RECAPITULATION.
148. At the opening of this chapter mention was made of the besetting sin of the analytical tendency, namely, to disregard the elements which provisionally had been set aside, and not restore them in the reconstruction of a synthetical explanation. Familiar experiences tell us that a stimulus applied to the skin is followed by a muscular movement, or a glandular secretion; sometimes this takes place without any conscious sensation; sometimes we are distinctly conscious of the stimulus; and sometimes we consciously will the movement. These facts the physiologist tries to unravel, and to trace the complicated processes involved. The neurologist of course confines himself exclusively to the neural processes; all the other processes are provisionally left out of account. But not only so: the analytical tendency is carried further, and even in the neural process the organs are neglected for the sake of the nervous tissue, and the nervous tissue for the sake of the nerve-cell. The consequence has been that we have an explanation offered us which runs thus:—
149. The nerve-cell is the supreme element, the origin of the nerve-fibre, and the fountain of nerve-force. The cells are connected one with another by means of fibres, and with muscles, glands, and centres also by means of fibres, which are merely channels for the nerve-force. A stimulus at the surface is carried by a sensory fibre to a cell in the centre; from that point it is carried by another fibre to another cell; and from that by a third fibre to a muscle: a reflex contraction results. This is the elementary “nervous arc.” But this arc has also higher arcs with which it is in connection: the sensory cell besides sending a fibre directly to a motor cell, also sends one upwards to the cerebral centres; and here again there is a nervous arc, so that the cerebral centre sends down an impulse on the motor cells, and the contraction which results is due to a volitional impulse. The transmission of the stimulation which in the first case was purely physical, becomes in the latter case psychical. The sensory impression is in one cell transformed into a sensation, in another cell into an idea, in a third cell into a volition.
150. This course is described with a precision and a confidence which induces the inexperienced reader to suppose that it is the transcript of actual observation. I venture to say that it is imaginary from beginning to end. I do not affirm that no such course is pursued, I only say no such course was ever demonstrated, but that at every stage the requisite facts of observation are either incomplete or contradictory. First, be it noted that the actions to be explained are never the actions of organs so simple as the description sets forth. It is not by single fibres and cells that the stimulus is effected, but by complex nerves and complex centres. Only by a diagrammatic artifice can the fibre represent the nerve, and the cell the centre. In reality the cells of the centre (supposing them to be the only agents) act in groups, and Anatomy should therefore show them to be mutually united in groups—which is what no Anatomy has succeeded in showing, unless the Neuroglia be called upon. Secondly, be it noted that the current scheme of the relations between cells and fibres is one founded on physiological postulates, not on observation. Thirdly, much of what is actually observed is very doubtful, because we do not know whether the appearances are normal, or due to modes of preparation and post-mortem changes. We cannot at present say, for instance, whether the fibrillated appearance of cell contents and axis cylinder represents the living structure or not. We may either suppose that the neuroplasmic pulp splits longitudinally into fibres, or that neuroplasmic threads resolve themselves into a homogeneous pulp—the axis cylinder may be a condensation of many fibrils, or the fibrils may be a resolution of the substance.
151. Let us contrast step by step the Imaginary Anatomy found in the text-books with the Objective Anatomy as at present disclosed by the researches of all the chief workers. Imaginary Anatomy assumes that the sensory fibre passes from a surface into the cells of the posterior horn of the spinal cord. Objective Anatomy sees the fibre pass into the gray substance, but declares that no direct entrance of a fibre into a cell is there visible.
Imaginary Anatomy assumes that from the sensory cells of the gray substance pass fibres in connection with the motor cells of the anterior horn, thus forming a direct channel through which the excitation of a sensory cell is transmitted to a motor cell. Objective Anatomy fails to discover any such direct channel—no such fibres are demonstrable.
Imaginary Anatomy assumes that from the motor cells issue fibres which descend to the muscles and glands, and carry there the motor impulses and the “mandates of the will.” Objective Anatomy fails to find at the utmost more than a probability that these cells are continued as fibres, a probability which is founded on the rare facts of cell processes having been seen extending into the roots of the nerves, and of a cell process having occasionally been seen elsewhere continuous with a dark-bordered fibre. Granting, however, that this probability represents the fact, we have thus only one part of the “nervous arc” which can be said to have been verified.
