Just at this point, the superficial petrosal nerve is connected with the facial. Tracing it forwards from the intumescence, it is seen to pass at once through the neighbouring hiatus Fallopii, and thus it immediately gains the interior of the skull. Within the cranium it passes forwards, downwards, and inwards, lying in a groove on the outer or anterior surface of the petrous bone, and situated be neath the Oasserian ganglion of the fifth nerve. According to some anatomists, it occasionally passes amongst or through the meshes of the gangliform structure. Still beneath and in ternal to the ganglion, it is next placed im mediately external to the internal carotid artery, where this vessel, emerging from the canal of the same name in the temporal bone, springs vertically upwards to form the com mencement of the posterior limb of the sig moid turn on the side of the sphenoid. It tnus enters the foramen lacerum basis cranii, perforating the cartilaginous substance which closes this bony orifice ; and in this manner it gains the posterior extremity of the vidian canal, which opens into the anterior aspect of this irregular opening. Finally, it continues along this canal to its anterior termination ; and is then prolonged horizontally forwards for a short distance to join Meckel's ganglion, which occupies this part of the spheno-palatine fossa.
Another nerve comes off from the same knee-shaped bend of the portio dura ; and as it appears from the same horizontal slit, or hiatus Fallopii, whence the preceding emerged, and occupies a very similar position with re spect to the temporal bone, it has also been named " superficial petrosal," but is distin guished, from its lesser size, as the "small petrosal nerve." From the hiatus it is con tinued forwards and slightly inwards for about half an inch, running along the same surface of the temporal bone, but placed a little ex ternal to the preceding nerve. Arriving at the greater wing of the sphenoid, it perforates the bone by an oblique and minute orifice, which is situated between the foramen rotun dam and ovale : and appearing on the inferior surface of the base of the skull, it immediately unites with the otic ganglion which lies on the inner surface of the third division of the filth. During the latter part of this course it is ac companied by a filament from Jacobson's nerve of the glosso-pharyngeal. This branch, however, leaves the tympanum by a special canal, and is next placed externally to the lesser petrosal nerve on the petrous bone ; but, finally, it joins or runs with it to enter the same ganglion.
A branch to the membrane which closes the fenestra ovalis is sometimes described as coming from the facial, where it passes, in the aqueduct, above this orifice.
The minute fitament to the stapedius muscle is the next branch of this nerve. It leaves the portio dura and aqumductus Fallopii at about the middle of their second or vertical curve, or nearly on a level with the base of the promontory ; it next enters a small canal in this prominence, which conducts it to the proper osseous cavity for the muscle: it then breaks up and is lost in its substance.
The chorda tympani, the next Connection of the portio dura, is a much larger nerve than any of the preceding branches; it leaves the trunk of the facial at a distance of about the third of an inch from the stylo-mastoid fora men. Tracing the portio dura in the upward direction, it is first seen to experience a slight thickening, and gradually, by the increasing laxity of the connecting areolar tissue, a to lerably large branch seems to extricate itself from the trunk at a very acute angle. Di verging still more, it now altogether quits the aqumductus Fallopii, and enters a short canal which is appropriated to it, and which is placed anteriorly and externally to the former cavity, occupying the base of the promontory. While yet at a considerable distance from the apex of this eminence, the nerve emerges from its canal by an orifice very near the osseous ring to which the tympanic membrane is fixed. It now crosses the tympanum from its an terior to its posterior part, and lying close to its outer wall, but covered by a reflection of its mucous membrane, and ascending as it goes, it passes between and at right angles to the long process of the incus and the handle of the malleus, to reach the Processes gracilis of the latter bone ; along this process it con tinues during the remainder of its course in the tympanum. It next leaves the anterior wall of this cavity, and occupies a minute canal in the petrous_ portion of the temporal bone ; but it is still in close proximity to this process of the inalleus, being only separated by a small interval of bone from the Glasserian fissure which contains it. It is next seen ex ternal to the cranium, after coming through the aperture of this canal anteriorly and in ternally to the fissure. In the remainder of its course it lies deeply in the pterygoid fossa beneath the ramus of' the inferior maxilla, and is directed for about an inch downwards, for wards, and inwards, beneath the spinous process of the sphenoid, and the internal la teral ligament of the lower jaw attached to it, to join, at an acute angle, the outer side of the gustatory branch from the third division of the fifth.