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

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.