The description of the facial nerve may be conveniently separated into three parts : each representing a distinct stage of its course, which is accurately defined by its anatomical relations to the skull. The first of these is intra-cranial, and extends from the surface of the encephalon to the termination of the in ternal auditory canal. The second is osseous, and reaches from the latter point to the stylo-mastoid foramen, which forms the exit of the nerve from the aqueduct of Fallopius. The third is extra-cranial, and includes its distribution on the exterior of the skull be yond this aperture.
In the cranium, the course of the facial nerve is comparatively short. From the restiform body it passes forwards, lying immediately be neath and in contact with the pons varolii, and taking the same direction as the portio mollis, which is external and posterior to it. It next enters the meatus auditorius internus in company' with this nerve ; and finally leaves it by passing through the aperture at the upper part of the termination of this canal, and entering the aqueduct of Fallopius.
Portio intermedia.—With this part of' the auditory and facial nerves a third portion is visibly associated; which is, in all probability, essentially distinct from both. Wrisberg first announced the existence of this nerve as a separate branch ; and from its occupying a position between the " portio mollis " and " portio dura" of the seventh, he named it the " portio media " or " interrnedia." It arises by two or three filaments from the restiform body, in the same locality as the neighbouring facial nerve, from which its deeper origin can scarcely be separated. Fo ville, however, describes its ultimate visible fibres as traceable to a situation which is in termediate between that of the facial on the one hand, and that of the auditory on the other. He thus considers these three nerves, the facial, intermediate, and auditory, as aris ing respectively from the anterior pyramid, the olivary column, and the restiform body ; or to use his own language, from the anterior, the lateral, and the posterior tracts of the me dulla oblongata. Morganti's view of its origin closely- approximates to this; but he places it more in connection with the vestibular nerve, and hence more externally. But whatever may be the differences of opinion as to its exact mode of commencement, it is tolerably agreed that it is in very close proxiniity to the facial nerve, so much so as at first hardly to be separable from it ; and that, at a further stage of its course, it is attached to the vesti bular branch of the auditory nerve. Con
cerning its behaviour subsequently to this point, anatomists are less unanimous. Thus, some imagine that it continues engaged in the auditory nerve, and accompanies it into the internal ear. Others regard it as returning to the facial nerve, and passing with it into the aqueduct of Fallopius. It is, however, suffi ciently evident, that the only correct foundation of any of these views must be anatomical ; and since this method of investigation requires not only the artificial unravelling of the trunks, but also necessitates a frequent reference to portions and branches of the facial, as yet un described, its consideration is deferred until these shall have received some notice.
In the temporal bone. — The facial nerve, entering the aqumduetus Fallopii from the internal auditory meatus, passes, tor a very short distance, in a direction forwards, out wards, and slightly upwards, until it reaches the margin of the hiatus Fallopii on the upper surface of the petrous bone : it then suddenly bends backwards upon itself in the horizontal plane. Its next curve differs from the preceding both in character and direction, being much more gradually effected, and occu pying a vertical plane. The termination of this bend reaches the perpendicular, and opens by the stylo-mastoid foramen on the under surface of the petrous bone, or between the styloid and mastoid processes. In the middle of this course the osseous tube of the aqumductus Fallopii projects into the cavity of the tympanum ; and the nerve thus passes successively along its roof, above the fenestra ovalis, and then behind the pyramid on the inner side of the cavity ; and, finally, down its posterior surface.
At the anterior of the acute angle formed by the first bend of the facial nerve in the aqueduct of Fallopius, it experiences a slight enlargement, which has been called, from its position and shape, the " intumescentia genu formis." A dense and strong neurilemma here ensheaths the nerve, being a prolongation of dura meter, which is sent inwards on a minute vessel from the middle meningeal artery to enter the canal at the hiatus Fal lopii. The swelling itself is of a greyish-red colour, but it is somewhat obscured by this thick covering of fibrous tissue. Its nature will be spoken of hereafter.