge only of the change of Feeling which follows on some other
change; he infers that it is originated by the action of some external
cause, and infers that a neural process precedes, or accompanies, the
feeling. Obviously there are two distinct questions here, involving
distinct methods. The physiologist is compelled to complete his
objective observations by subjective suggestions; compelled to add
Feeling to the terms of Matter and Motion, in spite of the radical
diversity of their aspects. The psychologist also is compelled to
complete his subjective observations by objective interpretations,
linking the internal changes to the external changes. A complete theory
must harmonize the two procedures.
78. In a subsequent Problem we shall have to examine the nature of
Sensation in its psychological aspect; here we have first to describe
its physiological aspect. To the psychologist, a sensation is simply a
fact of Consciousness; he has nothing whatever to do with the neural
process, which the physiologist considers to be the physical basis
of this fact; and he therefore regards the physiologists as talking
nonsense when they talk of “unconscious sensations,” the phrase being
to him equivalent to “unfelt feelings,” or “invisible light.” It is
quite otherwise with the physiologist, who viewing a sensation solely
as a neural process, the reaction of a sensory organ, can lawfully
speak of unconscious sensations, as the physicist can speak of
invisible rays of light,--meaning those rays which are of a different
order of undulation from the visible rays, and which may become visible
when the susceptibility of the retina is exalted. He knows that there
are different modes, and different complexities of neural process; to
one class he assigns consciousness, to the other unconsciousness. If he
would be severely precise, he would never speak of sensation at all,
but only of sensory reaction. But such precision would be pedantic and
idle. He wants the connotations of the term sensation, and therefore
uses it.
79. The functional activity of a gland is stimulated by a neural
process reflected from a centre; by a similar process a muscle is
called into action. No one supposes that the neural process is, in
the one case secretory, in the other motory: in both it is the same
process in the nerve; and our investigation of it would be greatly
hampered if we did not disengage it from all the suggestions hovering
around the ideas of secretion and muscular action. In like manner
we must disengage the neural process of a sensory reaction from all
the suggestions hovering around the idea of Consciousness, when that
term designates a complex of many reactions. In Problem III. we shall
enter more particularly into the distinction between Sensibility and
Consciousness; for the present it must suffice to say that great
ambiguity exists in the current usage of these terms. Sometimes
Consciousness stands as the equivalent of Sensibility; sometimes as a
particular _mode_ of Sensibility known as Reflection, Attention, and
Thought. The former meaning is an extension of the term similar to
that given to the word Rose, which originally meaning Red came to be
restricted to a particular red flower; and after other flowers of the
same kind were discovered which had yellow and white petals, instead
of red, the term rose still adhered even to these. “Yellow Rose” is
therefore as great a verbal solecism as unconscious sensation. We have
separated the redness from the rose, and can then say that the color
is one thing, the flower another. By a similar process of abstraction
we separate Consciousness from Sensation, and we can then say that
there are sensations without consciousness. In consequence of this,
psychologists often maintain that to have a sensation and be conscious
of it are two different states. We are said to hear a sound, and yet
not to be conscious of hearing it. The sound excites a movement, but
it does not excite our consciousness. Now although it is true that
there are roses which are not red, it is not true that there are
roses which have no color at all. Although it is true that there are
sensations which are not of the particular mode of Sensibility which
psychologists specially designate as Consciousness, it is not true
that there are sensations which are not modes of Sensibility.
80. And what is Sensibility which, on its subjective side, is
Sentience? In one sense it may be answered that we do not know. In
another sense it is that which we know most clearly and positively:
Sentience forms the substance of all knowledge. Being the ultimate of
knowledge, every effort must be vain which attempts to explain it by
reduction to simpler elements. The human mind, impatient of ultimates,
is always striving to pierce beyond the fundamental mysteries; and this
impatience leads to the attempts so often made to explain Sensibility
by reducing it to terms of Matter and Motion. But inasmuch as a clear
analysis of Matter and Motion displays that our knowledge of these is
simply a knowledge of modes of Feeling, the reduction of Sentience or
Sensibility to Matter and Motion is simply the reduction of Sensibility
to some of its modes. This point gained, a clear conception of the
advantages of introducing the ideas of Matter and Motion will result.
