686 NERVOUS SYSTEM OF THE HORSE 



THE EIGHTH OR AUDITORY NERVE 



The auditory nerve (N. acusticus) is connected with the lateral aspect of the 

 medulla just behind and external to the facial (Fig. 499). It has two roots, vesti- 

 bular and cochlear (Radix vestibularis et cochlearis). 



The auditory nerve consists of two distinct parts which might well be regarded as separate 

 nerves. The cochlear part mediates the sense of hearing, while the vestibular part is not auditory 

 in function, but is concerned in the sense of the position of the body and the mechanism of 

 equilibration. 



The nerve passes outward to the internal auditory meatus, which it enters 

 behind the facial nerve. In the meatus it divides into two nerves, of which the 

 upper is the vestibular and the lower is the cochlear nerve. 



1. The vestibular nerve (N. vestibuli) is chstributed to the utriculus, the saccu- 

 lus, and to the ampuUse of the semicircular canals, of the internal ear. In the in- 

 ternal auditory meatus the nerve is connected by filaments with the geniculate 

 ganglion of the facial nerve. At the bottom of the meatus it bears the vestibular 

 ganglion (G. vestibulare), from the cells of which the fibers of the nerve arise. 



2. The cochlear nerve (N. cochleae) detaches a filament to the sacculus, passes 

 through the lamina cril)rosa to the lal^yrinth, and is distriliuted to the organ of 

 Corti in the cochlea. 



The fibers of the vestibular nerve arise from the vestibular ganglion as central processes 

 (axones) of the bipolar cells of the ganglion. The peripheral processes (dendrites) of the cells 

 form arborizations about the deep ends of tlie hair-cells of the maculae and crista? acustica^ of the 

 utriculus, sacculus, and semicircular canals. The fibers enter the medulla, pass between the 

 restiform body and tlie spinal tract of the trigeminus, and spread out to end in the vestibular 

 nucleus in the floor of the fourth ventricle. Among the central connections of the vestibular 

 nerve are: (1) fibers which connect its nucleus with centers in the cerebellum (chiefly of the oppo- 

 site side); (2) the vestibulo-spinal tract, which conveys impulses to the motor cells of the ventral 

 columns of the spinal cord; (3) fibers which connect the nucleus with those of the abducent nerve 

 of the same side, the third and fourth nerves, and the motor part of the trigeminus of both sides. 



The fibers of the cochlear nerve are the central processes of the bipolar cells of the spiral 

 ganglion of the cochlea. The peripheral processes of these cells end in relation to the hair-cells 

 of the organ of Corti. Some of the nerve-fibers enter the ventral cochlear nucleus in the meilulla 

 close to the superficial origin of the nerve; others end in the dorsal nucleus of the tuberculum 

 acusticum at the lateral angle of the floor of the fourth ventricle. From the ventral nucleus 

 fibers pass in the corpus trapezoideum to the anterior olivary nucleus of the same and of the 

 opposite side. Thence tracts pass to the nuclei of the motor nerves of the eye, and through the 

 lateral fillet to the posterior quadrigeminal body and the internal geniculate body. The axones 

 of the cells of the dorsal nucleus pass largely (as the stria> acustica') over the restiform body and 

 across the floor of the fourth ventricle toward the median plane. They then turn ventrally, 

 cross to the opposite side, and are continued by the lateral fillet. From the mid-brain a tract 

 proceeds to the cortex of the temporal lot:)e of the hemisphere. 



THE NINTH OR GLOSSO-PHARYNGEAL NERVE 



The glosso-pharyngeal nerve (N. glossopharyngeus) is attached to the anterior 

 part of the lateral aspect of the medulla by several filaments (Fig. 499). The root- 

 bundles enter the furrow ventral to the restiform Ijody; they are separated by a 

 short interval from the origin of the facial nerve, but are not marked off behind from 

 the roots of the vagus. The bundles converge laterally to form a nerve which per- 

 forates the dura mater and emerges through the foramen lacerum posterius just 

 in front of the tenth nerve (Fig. 515). As it issues from the cranium the nerve 

 bears a considerable ovoid gray enlargement, the petrous ganglion (G. petrosum).* 

 It then curves downward and forward over the guttural pouch and behind the great 

 cornu of the hyoid bone, (crosses the deep face of the external carotid artery, and 

 divides into pharyngeal and lingual branches (Fig. 437). The collateral branches 

 are as follows: 



1. The tympanic nerve (N. tympanicus) (Fig. 515) arises from the petrous 

 ' This ganglion appears to be equivalent to the jugular or superior and petrous ganglia of man. 



