The action of water upon the nervous coat produces a phenomenon that is exhibited by few others of the animal textures. Far from softening it immediately and then reducing it to pulp, it seems in the beginning to increase its consistence. A nerve soaked in water becomes there harder and more resisting, and this state, at the ordinary temperature of cellars, continues for a month and a half, and even two months. It is only at the end of this time and frequently longer, that the texture of the nervous coat is gradually softened, and broken, and finally ends by being diffused like other macerated textures. I have not repeated this experiment in a very warm temperature, which has always succeeded in that of winter and spring. The coat of the nervous filaments has a very great resistance, because it is, in proportion to the medullary substance that it contains, infinitely thicker than the membranous canal of the spinal marrow. It is thus that the proportion between the thickness of the vascular parietes and the fluids they contain, is much less in the great trunks than in the small branches; the fluid considerably exceeds the solid in the first, there is at least an equality in the second. Thus a very small nerve would support a much greater weight than the spinal marrow. I believe that among the textures which are arranged in filaments or in elongated tubes, this and the arterial, next to the fibrous, afford the greatest resistance; they surpass the venous, the muscular, the serous, &c. _Medullary substance; its origin._ This substance occupies the interior of the nervous canal, in the same way as the substance of the spinal marrow fills the canal formed by the pia mater. This medullary substance is whitish, as that of the brain and spinal marrow; it gives the nerve its colour. It is in much greater proportion in the optic nerve than in any other; it is found exclusively in that part of it posterior to the junction of the two, as well as in the olfactory. It is so abundant in the auditory that it seems to form a great part of it. In general, I think at the origin in the osseous cavities, it predominates over the nervous coat, but in the course of the nerve, the nervous coat is the greatest. Hence the greater degree of resistance of the nerves in the second, compared to what they have in the first. This substance appears to be continuous with the medulla of the brain, the tuber annulare and its dependancies, and the spinal marrow. I think, no one can deny this continuity with the origin of the optic and olfactory nerves, in which more of this medullary substance is found than in the other nerves. In the auditory, also, it is very apparent; in the spinal marrow, by scraping this white substance from the internal surface of the pia mater, so as to leave the nerves adhering to this membrane, we see evidently at the place where these nerves go off, that there is an elongation penetrating their nervous coat.