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.
CHAPTER IV.
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.
CHAPTER V.
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.
CHAPTER VI.
WHAT IS TAUGHT BY EMBRYOLOGY?
97. Subject to the qualification expressed in the last chapter, stimulation of muscles and glands involves a neural process in ingoing nerve, centre, and outgoing nerve. These are the triple elements of the “nervous arc.” If muscles were directly exposed to external influences, they would be stimulated without the intervention of a centre; but as a matter of fact they never are thus exposed, being always protected by the skin. Did the skin-nerves pass directly to the muscles underneath, they would move those muscles, without the intervention of a centre; but as a matter of fact the skin-nerves pass directly to a centre, so that it is only through a centre that they can act upon the muscles. Were muscles and glands directly connected with sensitive surfaces, their activity would indeed be awakened by direct stimulation; but unless the muscles were so connected the one with the other, by anastomosis of fibres or continuity of tissue, that the movement of one was the movement of all, there would need to be some other channel by which their separate energies should be combined and co-ordinated. In the higher organisms anastomosis of muscles is rare, and the combination is effected by means of the nerves.
98. Although analysis distinguishes the two elements of the neuro-muscular system, assigning separate properties to the separate tissues, an interpretation of the phenomena demands a synthesis, so that a movement is to be conceived as always involving Sensibility, and a sensation as always involving Motility.[126] In like manner, although analysis distinguishes the various organs of the body, assigning separate functions to each, our interpretation demands their synthesis into an organism; and we have thus to explain how the whole has different parts, and how these different parts are brought into unity. Embryology helps us to complete the fragmentary indications of Anatomy and Physiology.
99. Take a newly laid egg, weigh it carefully, then hatch it, and when the chick emerges, weigh both chick and shell: you will find that there has been no increase of weight. The semifluid contents have become transformed into bones, muscles, nerves, tendons, feathers, beak, and claws, all without increase of substance. There has been differentiation of structure, nothing else. Oxygen has passed into it from without; carbonic acid has passed out of it. The molecular agitation of heat has been required for the rearrangements of the substance. Without oxygen there would have been no development. Without heat there would have been none. Had the shell been varnished, so as to prevent the due exchange of oxygen and carbonic acid, no chick would have been evolved. Had only one part of the shell been varnished, the embryo would have been deformed.
99a. The patient labors of many observers (how patient only those can conceive who have made such observations!) have detected something of this wondrous history, and enabled the mind to picture some of the incessant separations and reunions, chemical and morphological. Each stage of evolution presents itself as the consequence of a preceding stage, at once an emergence and a continuance; so that no transposition of stages is possible; each has its appointed place in the series (Problem I. § [107]). For in truth each stage is a process—the sum of a variety of co-operant conditions. We, looking forward, can foresee in each what it will become, as we foresee the man in the lineaments of the infant; but in this prevision we always presuppose that the regular course of development will proceed unchecked through the regular succession of special conditions: the infant becomes a man only when this succession is uninterrupted. Obvious as this seems, it is often disregarded; and the old metaphysical conception of potential powers obscures the real significance of Epigenesis. The potentiality of the cells of the germinal membrane is simply their capability of reaching successive stages of development under a definite series of co-operant conditions. We foresee the result, and personify our prevision. But that result will not take place unless all the precise changes that are needful serially precede it. A slight pressure in one direction, insufficient to alter the chemical composition of the tissue, may so alter its structure as to disturb the regular succession of forms necessary to the perfect evolution.
100. The egg is at first a microscopic cell, the nucleus of which divides and subdivides as it grows. The egg becomes a hollow sphere, the boundary wall of which is a single layer of cells, all so similar that to any means of appreciation we now possess they are indistinguishable. They are all the progeny of the original nucleus and yolk, or cell contents. Very soon, however, they begin to show distinguishable differences, not perhaps in kind, but in degree. The wall of this hollow sphere is rapidly converted into the germinal membrane, out of which the embryo is formed. Kowalewsky (confirmed by Balfour) has pointed out how in the Amphioxus the hollow sphere first assumes an oval shape, and then, by an indentation of the under side, with corresponding curvature of the upper side, presents somewhat the shape of a bowl. The curvature increases, and the curved ends approaching each other, the original cavity is reduced to a thin line separating the upper from the under surface. The cavity of the body is formed by the curving downwards of this double layer of the germinal membrane.
