VII N. Facialis
(Fig. 155).—The seventh or facial nerve arises from the trapezium at the caudal margin of the pons, craniad of the origin of the eighth nerve, as described in the account of the brain (Fig. 138, VII). It passes into the internal auditory meatus, traverses the facial canal through the petrous bone, and emerges at the stylomastoid foramen.
Within the facial canal the nerve bears an enlargement, the ganglion geniculi. It gives off within the canal a branch to the stapedius muscle, and the superficial petrosal and chorda tympani nerves.
The superficial petrosal (N. petrosus superficialis major) nerve passes into the hiatus facialis, through the canal which forms its continuation, and joins the sphenopalatine ganglion (p. 373).
The chorda tympani is given off two or three millimeters before the emergence of the facial at the stylomastoid foramen. It passes into the tympanic cavity, extends across it between the malleus and incus, passing close against the tensor tympani muscle, and leaves it (by a small cleft, the canal of Huguier) between the bulla and the squamous portion of the temporal. It then passes craniad (Fig. 154, c), ventrad of the root of the zygomatic process of the temporal, and joins the lingual nerve (Fig. 154, b) (branch of the third division of the fifth nerve).
On emerging from the stylomastoid foramen the facial nerve (Fig. 155, a) gives off at once a small branch (b) to the digastric muscle (1), and a larger branch, N. auricularis posterior (c), which passes dorsocaudad about the base of the ear, supplying some of its muscles. Another small branch (d) pierces the cartilaginous external ear and is distributed to its inner surface. The facial nerve then curves craniad about the proximal part of the cartilaginous external ear, and divides five or six millimeters from its emergence into two main branches, which may be designated as the dorsal (f) and ventral (e) rami. A third small branch may also rise from the point of union of the two; it is usually, however, a twig from the dorsal ramus, and passes to M. submentalis.
The dorsal ramus (f) sends two or three small branches to the cheek (including the one just mentioned), and divides into the temporal (g) and the zygomatic (h) branches. The former (g) passes along the cranial margin of the external ear, supplying the superficial muscles, and extends into the temporal region, where it lies deeper than the terminal branches of the lachrymal nerve (m) (from the fifth cranial). The zygomatic branch (h) passes across the malar bone to the caudal angle of the eye, sends branches into both eyelids, anastomoses with twigs from the lachrymal branch (m) of the fifth nerve, and passes along the medial side of the eye to the lateral surface of the nose, where it ramifies.
The ventral ramus (e) gives off a minute branch (i) to the stylohyoid muscle (2), then proceeds toward the angle of the mouth and divides into superior (k) and inferior (j) buccal branches, the former to the muscles of the upper lip and the contiguous regions, the latter to those of the lower lip and chin.
The seventh nerve thus supplies most of the muscles of the head except those of mastication, and of these it supplies the digastric.