694 FACIAL NERVE. 



may be rotated involuntarily, in order that it may not interfere with the vision 

 of the unaffected eye, the patient thus unconsciously placing himself in the position 

 of a person with but one eye. 



VII. FACIAL NERVE. 



The facial nerve arises with centrifugal fibers from the ganglion-cells of two 

 nuclei on the floor of the fourth ventricle. The anterior, smaller nucleus, which 

 is situated immediately behind the most posterior cells of the oculomotor nucleus, 

 is the origin for the muscular nerves of the eyelids and the structures about the 

 orbit. The posterior, larger nucleus is situated in the most ventral portion of 

 the tegmentum to the inner side of the ascending root of the fifth nerve. The 

 fibers that arise here surround the nucleus of the abducens and contain the nerve 

 for the muscles of the mouth and of the remainder of the face. In their passage 

 from the facial center in the cerebral cortex to the nuclei the fibers for the mouth 

 decussate, while the fibers for the eyes in part do not decussate. 



The nerve makes its appearance at the posterior margin of the pons to the 

 inner side of the auditory nerve. Between the two arises the thin intermediate 

 portion of Wrisberg, which sends most of its fibers to the facial nerve and the 

 remainder to the auditory. The filaments of origin of the intermediate portion 

 arise from the glossopharyngeal nucleus. The gustatory and the tactile fibers 

 possessed by the chorda tympani appear to enter the facial through these filaments. 

 These fibers have ganglion-cells in the geniculate ganglion. The intermediate por- 

 tion would thus be a separate division of the gustatory nerve, which unites with 

 the facial and passes through the tongue with the chorda. With the auditory 

 nerve the facial first enters the internal auditory canal and at the bottom of this, 

 separated from the former, it enters the facial or Fallopian canal. At first it has a 

 transverse direction as far as the hiatus of this canal. It then turns at a right 

 angle at the geniculate ganglion (Fig. 243, a), containing ganglion-cells, passing 

 over the tympanic cavity, to descend into the bone on the posterior aspect of 

 this cavity. Finally, it makes its exit at the stylomastoid foramen, penetrates 

 the parotid gland, and divides into its terminal branches, to be distributed in a 

 fan-shaped manner (pes anserinus major). 



The branches of the facial nerve (Fig. 243) are : 



1. The motor greater superficial petrosal nerve (j). It passes from 

 the geniculate ganglion through the hiatus out of the facial canal into the 

 cranial cavity, then downward upon the anterior surface of the petrous 

 bone, next through the sphenoidal fissure to the inferior surface of the 

 base of the skull, and finally through the Vidian canal to the spheno- 

 palatine ganglion. It is also possible that the nerve transmits sensory 

 fibers to the facial from the second division of the trigeminus. 



2. From the geniculate to the otic ganglion connecting fibers (ft) whose 

 course and function are described on p. 689. 



3. The motor branch to the stapedius muscle 0). 



4. The chorda tympani nerve (i i) arises before the exit of the facial 

 from the stylomastoid foramen (s), passes through the tympanic cavity, 

 above the tendon of the tensor tympani, between the handle of the mal- 

 leus and the long process of the incus, then through the petrotympanic 

 fissure externally to the base of the skull and downward at an acute 

 angle into the lingual nerve. In advance of this union an exchange of 

 fibers takes place between the chorda and the otic ganglion (m). Both 

 these, as well as the connection of the chorda with the lingual nerve, may 

 transmit sensory fibers to the chorda and later on to the facial nerve. 

 The chorda contains sensory fibers, for irritation of the nerve, which is pos- 

 sible in man when the tympanic membrane is destroyed, causes a stick- 

 ing and prickling sensation in the anterior lateral portion and at the tip 

 of the tongue. After division of the chorda O. Wolf found sensibility 

 for tactile and thermic irritations abolished in man in the same distri- 

 bution, and also gustatory sensation. 