Imaginary Anatomy further assumes that this nervous arc is connected with cerebral centres by means of fibres going upwards from the posterior cells, and fibres descending downwards to the anterior cells. Objective Anatomy sees nothing of the kind. It sees fibres entering the gray substance, and there lost to view in a mass of granular substance, fibrils, neuroblasts, and cells. There may be uninterrupted fibres passing upwards and downwards; but it is impossible to see them. And if we are told that physiological interpretations demand such a structure, we may fairly ask if this, and this only, is the structure which is adequate to the propagation of excitation? Now it seems to me that another kind of structure, and one more closely agreeing with what is observed, better answers the demands of Physiology. This will be more evident after the Laws of Nervous Action have been expounded in the succeeding chapter. Meanwhile we may remark that the arrangement of cells and fibres which is imagined as the mechanism of propagation and reflexion is absolutely irreconcilable with the teaching of Experiment: for the spinal cord may be cut through anywhere, without destruction of the transmission of sensory and motor excitations, provided only a small portion of gray substance be left to establish the continuity of the axis. Divide all the substance of the posterior half in one place, and all the substance of the anterior half in another, yet so long as there is a portion of gray substance left as a bridge between the lower and upper segments, the transmission of sensory and motor excitations will take place.
152. In other essential respects we have to note that the anatomical evidence for the current interpretations is absolutely deficient or contradictory. There is no adequate warrant for the assumption that all nerves have their origin in ganglia, all fibres in cells. Such evidence as at present exists is against that supposition, and in favor of the supposition that both cell and fibre are differentiations of a common neuroplasm, sometimes directly, sometimes indirectly continuous. Fibres, and plexuses of fibres, interspersed with cells irregularly distributed—now singly, now in small groups, now in larger and larger groups—constitute the figured elements of nerve-tissue; and even if we set aside the amorphous substance as indifferent or subordinate, we have still no ground for assigning the supremacy, much less the sole significance, to the cells. The grounds of this denial have been amply furnished in our exposition. For, let it be granted that nerve-cells are the origins of the fibres and the sources of their nutrition—a point which is eminently disputable—this would in no sense help the physiological hypothesis of the cell as the fountain of Neurility. If the fibre is simply the cell-contents drawn out longitudinally, if its essential element is identical with the essential element of the cell, then we can no more ascribe to the cell the exclusive property of Neurility than we can draw a lump of lead out into a wire, and then ascribe different properties to the thin end and the thick end. But on this point it is needless to speculate, since we have experimental evidence proving that the nerve-fibre has its Neurility even when separated from the cell, or even from the ganglion.
153. It is possible—I do not see sufficient evidence for a stronger assertion—that the cells are the nutritive sources of the fibres. They may represent the alimental rather than the instrumental activities of nervous life. (Compare Problem I. § [42].) My contention is that in any case they are not the supreme elements of the active tissue, and in no sense can they be considered as organs. Only confusion of ideas could for a moment permit such language, or could assign central functions to cells which are elements of tissue. If the cell be credited with such powers anywhere, it must be credited with them everywhere. Now I ask what conceivable central function can be ascribed to a cell which terminates the fibre in a peripheral ganglion, or which is merely an enlargement in the course of a fibre in a nerve-bundle? Besides the facts already adduced, let attention be called to this: If a nerve-bundle from the submucosa of the intestine be examined, there appear among the fibres many nuclei (neuroblasts), and occasionally cells, unipolar and bipolar. These cells—if we may trust the observations of Rouget on the earliest development of nerves, and of Sigmund Mayer on regenerated nerves—are simply more advanced stages of evolution of the neuroblasts; but whatever their genesis may be, there can be nothing in the nature of a central function assigned to them.