It will then be the familiar and indispensable method of explaining
the little known by the better known. The objective aspect of things
is commonly represented in the visible and palpable; because what we
can see we can also generally touch, and what we can touch we can
taste and smell; but we cannot touch an odor nor a sound; we cannot
see them; we can only connect the odorous and sonorous objects with
visible or palpable conditions. Everywhere we find sensations referred
to visible or palpable causes; and hence the desire to find this
objective basis for every change in Sensibility. The sensation, or
state of consciousness, is the ultimate fact; we can only _explain_ it
by describing its objective conditions.
81. Thus much on the philosophical side. Returning to our physiological
point, we must say that a sensation is, objectively, the reaction
of a sensory organ, or organism; subjectively, a change of feeling.
Objectively it is a phenomenon of movement, but distinguishable
from other phenomena by the speciality of its conditions. It is a
vital phenomenon, not a purely mechanical phenomenon. Although the
molecular movement conforms, of course, to mechanical principles, and
may be viewed abstractly as a purely mechanical result, yet, because
it takes place under conditions never found in machines, it has
characters which markedly separate it from the movements of machines.
Among these differential characters may be cited that of _selective
adaptation_,[112] which is most conspicuous in volition.
82. In the early stages of animal evolution there is no differentiation
into muscle and nerve. The whole organism is equally sensitive (or
irritable) in every part. Muscles appear, and then they are the most
sensitive parts. Nerves appear, and the seat of Sensibility has
been transferred to them; not that the muscles have lost theirs, but
their irritability is now represented by their dominant character of
Contractility, and the nerves have taken on the special office of
Sensibility. That is to say, while both muscle and nerve form integral
elements of the sensitive reaction, the process itself is analytically
conceived as a combination of two distinct properties, resident in two
distinct tissues.
83. Carrying further this analytical artifice, I propose to distinguish
the central organs as the seat of Sensibility, confining Neurility to
the peripheral nerves. In physiological reality both systems, central
and peripheral, are one; the separation is artificial. Strictly
speaking, therefore, Neurility--or nerve-action--is the general
property of nerve-tissue, central and peripheral. But since Neurility
may be manifested by nerves apart from centres, whereas Sensibility
demands the co-operation of both, and since we have often to consider
the central process in itself, without attending to the process in the
nerves, it is well to have two characteristic terms. I shall therefore
always use the term Sensibility for the reactions of the nervous
centres,--Sentience being its psychological equivalent; although the
reader will understand that in point of fact there is no break, nor
transformation, as the wave of change passes from sensory nerve to
centre, and from centre to motor nerve: there is one continuous process
of change. But just as we analytically distinguish the sensory from
the motor element of this indissoluble process, so we may distinguish
the ingoing and outgoing stages from the combining stage. Sensibility,
then, represents the property of _combining_ and grouping stimulations.
84. Fully aware of the misleading connotations of the term, and of the
difficulty which will be felt in disengaging it from these, especially
in reference to Consciousness, I have long hesitated before adopting
it. But the advantages greatly outweigh the disadvantages. Sensibility
has long been admitted to express the peculiar modes of reaction in
plants and animals low down in the scale. No one hesitates to speak of
a sensitive plant, or a sensitive surface. The tentacles of a polype
are said to be sensitive; though probably no one thereby means that
the polype has what psychologists mean by Consciousness. By employing
the general term Sensibility to designate the whole range of reactions
peculiar to the nerve-centres, when these special organs exist, it
will be possible to interpret all the physiological and psychological
phenomena observed in animals and men on one uniform method. The
observed variations will then be referable to varieties in organisms.
