There are two separate elements in the seventh nerve, a motor portion, the facial nerve proper, and a sensory part, the pars intermedia (nervus intermedius). They unite at the bottom of the internal auditory meatus.
The fibres arise from a nucleus in the lower part of the pons near the superior olive. From this nucleus, the fibres pass backwards and slightly medially, to hook round the nucleus of the abducens nerve from the lateral to the medial side. They next travel downwards, laterally, and forwards to their superficial origin.
The nerve fibres which issue from the cells of the geniculate ganglion of the facial divide into two groups. The peripheral set form the greater sup-ficial petrosal and the chorda tympani, while the central group become the pars intermedia, and entering the brain terminate in the upper part of the grey matter associated with the tractus solitarius.
The lower border of the pons, on the medial side of the auditory nerve, the pars intermedia resting between the latter and the facial nerve proper.
The facial passes into the internal auditory meatus, lying superficial to the pars intermedia and the auditory nerves; at the bottom of the meatus it enters the aqueductus Fallopii (facial canal), In the canal the facial is lodged during its passage through the petrous portion of the temporal bone. The course of the nerve is somewhat complex. It is first directed laterally between the cochlea and the vestibule, then backwards along the medial wall of the tympanum, and finally arches over the fenestra ovalis to proceed downwards on the posterior wall of the tympanum, immediately in front of the lateral semicircular canal. Emerging from the stylomastoid foramen the facial reaches the parotid gland, where it lies on a level with the lower margin of the tragus, and splits into two divisions, temporo-facial and cervico-facial, to supply the muscles of the face.
The geniculate ganglion is situated on the seventh nerve at the junction of the lateral and backward parts of its course.
(a) A communicating branch from the pars intermedia as it lies in the internal auditory meatus.
(b) The geniculate ganglion receives a twig from the vestibular trunk of the auditory nerve, and gives off three branches :-
(1) Greater superficial petrosal emerges from the hiatus Fallopii to join the deep petrosal from the carotid sympathetic plexus, thus forming the Vidian (see trigeminal nerve).
(2) A minute nerve to the tympanic branch of the glossopharyngeal. The union of the two forms the lesser superficial petrosal which goes to the otic ganglion.
(3) External superficial petrosal to middle meningeal sympathetic plexus.
(c) Nerve to stapedius muscle.
(d) Chorda tympani leaves the tympanum through the canal of Huguier (petro-tympanic fissure) to join the lingual beneath the external pterygoid (see trigeminal nerve).
(e) Communicating twig to Arnold's nerve (auricular branch of the vagus).
(f) In the neck three branches arise :-
(1) Posterior auricular.
(2) Nerve to stylo-hyoid.
(3) Nerve to posterior belly of digastric.
The posterior auricular divides into auricular and occipital branches. It supplies the occipito-frontalis (epicranius), auricularis posterior, and the intrinsic muscles of the auricle.
(g) In the parotid gland the branches of the facial form the parotid plexus. They are :-
(1) Temporo-facial division
Temporal Zygomatic Infra-orbital
(2) Cervico-facial division
The muscles innerved by these branches are shown in the table.
Orbicularis palpebrarum; corrugator super-cilii; auricularis anterior and superior; frontalis.
Orbicularis palpebrarum and zygomaticus major.
Muscles of nose and lower lip. Buccinator and muscles passing to angle of mouth.
Depressor anguli oris; depressor labii inferiors ; orbicularis oris; levator menti. Platysma.