154. It may be asked, What part can we assign to cells in neural actions if they are apolar, unipolar, and even when multipolar, isolated from each other, and from fibres? I confess that I have no answer ready, not even an hypothesis. Until some rational interpretation of the cell be given we must be content to hold an answer in suspense. What I would urge is that we are precipitate in assuming that the anatomical connection between one element and another must necessarily be that of a fibre. In a semi-fluid substance, such as neurine, continuity may be perfect without solid fibres: the amorphous substance and the plasmode may as well transmit waves of molecular motion from one part of the tissue to another, and therefore from cell to cell, or from cell to fibre, as a figured substance may. When the posterior root enters the gray substance of the cord, there is no more necessity for its fibres passing directly into the cells of that gray substance, in order to excite their activity, than there is for a wire to pass from the bell to the ear of the servant, who hears the vibrations of the bell through the pulsations of the intervening air upon her tympanum. Look at the structure of the retina, or the cerebellum, and you will find that the ganglionic cells which have processes passing in a direction contrary to that whence the stimulus arrives, have none where continuity of fibre and cell would be indispensable on the current hypothesis. Light stimulates the rods and cones, but there are no nerve-fibres, hitherto discovered, passing from these to the ganglionic cells; instead of that there is a ground-substance thickly interspersed with granules and nuclei. From the cells we see processes issue; to the cells none are seen arriving. So with the cerebellum. The large cells send their processes upwards to the surface; but downwards towards the white substance the processes are lost in the granular layer, which most histologists regard as connective tissue.
155. A mere glance at nervous tissue in any part will show that cells are far from forming the principal constituents. In the epidermis or a gland the cell is obviously the chief element, forming the bulk of the tissue, and being the characteristic agent. In nerve-tissue, as in connective tissue, the reverse is the case. We must therefore cease to regard the cell as having the importance now attached to it, and must rather throw the emphasis on the fibres and neuroglia.
156. Before quitting this subject let a word be said on the amazing classification which has attained wide acceptance (although rejected by the most eminent authorities), founded on the size of the cells—the large multipolar cells being specified as motor, the smaller cells as sensory, while those of an intermediate size are sympathetic. I forbear to dwell on the development of this notion which specifies sensational, ideational, and emotional cells, because this does not pretend to have a basis in observation; whereas there are anatomical facts which give a certain superficial plausibility to the original classification. The conception is profoundly unphysiological; yet, if the anatomical evidence were constant, one might give it another interpretation. The evidence is, however, not constant. Large cells are found in regions assigned to sensory nerves, and small cells in motor regions. In the spinal cord of the tortoise Stieda declares that the so-called motor cells are limited to the cervical and lumbar enlargements; all the rest of the motor region being absolutely destitute of them.[182] Again look at the cells of the retina—no one will assign motor functions to them—yet they are the same as those of the cerebellum and the anterior horns of the spinal cord. (It is worth a passing mention that the structure of the nervous parts of the retina more closely resembles that of the cerebellum than of the cerebrum.)
157. While our knowledge of the cell is thus far indeed from having the precision which the text-books display, and in no sense warrants the current physiological interpretations, our knowledge of fibres and neuroglia is also too incomplete for theoretic purposes. We know that the axis cylinder is the essential element; but we are still at a loss what part is to be assigned to the medullary sheath. There is indeed a popular hypothesis which pronounces it to be the means of insulating the fibre, and thus preserving the isolated conduction of nerve-force. Being of a fatty nature, this insulating office was readily suggested in agreement with the assumption that Neurility was Electricity. Now, without discussing whether Neurility is or is not Electricity, even admitting the former to be satisfactorily proved, I must remark that the admission still leaves the medullary sheath incapable of fulfilling the supposed office, since not only is there no such sheath in most of the invertebrates and in the sympathetic nerves of vertebrates, but even in those nerves which have the sheath it is precisely in places where the insulation would be most needed—namely, just before the terminations of the fibres in muscles and in centres—that the sheath is absent. This is as if we tried to conduct water through a pipe which fell short at both ends—before it left the cistern, and before it reached the spot to be watered. If there is a tendency in Neurility to spread wherever it is not insulated by a medullary sheath, then before reaching the centres and the muscles, it must, on the insulating hypothesis, dribble away!