85. Suppose, for illustration, an organism like the human except
that it is wholly deficient in Sight, Hearing, Taste, and Smell. It
has no sense but Touch--or the general reaction under contact with
external objects. It will move on being stimulated, and will combine
its movements differently under different stimulations. It will feel,
and logically combine its feelings. But its mass of feeling will be
made of far simpler elements than ours; its combinations fewer; and
the contents of its Consciousness so very different from ours that
we are unable to conceive what it will be like; we can only be sure
that it will _not_ be very like our own. This truncated Organism will
have its Sensibility; and we must assign this property to its central
nerve-tissue, as we assign our own. If now we descend lower, and
suppose an organism with no centres whatever, but which nevertheless
displays evidence of Sensibility--feelings and combinations of
movements--we must then conclude that the property specialized in
a particular tissue of the highly differentiated organism is here
diffused throughout.
It is obvious that the sensations or feelings of these supposed
organisms will have a common character with the feelings of more highly
differentiated organisms, although the modes of manifestation are so
various. If we recognize a common character in muscular movements so
various as the rhythmic pulsation of the heart, the larger rhythm of
inspiration and expiration, the restless movements of the eye and
tongue, the complexities of manipulation, the consensus of movements in
flying, swimming, walking, speaking, singing, etc., so may we recognize
a common character in all the varieties of sensation. The special
character of a movement depends on the moving organ. The special
character of a sensation depends on the sensory organ. Contractility
is the abstract term which expresses all possible varieties of
contraction. Sensibility--or Sentience--is the abstract term which
expresses all possible varieties of sensation.
86. The view here propounded may find a more ready acceptance when its
application to all physiological questions has been tested, and it is
seen to give coherence to many scattered and hitherto irreconcilable
facts. Meanwhile let a glance be taken at the inconsistencies of the
current doctrine. That doctrine declares one half of the gray substance
of the spinal cord to be capable only of _receiving_ a sensitive
stimulation, the other half capable only of _originating_ a motor
stimulation. We might with equal propriety declare that one half of a
muscle is capable only of receiving a contractile stimulation, and the
other half of contracting. The ingoing nerve, passing from the surface
to the posterior part of the spinal cord, excites the activity of the
gray substance into which it penetrates; with the anterior part of
this gray substance an outgoing nerve is connected, and through it
the excitation is propagated to a muscle: contraction results. Such
are the facts. In our analysis we separate the sensory from the motor
aspect, and we then imagine that this ideal distinction represents a
real separation. We suppose a phenomenon of Sensibility independent of
a phenomenon of Contractility--suppose the one to be “transformed” into
the other--and we then marvel “how during this passage the excitation
changes its nature.”[113]
87. Before exerting ingenuity in explaining a fact, it is always well
to make sure that the fact itself is correctly stated. _Does_ the
neural excitation change its nature in passing from the posterior to
the anterior gray substance? I can see no evidence of it. Indeed the
statement seems to confound a neural process with a muscular process.
The neural process is one continuous excitation along the whole line
of ingoing nerve, centre, and outgoing nerve, which nowhere ceases
or changes into another process, until the excitation of the muscle
introduces a new factor. So long as the excitation keeps within the
nerve-tissue, it is one and the same process of change; its issue
in a contraction, a secretion, or a change in the conditions of
consciousness, depends on the organs it stimulates.
88. I have already called attention to the artificial nature of all our
distinctions, and the necessity of such artifices. They are products of
that
“Secondary power
By which we multiply distinctions, then
Deem that our puny boundaries are things
That we perceive, and not that we have made.”[114]
The distinction of Central and Peripheral systems is not simply
anatomical, it has a physiological justification in this, that the
Central System is the organ of connection. Any one part of it directly
excited by an ingoing nerve propagates that excitation throughout
the whole central mass, and thus affects every part of the organism.
Therefore we place Sensibility in it.
But this general Property subserves various Functions, according as the
Central System is variously related to different organs. This fact has
given rise to the idea that different portions of the cerebro-spinal
axis have different properties--which is a serious error. What is
certain is that the Cerebrum must have a different function from that
of the Thalami, and the Cerebellum one different from the Medulla
Oblongata; while that of the Medulla Spinalis is different from all.
Precisely on the same grounds that a muscle-nerve has a different
office from a skin-nerve, or the pneumogastric from the acoustic.
But all nerves have one Neurility in common; all centres have one
Sensibility in common.