The =facial nerve= (N. facialis) has its superficial origin at the
lateral part of the corpus trapezoideum, immediately behind the pons
(Fig. 499). It passes outward in front of the eighth nerve and enters
the internal auditory meatus. At the bottom of the meatus the two nerves
part company, the facial coursing in the facial canal of the petrous
temporal bone. The canal and nerve are at first directed outward between
the vestibule and the cochlea, then curve backward and downward in the
posterior wall of the tympanum to end at the stylo-mastoid foramen. The
bend formed by the nerve is called the =knee= (Geniculum n. facialis)
and bears at its highest point the round =geniculate ganglion= (G.
geniculi).
The nerve consists of two parts, motor and sensory. The =motor part=
constitutes the bulk of the nerve. Its deep origin is from the cells
of the =facial nucleus=, which is situated in the medulla above the
facial tubercle. On leaving the nucleus the root-fibers pass upward
and inward, incline forward close to the median plane, and then bend
sharply downward to the point of emergence. The highest point of the
bend is subjacent to the gray matter of the eminentia teres in the
floor of the fourth ventricle, and the abducent nucleus lies in the
concavity of the curve. The small =sensory part= (N. intermedius)
consists of axones of cells of the geniculate ganglion, which is
interposed on the facial nerve as it bends downward in the facial
canal. The fibers of this part after entering the medulla, pass to the
nucleus of termination which it shares with the ninth and tenth
nerves. The peripheral fibers from the geniculate ganglion constitute
the chorda tympani.
After its emergence through the stylo-mastoid foramen (Fig. 514) the
nerve passes downward, forward, and outward on the guttural pouch under
cover of the parotid gland, and crosses between the origin of the
superficial temporal and internal maxillary arteries internally and the
superficial temporal vein externally. It then crosses the posterior
border of the ramus of the mandible ventral to the transverse facial
artery and about an inch and a half (ca. 3.5 to 4 cm.) below the
articulation of the jaw. Emerging from beneath the parotid gland upon
the masseter muscle, it receives the lower branch of the superficial
temporal nerve, and divides into superior and inferior buccal branches.
The following collateral branches are given off, the first five being
detached within the facial canal, and the others between the
stylo-mastoid foramen and the border of the jaw.
1. The =great superficial petrosal nerve= (N. petrosus superficialis
major) arises from the geniculate ganglion.[203] It passes through the
petrosal canal, contributes a filament to the tympanic plexus, receives
the great deep petrosal nerve from the carotid plexus of the
sympathetic, emerges through the foramen lacerum, and is continued as
the Vidian nerve to the sphenopalatine plexus and ganglia (Fig. 515).
2. A delicate branch (R. anastomoticus cum plexu tympanico) emerges from
the geniculate ganglion and unites with a filament issuing from the
tympanic plexus to form the =small superficial petrosal nerve= (N.
petrosus superficialis minor); this ends in the otic ganglion.
3. The =stapedial nerve= (N. stapedius) (Fig. 515) is a short filament
detached from the facial nerve as it turns down in the facial canal. It
innervates the stapedius muscle.
4. The =chorda tympani= (Fig. 515) is a small nerve which arises a
little below the preceding and pursues a recurrent course in a small
canal in the mastoid part of the temporal bone to reach the tympanic
cavity. It traverses the latter, passing between the handle of the
malleus and the long branch of the incus. Emerging through the
petro-tympanic fissure, the nerve passes downward and forward, crosses
beneath the internal maxillary artery, and joins the lingual nerve. It
sends twigs to the submaxillary and sublingual glands, and through its
incorporation with the lingual nerve furnishes fibers to the mucous
membrane of the anterior two-thirds of the tongue which are believed to
mediate the sense of taste.
5. Anastomotic filaments unite with the auricular branch of the vagus
near the stylo-mastoid foramen.
6. The =posterior auricular nerve= (N. auricularis posterior) arises
from the facial at its emergence from the facial canal (Fig. 514). It
runs upward and backward with the posterior auricular artery under cover
of the parotid gland and supplies the posterior auricular muscles and
the skin of the convex surface of the external ear. It anastomoses with
branches of the first and second cervical nerves.
7. The =internal auricular nerve= (N. auricularis internus) springs from
the facial close to or in common with the preceding (Fig. 514). It
ascends in the parotid gland just behind the styloid process of the
conchal cartilage, passes through an opening in the cartilage, and
ramifies in the skin of the concave surface of the ear.
8. The =digastric branch= (R. digastricus) (Fig. 514) arises from the
facial below the auricular nerves. Its branches innervate the posterior
belly of the digastricus, the stylo-hyoideus, and the occipito-hyoideus.
At its origin it gives off a small branch which forms a loop around the
great auricular artery or its posterior branch and rejoins the trunk.
9. The =auriculo-palpebral nerve= (N. auriculo-palpebralis) (Fig. 514)
arises from the upper edge of the facial near the posterior border of
the ramus. It ascends in the parotid gland behind the superficial
temporal artery, and terminates in anterior auricular and temporal
branches. The =anterior auricular branches= form with branches of the
trigeminus the =anterior auricular plexus=. They innervate the anterior
auricular and parotido-auricularis muscles. The =temporal branch= runs
forward and inward over the temporal muscle to the inner canthus of the
eye, forms a plexus with the terminal branches of the ophthalmic nerve,
and is distributed to the orbicularis oculi, corrugator supercilii, and
levator nasolabialis.
10. The =cervical branch= (R. colli) (Fig. 435) arises from the ventral
border of the facial opposite to the preceding nerve. It emerges
obliquely through the parotid gland, passes downward and backward on or
near the jugular vein, and anastomoses with the cutaneous branches of
the cervical nerves. It gives branches to the parotido-auricularis and
the cervical panniculus. In its course along the neck the nerve is
reinforced by twigs from the cutaneous branches of the second to the
sixth cervical nerves.
11. Small branches are detached to the guttural pouch and the parotid
gland. The latter (Rami parotidei) concur with branches of the
superficial temporal nerve in forming the =parotid plexus=.
The facial nerve usually terminates after a short course on the surface
of the masseter by dividing into two buccal branches (Figs. 435, 461).
1. The =superior buccal nerve= (N. buccalis dorsalis) passes forward on
the upper part of the masseter, dips under the zygomaticus, and
continues along the lower border of the dilatator naris lateralis. It
then runs under the last-named muscle and anastomoses with branches of
the infraorbital nerve, and is distributed to the muscles of the cheek,
upper lip, and nostril.
2. The =inferior buccal nerve= (N. buccalis ventralis) crosses the
masseter obliquely and continues forward along the depressor labii
inferioris. It is connected by variable anastomotic branches with the
superior nerve. It gives collateral branches to the panniculus,
buccinator, and depressor labii inferioris, and ramifies with the
terminal branches of the inferior alveolar nerve in the lower lip.
The buccal nerves are subject to much variation in regard to their
course, anastomoses, and relations to the sensory components derived
from the superficial temporal nerve. Their distribution is constant.
The point at which the branch of the superficial temporal nerve joins
the facial is variable.