158. The facts expressed in the “law of isolated conduction” are important, and are difficult of explanation; but it is obvious that they cannot be referred to the presence of the medullary sheath. Nor indeed will any insight into the propagation of stimulation through the central axis be intelligible until we have reformed our anatomical theories, and taken the Neuroglia into account. The theory which connects every fibre directly with a cell, and every cell with another by anastomosis—even were it demonstrated—would not explain the law of isolated conduction. Butzke cogently remarks[183] that such a disposition of the elements should render all neural paths invariable; whereas the fact is that they are very variable. We learn to perform actions, and then we unlearn them; the paths are traversed now in one direction, now in another. Fluctuation is the characteristic of central combinations. And for this fluctuating combination of elements a corresponding diversity is required in the possible channels. This seems to be furnished by the network of the Neuroglia. See the representation copied from Butzke’s plate, and note how the cell-process blends with the meshes of the Neuroglia. Is it fanciful to regard this network of fibrils as having somewhat the relation of capillaries to blood-vessels? Did we not experimentally know that the capillaries are terminal blood-vessels, we should not suspect it from mere examination of the structure.
159. Having insisted that our knowledge is insufficient for any explanation of the “law of isolated conduction,” I can only suggest a path of research which may lead to some result. What we know is that some stimulations are propagated from one end of the cerebro-spinal axis to the other in definitely restricted paths, while others are irradiated along many paths. In the succeeding chapter this will be more fully considered; what we have here to note is that the manifold irradiations of a stimulation have an anatomical substratum in the manifold sub-divisions of the network of fibrils and the amorphous substance in which they penetrate.
Fig. 26.—Nerve-cells with processes terminating in neuroglia.
160. In conclusion, I would say, let no one place a too great confidence in the reigning doctrines respecting the elementary structure of the nervous system, but accept every statement as a “working hypothesis” which has its value in so far as it links together verified facts, or suggests new research, but is wholly without value in so far as it is made a basis of deductions not otherwise verified. Hypotheses are indispensable to research, but they must be accompanied by vigilant scepticism. Imagination is only an enemy to Science when Scepticism is asleep.
CHAPTER VIII.
THE LAWS OF NERVOUS ACTIVITY.
161. The foregoing remarks have had the object of showing how little substantial aid Psychology can at present derive from what is known of the elementary structure of the nervous system, indispensable as an accurate knowledge of that structure must be to a complete analysis of its functions. This caution has been specially addressed to those medical and psychological students whose researches leave them insufficient leisure to pursue microscopical investigations for themselves, and who are therefore forced to rely on second-hand knowledge, which is usually defective in the many qualifying considerations which keep scepticism vigilant. Relying on positive statements, and delusive diagrams which only display what the observer imagines, not what he actually sees, they construct on such data theories of disease, or of mental processes; or else they translate observed facts into the terms of this imaginary anatomy, and offer the translation as a new contribution to Science.
162. But little aid as can at present be derived from the teaching of the microscope, some aid Psychology may even now derive from it. The teaching will often serve, for instance, to correct the precipitate conclusions of subjective analysis, which present artificial distinctions as real distinctions, separating what Nature has united. It will show certain organic connections not previously suspected; and since whatever is organically connected cannot functionally be separated, such sharply marked analytical distinctions as those of periphery and centre, or of sensation and motion, must be only regarded as artificial aids. The demonstration of the indissoluble union of the tissues is a demonstration of their functional co-operation. So also the anatomical demonstration of the similarity and continuity of all parts of the central system sets aside the analytical separation of one centre from another, except as a convenient artifice; proving that cerebral substance is one with spinal substance, having the same properties, the same laws of action.
For the present, Psychology must seek objective aid from Physiology and Pathology rather than from elementary Anatomy. In the paragraphs which are to follow I shall endeavor to select the chief laws of nervous activity which the researches of physiologists and pathologists disclose. By these laws we may direct and control psychological research.