THE ABDUCENT NERVE THE FACIAL NERVE 801 



downward and forward, lying at first between the ramus of the mandible and the 

 medial pterygoid muscle, then on the medial face of the mylo-hyoid. On reaching 

 the root of the tongue it divides into superficial and deep branches. The super- 

 ficial branch (Ramus superficialis) runs forward on the stylo-glossus and accom- 

 panies the mandibular duct on the deep face of the sublingual gland. It supplies 

 the mucous membrane of the tongue and the floor of the mouth. At the root of the 

 tongue it gives off a recurrent branch to the isthmus faucium, which communicates 

 with the lingual branch of the glosso-pharyngeal nerve. The larger deep branch 

 (Ramus profundus) turns around the lower edge of the stylo-glossus and hyo- 

 glossus, passes upward and forward between the latter muscle and the genio- 

 glossus, and continues forward on the latter to the tip of the tongue. At the turn 

 it gives off one or more ascending branches which ramify on the deep face of the 

 hyo-glossus. It gives branches to the mucous membrane and the fungiform papillae 

 of the tongue, and anastomoses with branches of the hypoglossal nerve and with 

 the superficial branch. Minute ganglia occur on the finer branches of the lingual 

 nerve. The chorda tympani branch of the facial joins the lingual nerve at the origin 

 of the latter and is incorporated with it in its distribution to the tongue. 



THE ABDUCENT NERVE 



The abducent nerve (N. abducens) emerges from the brain behind the pons 

 and just lateral to the pjo-amid (Fig. 629). It passes forward across the pons, 

 pierces the dura mater, and accompanies the third and ophthalmic nerves, below 

 which it emerges through the foramen orbitale. In the orbit it divides into two 

 branches which innervate the rectus lateralis and the retractor muscle of the eye- 

 ball. Within the cranium it receives filaments from the carotid plexus of the sym- 

 pathetic. 



The fibers of the abducent nerve are axones of the large multipolar cells of the abducent 

 nucleus which is situated beneath the eminentia teres of the floor of the fourth ventricle. The 

 nucleus Ues within the loop formed by the fibers of origin of the facial nerve. It is connected 

 with (a) the anterior olive; (6) the oculomotor nucleus of the opposite side; (c) the motor area of 

 the cerebral cortex by means of the pyramidal tract of the opposite side. 



THE FACIAL NERVE 



The facial nerve (N. facialis) has its superficial origin at the lateral part of the 

 corpus trapezoideum, immediately behind the pons (Fig. 629). It passes outward 

 in front of the eighth nerve and enters the internal acoustic 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 ventrally 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 meduUa 

 above the facial tubercle. On leaving the nucleus the root-fibers pass dorso-mediaUy, inchne for- 

 ward 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 Ues in the concavity of the curve. The small sensory 

 part (N. intermedins) consists of axones of cells of the geniculate ganghon, 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 ganghon constitute the chorda tympani. 



After its emergence through the stylo-mastoid foramen (Fig. 646) the nerve 

 passes ventrally, forward, and outward on the guttural pouch under cover of the 

 51 



