This nerve arises from a gray nucleus in the floor of the fourth ventricle. It passes, with the other part of the seventh (portio mollis) or auditory nerve, into the internal auditory meatus of the temporal bone. It first passes out toward the hiatus, then turns at a right angle to form a knee-like swelling (geniculate ganglion), and then runs backward along the top of the inner wall of the drum, and passing downward through a special canal in the bone, comes out at the stylo-mastoid foramen, and finally spreads out on the side of the face. It is essentially an efferent nerve, being partly motor and partly secretory, though its connections have caused afferent functions to be ascribed to it. Its distribution may be thus briefly summarized: -
(1) To the muscles of the forehead, eyelids, nose, cheek, mouth, chin, outer ear and the platysma, which may be grouped together as the muse lea of expression. (2) To some muscles of mastication, viz., buccinator, posterior belly of digastric, and the stylohyoid - all the foregoing being supplied by external branches - while in the temporal bone it gives a branch to (3) the stapedius muscle, and also a branch from the geniculate ganglion, named the great superficial petrosal nerve, which, after a circuitous course, is supplied to the elevator and azygos muscles of the palate and uvula.
(1) To the parotid gland by the small superficial petrosal nerve, which sends a branch to the otic ganglion, whence the fibres pass to the auriculo-temporal nerve, and then on to the gland. (2) To the submaxillary gland by the chorda tympani, which, after having traversed the tympanum, leaves the ear by a fissure at its anterior extremity, then joins the lingual branch of the fifth to separate from it and pass into the submaxillary ganglion, which lies in close relation to the gland (compare Figs. 64 and 65).
Vasomotor Or Vaso-Inhibitory influences are chiefly connected with the motor and secretory functions, since dilatation of the vessels of muscles and glands accompanies the motion and secretion that follows stimulation of the nerves going to them.
The Following Afferent Impulses are said to travel along the track of the portio dura and its branches: (1) Special taste sensations, which are chiefly located in the chorda tympani branch, may be explained by the branches of communication which pass from the trunk and petrous ganglion of the glossopharyngeal to the portio dura at its exit from the foramen, or by the connection in the drum of the ear between the tympanic branch of the glosso-pharyngeal and the geniculate ganglion of the portio dura through the lesser superficial petrosal nerve. (2) Ordinary sensations, which are also located in the chorda tympani, are said to traverse this nerve in an afferent direction until it comes near the otic ganglion, when the sensory fibres leave the chorda and pass to the inferior division of the 'fifth nerve through the otic ganglion.
Injury of the facial nerve in any of the deeper parts of its course gives rise to the striking group of symptoms known as facial paralysis, the details of which are too long to be given here. When it is remembered that muscles aiding in expression, mastication, deglutition, hearing, smelling, and speaking are paralyzed, and that taste, salivary secretion, and possibly ordinary sensation are impaired, one can form some idea of the complex pathological picture such a case presents.