Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗

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