If one of the very large cells be taken from the ganglion of a living mollusc, and be gently pressed till it bursts, the discharged contents will be seen to be of a hyaline viscid substance, with fine granules but no trace of fibres. Yet we must not rashly generalize from this, and declare that in the vertebrate cells the substance is not also fibrillated. As a good deal of speculation rests on the assumption of the fibrillated cell-contents, I have thought it worth while to note the uncertainty which hovers round it. 122. Among the uncertainties must be reckoned the question as to the cell-processes. The existence of apolar and unipolar cells is flatly denied by many writers, who assert that the appearances are due to the fragility of the processes. Fragile the processes are, and evidence of their having been broken off meet us in every preparation; but the denial of apolar and unipolar cells seems to me only an example of the tendency to substitute hypothesis for observation (§ 114). The "postulate" which some seem to regard as a "necessity of thought" that every nervecell shall have at least two fibres, one ingoing, the other outgoing, is allowed to override the plain evidence.* It originated in the fact first noticed by Wagner and Charles Robin that certain cells in the spinal ganglia of fishes are bipolar. The fact was rapidly generalized, in spite of its not being verified in other places; the generalization was accepted because (by a strange process of reasoning running counter to all physiological knowledge) it was thought to furnish an elementary illustration of the reflex process. As the centre had its ingoing and outgoing nerve, so the cell was held to be a centre "writ small," * An eminent friend of mine was one day insisting to me that the physiological postulate made it impossible for a nerve-cell to be without its ingoing and outgoing fibres; and he was not a little astounded when I replied, "Come into my workroom and I will show you a thousand." and required its two fibres. No one paused to ask, how a cell placed in the track of an ingoing nerve could fulfil this office of a reflex centre; no one supposed that the portion of the sensory fibre which continued its course, after the interruption of the cell, was a motor fibre. What does Observation teach? It teaches that at first all nerve-cells are apolar. Even in the cortex of the cerebrum, where (unless we include the nuclei and grain-like corpuscles under cells) all the cells are finally multipolar, there is not one which has a process, up to the seventh or eighth day of incubation (in the chick); from that day, and onwards, cells with one process appear; later on, cells with two, and later still, with three. By this time all the apolar cells have disappeared. They may therefore be regarded as cells in their infancy. However that may be, we must accept the fact that apolar cells exist; whether they can co-operate in neural functions, is a question which must be decided after the mode of operation of cells is placed beyond a doubt. 123. If apolar cells are embryonic forms of cells which afterwards become multipolar, this interpretation will not suffice for the unipolar cells. They are not only abundant, but are mature forms in some organs, and in some animals; though in some organs they may truly be regarded as embryonic. Thus in the human embryo up to the fourth month all the cells of the spinal cord are said to be unipolar,* later on they become multipolar. But in birds, rabbits, dogs, and even man, the cells in the spinal ganglia are mainly (if not wholly) unipolar;† nor is there * EICHHORST in Virchow's Archiv, 1875, LXIV. p. 432. + AUERBACH (Ueber einen Plexus Myentericus, 1862) describes the ganglia as filled with apolar cells, among which only a few are unipolar. STIEDA (Centralnervensystem der Vögel, 1868) finds both apolar and unipolar cells in the spinal ganglia of birds. AXMANN (De Gangliorum Systematis Structura penitiori, 1847) says the spinal cells are all unipolar. SCHWALBE (Archiv für mikros. Anat., 1868) and COURVOISIER (ibid., any difficulty in observing the same fact in the œsophageal ganglia of molluscs (see Fig. 22). Such are the observations. They have indeed been forced into agreement with the bipolar postulate, by the assumption that the single process branches into two, one afferent, the other efferent. But before making observation thus pliant to suit hypothesis, it would be well to look more closely into the evidence for the hypothesis itself. For my own part, I fail to see the justification of the postulate; whereas the existence of unipolar cells is an observation which has been amply verified. 124. Bipolar cells abound; multipolar cells are still more abundant; and these are the cells found in the gray substance of the neural axis. Deiters, in his epoch-making work,† propounded an hypothetic schema which has been widely accepted. Finding that the large cells in the anterior horn of the spinal cord gave off processes of different kinds, one branched, the other unbranched, he held that the latter process was the origin of the axis 1869) say the same. So also RANVIER, Comptes Rendus, 1875. KöLLIKER (Gewebelehre) speaks decidedly in favor of both apolar and unipolar cells, but thinks the apolar are embryonic. PAGLIANI (Saggio sullo Stato attuale delle Cognizioni della Fisiologia intorno al Sistema nervoso, 1873), who represents the views of MOLESCHOTT, admits the existence of apolar and unipolar cells. The authors just cited are those I happen to have before me during the rewriting of this chapter, and the list might easily be extended if needful. AUERBACH, BIDDER, and SCHWEIgger-Seidel describe unipolar cells which in some places present the aspect of bipolar cells simply because two cells lie together, their single poles having opposite directions. I will add that the bipolar cells do not really render the physiological interpretation a whit more easy than the unipolar, for they are simply cells which form enlargements in the course of the nervefibres. * When Dr. BEALE says "that it is probable no nerve-cell exists which has only one single fibre connected with it" (Bioplasm, p. 186), he has no doubt this in his mind; since he would not, I presume, deny that there are cells each with a single process. + DEITERS, Untersuchungen über Gehirn und Rückenmark, 1865. Fig. 18. Supposed union of two nerve-cells and a fibre. The processes subdivide into a minute network, in which the fibre also loses itself. cylinder of a nerve-fibre, whereas the branched process was protoplasm which divided and subdivided, and formed the connection between one cell and another. Gerlach has modified this by supposing that the minute fibrils of the branching process reunite and form an axis cylinder (Fig. 18). There is no doubt that some processes terminate in a fine network; and there is a probability (not more) that the unbranched process is always continuous with the axis cylinder of a motor nerve, as we know it sometimes is with that of a dark-bordered fibre in the white substances. This, though probable, is, however, very far from having been demonstrated. Once or twice Kölliker, Max Schultze, and Gerlach have followed this unbranched process as far as the root of a motor nerve; and they infer that although it could not be traced further, yet it did really join an axis cylinder there. In support of this inference came the observations of Koschennikoff,* that in the cerebrum and cerebellum, processes were twice seen continuous with dark-bordered nerve-fibres. But the extreme rarity of such observations amid thousands of cells. is itself a ground for hesitation in accepting a generalized interpretation, the more so since we have Henle's observation of the similar entrance of a branched process into the root. Now it must be remembered that the branched process is by no anatomist at present regarded as the origin of the axis cylinder; so that if it can enter the root without being the origin of a nerve-fibre, we are not entitled to assume that the entrance of the unbranched process has any other significance (on this head compare § 145), especially when we reflect that no trustworthy observer now professes to have followed a nerve-fibre of the posterior root right into a multipolar cell. Figures, * Archiv für mikros. Anat., 1869, p. 217. Compare also BUTZKE, Archiv für Psychiatrie, 1872, p. 584. + HENLE, Nervenlehre, 1871, p. 58, Fig. 21. |