101. This is not precisely the course observable in other vertebrates; but in all, the germinal membrane, which lies like a watch-glass on the surface of the yolk, is recognizable as two distinct layers of very similar cells. What do these represent? They are the starting-points of the two great systems: Instrumental and Alimental. The one yields the dermal surface; the other the mucous membrane. Each follows an independent though analogous career. The yolk furnishes nutrient material to the germinal membrane, and so passes more or less directly into the tissues; but unlike the germinal membrane, it is not itself to any great extent the seat of generation by segmentation. There are two yolks: the yellow and the white (which must not be confounded with what is called the white of egg); and their disposition may be seen in the diagram ([Fig. 14]) copied from Foster and Balfour’s work. The importance of the white yolk is that it passes insensibly into a distinct layer of the germinal membrane, between the two primary layers.[127] Each of the three layers of the germinal membrane has its specific character assigned to it by embryologists, who, however, are not all in agreement. Some authorities regard the topmost layer as the origin of the nervous system, the epidermis, with hair, feathers, nails, horns, the cornea and lens of the eye, etc. To the middle layer are assigned the muscular and osseous systems, the sexual organs, etc. To the innermost layer, the alimentary canal, with liver, pancreas, gastric and enteric glands. Other authorities are in favor of two primary layers: one for the nervous, muscular, osseous, and dermal systems; the other for the viscera and unstriped muscles. Between these two layers, a third gradually forms, which is specially characterized as the vascular.
Fig. 14.—Diagrammatic section of an unincubated hen’s egg. bl, blastoderm; w y, white yolk; y y, yellow yolk; v t, vitelline membrane; x and w, layers of albumen; ch l, chalaza; a ch, air-chamber; i s m, internal layer of shell membrane; s m, external layer; s, shell.
102. Messrs. Foster and Balfour, avoiding the controverted designations of serous, vascular, and mucous layers, or of sensorial, motor germinative, and glandular layers, employ designations which are independent of theoretic interpretation, and simply describe the position of the layers, namely, epiblast for the upper, mesoblast for the middle, and hypoblast for the under layer. From the epiblast they derive the epidermis and central nervous system (or would even limit the latter to the central gray matter), together with some parts of the sense-organs. From the mesoblast, the muscles, nerves (and probably white matter of the centres), bones, connective tissue, and blood-vessels. From the hypoblast, the epithelial lining of the alimentary canal, trachea, bronchial tubes, as well as the liver, pancreas, etc.[128] Kölliker’s suggestion is much to the same effect, namely, that the three layers may be viewed as two epithelial layers, between which subsequently arises a third, the origin of nerves, muscles, bones, connective tissue, and vessels.[129]
103. The way in which the history may be epitomized is briefly this: There are two germinal membranes, respectively representing the Instrumental and Alimental Systems. Each membrane differentiates, by different appropriations of the yolk substance, into three primary layers, epithelial, neural, and muscular. In the epiblast, or upper membrane, these layers represent: 1°, the future epidermis with its derivatives—hair, feathers, nails, skin glands, and chromatophores; 2°, the future nervous tissue; 3°, the future muscular tissue.[130] (Bone, dermis, connective tissue, and blood-corpuscles are subsequent formations.)
The hypoblast, or under membrane, in an inverted order presents a similar arrangement: 1°, the unstriped muscular tissue of viscera and vessels; 2°, the nervous tissue of the sympathetic system; 3°, the epithelial lining of the alimentary canal with its glands.
Fundamentally alike as these two membranes are, they have specific differences; but in both we may represent to ourselves the embryological unit constituted by an epithelial cell, a nerve-cell, and a muscle-cell. All the other cells and tissues are adjuncts, necessary, indeed, to the working of the vital mechanism, but subordinated to the higher organites.
104. This conception may be compared with that of His in the division of Archiblast and Parablast assigned by him to the germ and accessory germ.[131] We can imagine, he says, the whole of the connective substances removed from the organism, and thus leave behind a scaffolding in which brain and spinal cord would be the axis, surrounded by muscles, glands, and epithelium, and nerves as connecting threads. All these parts stand in more or less direct relation to the nervous system. All are continuous. By a similar abstraction we can imagine this organic system removed, and leave behind the connected scaffolding which is formed from the accessory germ; but this latter has only mechanical significance; the truly vital functions belong to the other system.
105. The researches of modern histologists have all converged towards the conclusion that the organs of Sense are modifications of the surface, with epithelial cells which on the one side are connected with terminal hairs, or other elements adapted to the reception of stimuli, and are connected on the other side through nerve-fibres with the perceptive centres. It has been shown that nerve-fibres often terminate in (or among) epithelial cells—sensory fibres at the surface, and motor-fibres in the glands.[132] Whether the fibres actually penetrate the substance of the cell, or not, is still disputed. Enough for our present purpose to understand that there is a physiological connection between the two, and above all that sensory nerves are normally stimulated through some epithelial structure or other.
