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.
DIFFICULTIES OF THE INVESTIGATION.
113. So great and manifold are the difficulties of the search, that although hundreds of patient observers have during the last forty years been incessantly occupied with the elementary structure of the nervous system, very little has been finally established. Indeed, we may still repeat Lotze’s sarcasm, that “microscopic theories have an average of five years’ duration.” This need not damp our ardor, though it ought to check a too precipitate confidence. Nothing at the present moment needs more recognition by the student than that the statements confidently repeated in text-books and monographs are very often for the most part only ingenious guesses, in which Observation is to Imagination what the bread was to the sack in Falstaff’s tavern bill. Medical men and psychologists ought to be warned against founding theories of disease, or of mental processes, on such very insecure bases; and physiological students will do well to remember the large admixture of Hypothesis which every description of the nervous system now contains. Not that the potent aid of Hypothesis is to be undervalued; but its limits must be defined. It may be used as a finger-post, not as a foundation. It may suggest a direction in which truth may be sought; it cannot take the place of Observation. It may link together scattered facts; it must not take the place of a fact. We are glad of corks until we have learned to swim. We are glad of a suggestion which will for the nonce fill up the gaps left by observation, and hold the facts intelligibly together. And both as suggestion and colligation, Hypothesis is indispensable. Indeed, every discovery is a verified hypothesis; and there is no discovery until verification has been gained: up to this point it was a guess, which might have been erroneous—a torchbearer sent out to look for a missing child in one direction, while the child was wandering in another; only when he finds the child can we acknowledge that the torchbearer pursued the right path. Hypothesis satisfies the intellectual need of an explanation, but we must be wary lest we accept this fulfilment of a need as equivalent to an enlargement of knowledge; we must not accept explanation as demonstration, and suppose that because we can form a mental picture of the possible stages of an event, therefore this picture represents the actual stages. Let us be alert, forewarned against the tendency to seek evidence in support of a conclusion, instead of seeking to unfold the conclusion step by step from the evidence. To seek for evidence in support of a guess is very different from seeking it in support of a conclusion; which latter practice is like that of people asking advice, and only following it when it chimes in with their desires.
114. Is not the warning needed, when we find anatomists guided by certain “physiological postulates,” and consequently seeing only what these postulates demand? For example, there is the postulate of “isolated conduction,” which is said to require that every nerve-fibre should pursue its course singly from centre to periphery. Accordingly the fibres are described as unbranched. Whatever may be the demand of the postulate, or the felt necessity of the deduction, the fact is that nerve-fibres do branch off during their course at various points; nay, it is doubtful whether any lengthy fibre is unbranched. Other postulates demand what fact plainly denies. It is said to be “necessary” that every cell should have at least two fibres, and that sensory and motor nerves should be directly connected through their respective cells. These things cannot be seen, but they are described with unhesitating precision. Diagrams are published in which the sensory fibres pass into the cells of the posterior horn of the spinal cord, and these cells send off prolongations to the cells of the anterior horn, and thence the motor fibres pass out to the muscles: an absolutely impossible arrangement, according to our present data! Again, the postulate that nerve-force originates in the cells, and that nerve-functions depend on cells, required that the cells should be most abundant where the function was most energetic. Of course they were found most abundant in the required places—no notice whatever being taken of the facts which directly contradicted the deduction.