Auditory nerve.—The auditory nerve exhibits characters sufficiently distinct from the portio dura of the seventh, beside which it lies, to have led the anatomists of former days to separate it under the name of portio mollis. In fact, it pos sesses all the appearance of cerebral substance, and it wants the fasciculated disposition which mere nerves exhibit. Its fibres are delicate and very prone to become varicose, and, as in the case of the olfactory process, it passes out of the cranium, not as a trunk, but by means of several minute filaments of various size which pierce the foramina of the cribriform floor of the internal auditory foramen. Most observers express themselves in favour of the opinion that the terminal filaments are disposed in a looped forrn upon the membranous laby rinth and the cochlea. Valentin describes and delineates a plexiform arrangement, with loop ugs ofsome of the primitive fibres ; others ofthem, however, he says, do not affect this disposition, but appear to have free extremities. And this de scription corresponds with that which lIenle has given. This author states that from researches which he has made upon the lamina spiralis of mammifera and the ampullm of the frog, he has no doubt of the existence of fibres which pass from one fascicle to another in a looped form ; but he finds it difficult to determine whether all the tubes contained in each fascicle form similar loops. Wagner delineates the looped arrangement, and Pappenheim adopts the same view. Mr. Wharton Jones states that the tubular structure of the nertais fila ments ceases among grains of nervoM matter, arranged into a sort of expansion, (see ORGA N OP HEARING), and lie denies the existence of an arrangement in loops. My own observation leads me to concur in this description ; and I would add that there seem to be here, as in the retina, some elements of the grey nervous matter scattered among the primitive filaments. This fact did not escape Valentin, for he remarks the aistence of " very large globules" among the primitive fibres, similar to what he and Purkinje had noticed in the grey matter of the olfactory bulbs.* If this view of the peri pheral expansion of the auditory nerve be cor rect, its analogy with the optic is very obvious; and it may be conjectured of the ear, as in reference to the eye, that around a process from the brain an apparatus has been organized fitted to transmit and modify sonorous undulations.
In the present state of observation we should not be justified in making any positive state ment with reference to either the central or peripheral connexions of the nerves, beyond the following : that at the centres the grey and white elements are always associated, and that nerves may be truly said to arise out of grey matter ; and that at the periphery, the nervous fibres, which in their prog-ress from centre to circumference were bound together, become separated, and connect themselves, probably by intimate ad hesion, with the elementary parts of the tissues and organs to which they are distributed.
Of the ganglionic nerves.—Without more exact information respecting the minute ana tomy of these nerves, our knowledge of the peculiar function of the ganglionic system must be very incomplete. The following questions suggest themselves in reference to this system. I. Are its anatomical characters sufficiently distinct from those of the cerebro-spinal system to warrant us in separating it from that system, if only for purposes of description ? 2. Is it
an independent system, as some have conjec tured, giving fibres to the cerebro-spinal nerves as well as receiving some from them. 3. If it be an independent system, wherein consist the peculiar features by which its fibres are to be dis tinguished from those of cerebro-spinal nerves ? There are many features belonging to this system which justify its separation from that of the brain and spinal cord. The great number of ganglions connected with it, suggests the propriety of designating it ganglionic system, nor does the existence of ganglions on the pos terior roots of spinal nerves render this appella tion less proper; for in this system every nerve, nay every fibre, is connected with or passes through one or more ganglions. The external aspect of these nerves is very characteristic. Their neurilemma is very dense, and has more of the silvery appearance of white fibrous tissue than the sheaths of cerehro-spinal nerves; they want the fasciculated character of the latter nerves, and their colour has a diffused greyish or greyish red hue. The smaller ramifications are exceedingly delicate and appear to be soft, and therefore have been classed arnong the nervi molles by anatomists. In its peripheral distribution this nerve is prone to attach itself to the coats of bloodvessels, so much so, in fact, as to give it the character of an arterial or venous nerve; for, with a very few exceptions, it is always conveyed to organs along the bloodvessels which are distributed to diem. In its distribution it is entirely or almost con fined to the trunk, and probably has no con nexion with the extremities; or, if it have, that connexion must be by very few fibres, and those attached exclusively to the larger trunks of bloodvessels. The peripheral ramifications of this nerve are always plexifonn, and being distributed on some non-symmetrical parts, the plexuses which are derived from opposite sides of the body meet and anastomose along the mesial plane. The solar plexus, for example, derives filaments from the right and left trunks of the sympathetic, and the plexuses which accompany the superior and inferior mesenteric arterie.s, are also supplied from each side. Of the precise nature of these plexuses nothing is known: it is obvious, however, that their me dian anastomoses constitute a very peculiar feature, which strikingly distinguishes the sym pathetic from the cerebro-spinal nerves, which do not anastomose along the mesial line. If in these anastomoses the looped arrangement exist, it might be conjectured to form a commissural connection between opposite and symmetrical portions of the sympathetic or of the brain or spinal cord.
To determine the independence of this por tion of the nervous system on the bmin and spinal cord, it would be necessary to shew either that it possessed peculiar fibres distinct in characters from the cerebro-spinal fibres, which orig,inated in the ganglia, and were occasion ally bound up with cerebro-spinal nerves, or that fibres belonging to the ganglionic nerves, although exhibiting no essential difference from the cerebro-spinal, had their origin from the gan glia and not from the brain or spinal cord. The present state of the investigations into this sub ject does not enable us to determine these points; but there can be no doubt that at least a large proportion of the fibres which compose the sympathetic exhibit no essential difference from those of the cerebro-spinal nerves.