Note 174 — Chapter VII · THE ELEMENTARY STRUCTURE OF THE NERVOUS SYSTEM.

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.