684 NERVOUS SYSTEM OF THE HORSE 



THE SEVENTH OR FACIAL NERVE 



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, whicli is situated in the medulla 

 above the facial tubercle. On leaving the nucleus the root-fibers jiass upward and inward, in- 

 cline forward close to the median plane, and then bend sharply downward to the point of emer- 

 gence. The highest point of the bend is subjacent to the gray matter of the eminent ia teres in 

 the floor of the fourth ventricle, and the abducent nucleus lies in the concavity of the curve. 

 The small sensory part (N. intermedins) 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 supe- 

 rior and inferior buccal branches. The following collateral Ijranches 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.' It i)asses 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) entergcs 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 stape- 

 dius 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 l^ranch of the incus. Emerging 

 through the petrotympanic 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 filjcrs to the mucous membrane of the antiu'ior two-thirds 

 of the tongu(^ which are Ijelieved to mediate the sense of taste. 



^ Although this nerve springs directly from the ganglion, it contains motor as well as sensory 

 fibers. 



