On dissecting out this origin, however, it may be separated into two portions or roots. One of these immediately penetrates the res tiform body at a right angle to its surface, and sinks into the central grey matter of the me dulla oblongata : while the other, continuing backwards superficially to the restiform body, winds round it to reach the floor of the fourth ventricle, where this structure is deficient. By this latter root, •the nerve seems to be directly continuous with the tranverse white fascicles of the calamus scriptorius; and near the middle line, it sinks into the posterior part of the same grey mass of the olivary columns, into which the other portion was followed.
But considerable variations appear to prevail in the degree of the visible continuity of this root with these transverse white strize. Thus Meckel and Prochasita remarked that they are sometimes wanting ; while Longet confirms their statements, and adds, from the experience of himself, Serres, and others, cases which show that not only is their number variable vvithin certain limits, but that, even where present, they may not unite with the root of the auditory nerve, but may' curve upwards at their extremity, and pass up the posterior surface of the mesocephale. One or two ex aminations made by the author of this article seem to show that this is by no means unusual.
Other and more complicated origins have been ascribed by various anatomists to the au ditory nerve. Thus, according to Fovillet, a thin and white nervous lamina, which is con tinued from its roots and from those of the fifth nerve, is spread over, and as it werelines, the interior of the cortical grey matter of the cere bellum, in addition to covering the whole sur face of the fourth ventricle and medullary velum. But in this and other descriptions of a like tendency, it seems difficult to distinguish bow much of the connection observed was referrible to a mere physical contiguity of the soft nervous matter, apart from that unbroken continuity of nerve-tubules which we are pro bably justified in predicating of the cerebral nerves and their rnore immediate processes of origin.
From the place of its first appearance at the surface of the encephalon, the nerve passes, in a direction which is at once for wards, outwards, and upwards, to the inner surface of the petrous portion of the tem poral bone, where it enters the internal audi tory meatus. In this course, the flocculus, an isolated lobule of the cerebellum, is in close proximity with its outer side ; while on its inner side, and in front of it, is the portio dura, which slightly grooves this surface of the somewhat flattened auditory nerve.
After entering the auditory canal, it con tinues along it to its termination ; and, finally, at the bottom of the meatus, it divides into two branches. The anterior of these is dis tributed to the cochlea : and the posterior, which exhibits a small gangliform enlarge ment, supplies the vestibule, dividing into three branches ; one for the posterior vertical canal, another for the sacculus, and a third for the utriculus, and remaining semicircular canals. These several divisions perforate the numerous foramina which are found at the bottom of the meatus to enter the internal ear ; but as an account of their further ar rangement with respect to the parts they sup ply would require a description of the auditory apparatus itself, the reader is referred to a previous article, " ORGAN OF HEARING," ill which these details will be found included.
The facial nerve, the " portio dura " of the seventh pair, emerges from the same depression in the restiform body which was above de scribed as giving rise to the auditory. It is of a much firmer and harder consistence than the latter, the tubules which compose it being connected by, and included in, a firm and strong neurilemma. Its real origin is generally referred to that central grey matter of the olivary columns to which so many of the encephalic nerves are traced. It is difficult to follow it any depth beyond these in a satis factory manner ; but Foville considers that it may be traced in the transverse direction around the olivary column and anterior pyra mid, and hidden beneath the lower margin of the pons varolii, to an origin from the inner border of the pyramid. He corroborates this by a reference to its comparative anatomy ; and states that the various stages of this course are successively laid bare by that diminished development of the lower arches of the pons which occurs in many of the mammalia. The description given by Morganti* somewhat differs from this, since he describes its roots as radiating by many filaments, ascending, descending, and transverse ; and the latter joining more deeply the central grey substance of the medulla oblongata near the floor of the fourth ventricle.