Fig. 15.—Transverse section of a Blastoderm incubated for eighteen hours. The section passes through the medullary groove, m e. A, epiblast. B, mesoblast. C, hypoblast. m f, medullary fold, c h, notochord.
106. And this becomes clear when we go back to the earliest indications of development. Look at [Fig. 15], representing a transverse section of the germinal membranes in a chick after eighteen hours’ incubation. Here the three layers, A, B, and C, have the aspect of simple cells very slightly differing among each other. Yet since each layer has ultimately a progeny which is characteristically distinguishable, we may speak of each not as what it now is, but what it will become. Although the most expert embryologist is often unable to distinguish the embryo of a reptile from that of a bird or of a mammal, at certain stages of evolution, so closely does the one resemble the other, yet inasmuch as the embryo of a reptile does not, cannot become a bird, nor that of a bird a mammal, he is justified in looking forward to what each will become, and in calling each embryo by its future name. On the same ground, although we cannot point to any such distinction between the layers of the blastoderm as I have indicated in the separation of Instrumental and Alimental Systems, nor specify any characters by which the cells can be recognized as epithelial, neural, and muscular, yet a forward glance prefigures these divisions. We know that the first result of the segmentation of the yolk is the formation of cells all alike, which in turn grow and subdivide into other cells. We know that these cells become variously modified both in composition and structure, and that by such differentiations the simple organism becomes a complex of organs.
107. But here it is needful to recall a consideration sometimes disregarded, especially by those who speak of Differentiation as if it were some magical Formative Principle, quite independent of the state of the organized substance which is formed. There is a luminous conception—first announced by Goethe, and subsequently developed by Milne Edwards—which regards the organism as increasing in power and complexity by a physiological “division of labor,” very similar to that division of employments which characterizes the developed social organism. But the metaphor has sometimes been misleading; it has been interpreted as indicating that Function creates Organ (see Problem I. § [88]), and as if Differentiation itself were something more than the expression of the changes resulting from the introduction of different elements. In the Social Organism a “division of labor” presupposes that laborers with their labor-materials are already existing; the change is one of rearrangement: instead of each laborer employing his skill in doing many kinds of work, he restricts it to one kind, which he is then able to do with less loss of time and power. Thus is social power multiplied without increase of population, and the social organism becomes more complex by the differentiation of its organs. It is not precisely thus with the Animal Organism during its evolution. Indeed to suppose that the differentiation of the germinal membrane into special tissues and organs takes place by any such division of employments, is to fall into the ancient error of assuming the organism to exist preformed in the ovum. The unequivocal teaching of Epigenesis is that each part is produced out of the elements furnished by previous parts; and for every differentiation there must be a difference in composition, structure, or texture—the first condition being more important than the second, the second more important than the third. The word protoplasm has almost as wide a generality as the word animal, and is often used in forgetfulness of its specific values: the protoplasm of a nerve-cell is not the same as that of a blood-cell, a muscle-cell, or a connective-tissue cell, any more than a bee is a butterfly, or a prawn a lobster. No sooner has the specific character been acquired, no sooner is one organite formed by differentiation, than there is an absolute barrier against any transformation of it into any other kind of organite. The nerve-cell, muscle-cell, and epithelial cell have a common starting-point, and a community of substance; but the one can no more be transformed into the other than a mollusc can be transformed into a crustacean. In the homogeneous cellular mass which subsequently becomes the “vertebral plates,” a group of cells is very early differentiated: this is the rudimentary spinal ganglion, which becomes enveloped in a membrane, and then pursues a widely different course from that of the other cells surrounding it, so that “the same cell which was formerly an element of the vertebral plate now becomes a nerve-cell, while its neighbors become cartilage-cells.”[133] Indeed all the hypotheses of transformation of tissues by means of Differentiation are as unscientific as the hypotheses of the transformation of animals. In the organism, as in the Cosmos, typical forms once attained are retained. There probably was a time in the history of the animal series when masses of protoplasm by appropriating different materials from the surrounding medium were differentiated into organisms more complex and more powerful than any which existed before. But it is obvious that from a common starting-point there could have been no variations in development without the introduction of new elements of composition: there might have been many modifications of structure, but unless these facilitated modifications of composition, there could never have resulted the striking differences observed in animal organisms.[134]
108. To return from this digression, we may liken the three primary layers of the germinal membranes to the scattered and slightly different masses of protoplasm out of which the animal kingdom was developed. In this early stage there are no individualized organites—no nerve-cells or muscle-cells. They are cells ready to receive modifications both of composition and structure, appropriating slightly different elements from the yolk, and according to such appropriation acquiring different properties. And this is necessarily so, since the different cells have not exactly the same relation to the yolk, nor are they in exactly the same relation to the incident forces which determine the molecular changes. The uppermost layer (epiblast) under such variations develops into epithelium and central nerve-tissue; the epithelial cell cannot develop into a nerve-cell, the two organites are markedly unlike, yet both spring from a common root. Another modification results in the development of muscle-cells from the inner layer.
