As the cranium may be shewn to be com posed of the elements of three vertebrw, it has been attempted to prove that among these nerves some may be classed with the vertebral or spinal nerves. The fifth is obviously of this kind from its anatomical characters, namely, two roots; one, small, ganglionless ; the other large, ganglionic ; but with the former, which is analogous to the anterior root of a spinal nerve, the third, fourth, and sixth nerves may be conjoined from their similarity in structure and distribution. Thus one cranio-vertebral nerve is formed, the anterior root of which consists of the small portion of the fifth, the third, fourth, and sixth nerves ; and the pos terior or sensitive root, of the large portion of the fifth. A second cranio-vertebral nerve consists of the eighth pair, to which might be added the facial contributing to its motor por tion ; and a third is formed by the hypoglossal. The analogy, especially in the latter case, is far from being very obvious.
Sketch of the microscopical anatomy of the spinal cord and brain.—We conclude our ac count of the anatomy of the spinal cord and brain with a rapid glance at the present state of our knowledge of their minute anatomy as revealed by microscopical observation.
The elements of the two kinds of nervous matter, fibrous and vesieular, have been al ready sufficiently described. We shall only remark here that the great object of the ana tomises research should be to find out the precise manner in which the nerve-fibres are united with the nerve-vesicles. Of their in timate connection there can be no doubt,--7 much less of the influence which they are capable of exerting mutually upon each other.* Among the peculiarities of the fibrous mat ter in the centres it may be here stated that the fibres pass through a much greater range of size than in the nerves; that here we meet with nerve-tubes of the largest size, and, on the other hand, with minute fibres which seem to be con tinuous with the branching processes of the caudate nerve-vesicles. These fibres are per fectly transparent and differ from the nerve-tubes in the absence of any of the white substance of Schwann, and of the tubular membrane.
Some idea of the relation of the vesicular and fibrous matter in different parts of the cere bro-spinal centre may be formed by examining thin sections of the several portions of them made in various directions. It is impossible to make thesp sections sufficiently thin to enable us to explore a large surface with a high power, for which great transparency is necessary. Such sections, however, may be examined with lovv powers, as Stilling and Wallach have done. It is Important, however, to notice that the ap pearances observed in this way afford no certain indication of the course and direction of the' nerve-fibres, nor of the situation of the finer elements of the vesicular matter. The nerve
tubes are too minute to admit of being followed with an object-glass which magnifies less than from two hundred to three hundred diameters; yet Stilling's researches have been made with a power of no more than ten or twelve diameters.
The fibrous matter of the spinal cord consists of some fibres which pass either in a vertical direction, or obliquely, taking a long course, and deviatingbut slightly from the parallel to the axis of the cord. The fibres of the posterior columns are the most obviously longitudinal, and those which lie quite on the surface of the antero-lateral columns follow very much the same direction. Among the elements of the grey matter, fibres are found in great numbers, the direction of which is probably oblique or transverse, as considerable portions of them may be secn taking such a direction when a piece of grey matter, cut transversely, is exa mined under the microscope.
The grey matter of the cord contains caudate and spherical vesicles imbedded in their usual granular matrix. They are found in the horns as well as in the comrnissure. The caudate vesicles are most numerous, and distend in the anterior horn and at the root of the posterior one. The remainder of the posterior horn arid the gelatinous substance which is found at its posterior border, resemble very closely in struc ture the grey matter of the cerebral convolu tions.
By examining thin transverse sections of the cord, carefully hardened by immersion in spirits, a good view of the relative disposition of the grey and fibrous substances may be ob tained. Stilling has carried investigations of this kind to a great extent, and has published some beautiful plates, which are quite true to nature. Fig. 396 is copied from one of them.
It is impossible, however, to obtain any information from such examinations, except of the most general kind. On referring to the figure, the reader will perceive several lines, of the same colour and appearance as the central mass, to radiate from each horn of the grey matter to the surface of the cord, and not only to its external surface, but to that of its fissures. At whatever part of the cord the section be made, whether on a level with the roots of the nerves or between their points of emergence, the same appearance of radiating lines is seen, and the radiation will be found to extend be tween the central grey matter and whatever part of the surface of the cord the pia mater comes into contact with.