SEVENTH PAIR OF NERVES (Sie benter Nerv, Germ. ; Le Septidme N-erf, Fr.).
In laying the foundations of the natural sci ences, few circumstances would seem to have occasioned more serious and permanent embar rassments than the immediate necessity of in dicating the various new objects which they presented by specific names, and the difficulty of finding suitable ones. A nomenclature based upon their properties, would, perhaps, readily have suggested itself; but, generally speaking, the recognition of the object so greatly preceded the discovery of its proper ties, that this attempt was almost impossible. In the science of anatomy, this was especially the case; and a large proportion of the human structures were named, either according to their form, or, if this was not sufficiently de fined, by their real or fancied resemblance to some previously known object; or failing this, by the proper names of their discoverers, however polysyllabic or uncouth they might happen to be.
In some one or other of these modes, the different parts of the complicated nervous centre received their various designations. But the cerebral nerves, although very similar to each other in the outward properties of their shape, size, and appearance, yet offered, by their fewness, a sufficient ground of distinction in the application of the ordinal numbers. By denying the claims of the olfactory lobes, and overlooking the fourth and sixth, the earlier anatomists made a smaller number ; but the arrangement of Willis, which is more usually adopted in the present day, counts nine of these soft round cords, ana reckons them from before backwards.
Yet even this apparently simple method of distinction comes far short of real accuracy. Thus, some of the -so.called nerves offer the essential structure of nervous centres ; and include, in addition to the ordinary nerve fibres, those globular voticles which modern physiology recognizes as the generators of the nervous force. In others of them, the limited resemblance implied by this numerical ar rangement is modified by their arising as two or more roots, which subsequently, by their junction, form one nerve. While in the seventh
nerve, which forms the subject of the present article, the close proximity of two such cords during a part of their course has led to their being united under one name ; although in their distribution, properties, and functions, they present a marked contrast with each other.
The facial and auditory nerves proceed to gether from the medulla oblongata to the bottom of the meatus auditorius internus in the petrous portion of the temporal bone. Up to this point they are included in the term seventh nerve; but beyond this situation their courses widely diverge. In conformity with these differences, the following short account will describe them separately from each other. It will first mention such of their anatomical features as are manifest on simply 4aying bare their surface, and will afterwards refer to the appearances afforded by a more artificial dissection or separation of their fibres. Subsequently we shall briefly examine the bearing of these their structural peculiari ties, and the effect of their morbid changes, with a view to attempting the deduction of their function.
The auditory nerve is of a considerably softer texture than the facial ; a difference which is in great part attributable to the much more delicate neurilemma hy which it is enveloped, bnt which is, no doubt, to some extent the result of a peculiarity of its constituent nerve fibres. From the fact of its lesser consistence, it is frequently termed the "portly mollis" of the seventh nerve.
Its apparent origin is close to that of the facial. At the upper part of the lateral surface of the inedulla oblongata, a somewhat trian gular depression exists, which is bounded in front by the olivary body, above by the lower border of the pons varolii, and behind by the restiform body. This shallow cavity has been termed by Vicq d' Azyr " the fossa of the olivary eminence," and in it appears the com mencement of the auditory nerve.