[172] Weismann, Die nachembryonale Entwick. der Musciden, in the Zeitschrift für Wissen. Zoologie, 1864, Bd. XIV. Heft III.
[173] The suggestion in the text has since received a striking confirmation in the observations of Sigmund Mayer on the regeneration of nerves. The nerve when divided rapidly undergoes fatty degeneration, which is succeeded by a transformation of the myeline and axis cylinder into a homogeneous mass; in this resolved pulp new longitudinal lines of division appear, which subsequently become new fibres, and new nuclei are developed in the remains of the untransformed substance.—Archiv für Psychiatrie, Bd. VI. Heft II.
[174] Strong confirmation of various statements in the text, since they were written, has been furnished by the researches of Eichhorst, published in Virchow’s Archiv, LXIV. Our knowledge of the development of nerve-tissue in human embryos is so scanty that these researches have a great value. Eichhorst describes the striation of the cells in the cord to begin only at the fourth month; up to this time they are, what I find most invertebrate cells to be, granular, not fibrillar. There is very slight branching of the cell processes until the ninth or tenth month, when the multipolar aspect first appears; the cells are unipolar up to the end of the fourth month. The connection between the white columns and the gray columns is very loose up to the fifth month; and the two are easily separated. Subsequently the union is closer. The substance of the white columns readily separates into bundles and fibres, but that of the gray columns falls into a granular detritus if attempted to be teased out with needles. But after the fifth month this is no longer so. Instead of a granular detritus there appears a network of fine fibres and fibrils. Although the white posterior columns are developed before the fifth month, not a single cell can be seen in the posterior gray columns until the second half of the ninth month. (Yet the fibres are imagined to arise in the cells!) The passage from the granular to the fibrillar state is the same in the cell substance and the neuroglia. The nerve-fibre, as distinguished from a naked axis cylinder, does not appear till the fourth month. It is at first a bipolar prolongation of the nucleus. As it elongates, the nucleus seems to sit on it, and so loosely that it is easily shifted away by pressure on the covering glass. Finally the fibre separates entirely from the nucleus, and then begins to clothe itself with the medullary sheath. Very curious is the observation that so long as the axis cylinder is naked it is never varicose, but with the development of the medulla the primitive axis becomes fluid.
[175] Mayer, Op. cit., 393. I cannot, however, agree with Mayer when he says that the continuity of a nerve-fibre with a cell has never been distinctly shown (p. 395); in the Invertebrata and in the Electric fishes such a continuity is undeniable; and it has occasionally been seen in Vertebrata.
[176] Ranvier, in the Comptes Rendus, 1875, Vol. LXXXI. p. 1276. This observation throws light on the fact that cell processes are sometimes seen entering nerve-roots (§ [124]).
The very remarkable observations of Mr. F. Balfour, On the Development of the Spinal Nerves in Elasmobranch Fishes (Philos. Trans., Vol. CLXVI. p. 1), show that the spinal root, ganglion, and nerve-trunk arise from histological changes in a mass of cells at first all alike; not that ganglion-cells are formed and from their processes elongate into fibres. The nerve, he says, forms a continuation of its root rather than of its ganglion (p. 181); which accords with Ranvier’s view.
[177] In the Handbuch der menschlichen Anatomie of W. Krause, which has just appeared, I am pleased to find a similar view, p. 376.
[178] On this point consult Axel Key and Retzius, in the Archiv für mikros. Anat., 1873, p. 308, where the nutritive disturbance is assigned to the fact that the lymph can no longer take its normal course. Waller’s observations on the degeneration of the optic nerves, with preservation of the integrity of the retina, after division of the nerves (Proceedings of Royal Society, 1856, p. 10), cannot be urged in support of his view, because Berlin and Lebert’s observations are directly contradictory of his. Saemisch und Graefe, Handbuch der Augenheilkunde, II. 346. It is said by Krenchel that if the nerves be divided, so as to prevent disturbances in the circulation, no peripheral degeneration takes place (cited by Engelmann in Pflüger’s Archiv, 1875, p. 477).
[179] Schiff, Lehrbuch der Physiologie, pp. 120, 121.
[180] Kölliker, Gewebelehre, 317. Schwalbe, Archiv für mikros. Anat., 1868, p. 51.