109. Hence we can understand how the surface is sensitive even in organisms that are without nerve-tissue; and also how even in the highest organisms there is an intimate blending of epithelial with neural tissues. The same indication explains the existence of neuro-muscular cells in the Hydra, recorded by Kleinenberg, and of neuro-muscular fibres in the Beroë, by Eimer.[135] In the simpler organisms the surface is at once protective, sensitive, and absorbent. It shuts off the animal from the external medium, and thus individualizes it; at the same time it connects this individual with the medium; for it is the channel through which the medium acts, both as food and stimulus. The first morphological change is one whereby a part of the surface is bent inwards, and forms the lining of the body’s cavity. Soon there follows such a modification of structure between the outer and inner surfaces (ectoderm and endoderm) that the one is mainly sensitive and protective, the other mainly protective and absorbent. The outer surface continues indeed to absorb, but its part in this function is insignificant compared with that of the inner surface, which not only absorbs but secretes fluids essential to assimilation. The inner surface, although sensitive, is subjected to less various stimulation, and its sensibility is more uniform.
110. The uppermost of the primary layers we have seen to be epithelial; and we know that the first lines of the central nervous system are laid there. A depression called the medullary groove is the first indication of the future cerebro-spinal axis. Some writers—Kölliker, for instance—regard this medullary groove as continuous with but different from the epithelial layer; others maintain that it lies underneath the epithelium, just as we see it in later stages, when the differentiation between epithelial and nerve cell has taken place. Since no one disputes the fact that when the groove becomes a closed canal its lining is epithelial, one of two conclusions is inevitable: either the cells of the primary layer develop in the two diverse directions, epithelial and neural; or else epithelial cells can be developed on the surface of neural cells and out of them. The latter conclusion is one which, involving the conception of transformation, would seem to be put out of court. I think, then, we must admit that the under side of the primary layer of cells becomes differentiated into nerve-cells; and this is in accordance with the observations of Messrs. Foster and Balfour.[136]
111. While there is this intimate morphological and physiological blending of epithelial and neural organites, there is an analogous relation between neural and muscular organites. As the neural layer lies under the epithelial, the muscular lies under the neural. The surface stimulation passes to the centre, and is reflected on the muscles. Embryology thus teaches why a stimulus from the external medium must be propagated to a nerve-centre before it reaches the muscles; and why a stimulus on one part of the surface may set all the organism in movement, by passing through a centre which co-ordinates all movements. This, of course, only applies to the higher organisms. In the simpler structures the sensitive surface is directly continuous with the motor organs.
It is unnecessary here to pursue this interesting branch of our subject; nor need we follow the analogous evolution of the second germinal membrane representing the Alimental System. Our attention must be given to what is known and inferred respecting the elementary structure of the nerves and centres, on which mainly the interest of the psychologist settles, since to him the whole of Physiology is merged in nerve actions.
CHAPTER VII.
THE ELEMENTARY STRUCTURE OF THE NERVOUS SYSTEM.
112. The progress of science involves an ever-increasing Analysis. Investigation is more and more directed towards the separated details of the phenomena previously studied as events; the observed facts are resolved into their component factors, complex wholes into their simpler elements, the organism into organs and tissues. But while the analytical process is thus indispensable, it is, as I have often to insist, beset with an attendant danger, namely, that in drawing the attention away from one group of factors to fix it exclusively on another, there is a tendency to forget this artifice, and instead of restoring the factors provisionally left out of account, we attempt a reconstruction in oblivion of these omitted factors. Hence, instead of studying the properties of a tissue in all the elements of that tissue, and the functions of an organ in the anatomical connections of that organ, a single element of the tissue is made to replace the whole, and very soon the function of the organ is assigned to this particular element. The “superstition of the nerve-cell” is a striking illustration. The cell has usurped the place of the tissue, and has come to be credited with central functions; so that wherever anatomists have detected ganglionic cells, physiologists have not hesitated to place central functions. By such interpretations the heart and intestines, the glands and blood-vessels, have, erroneously, I think, their actions assigned to ganglionic cells.
It is unnecessary to point out the radical misconception which thus vitiates a great mass of anatomical exposition and physiological speculation. I only call the reader’s attention to the point at the outset of the brief survey we have now to make of what is known respecting the elementary structure of the nervous system.