between the lower lip and chin, and supplies the depressor anguli oris, depressor 
labui inferioris, and orbicularis oris. It communicates with the mental branch of 
the inferior dental nerve. 
The infra-mandibular branch emerges from the parotid gland near its lower 
end, and sweeps forwards below the angle of the jaw to the front of the neck. It 
supplies the platysma myoides, and forms loops of communication with the super- 
ficial cervical nerve from the cervical plexus. 
THE EIGHTH OR AUDITORY NERVE. 647 
THE EIGHTH oR AUDITORY NERVE. 
The auditory nerve (n. acusticus) arises from the brain by two roots, mesial and 
lateral. The mesial root (radix vestibularis) emerges between the olive and the 
restiform body. The lateral root (radix cochlearis), continuous through the cochlear 
nucleus with the striz acustice of the fourth ventricle, winds round the outer side 
of the restiform body (for deep connexions, see p. 480). The two roots become 
incorporated to form the trunk of the nerve, which is attached to the brain on the 
outer side of the facial nerve and pars intermedia at the posterior border of the 
pons Varoli (Fig. 452, p. 633 
The nerve extends outwards through the internal auditory meatus, lying beneath 
the facial nerve and pars intermedia (Fig. 457, p.636). In the meatus its two com- 
ponent parts separate from one another, 
forming a superior or vestibular trunk 
continuous with the mesial root, and an 
inferior or cochlear trunk continuous with 
the lateral root. These trunks again sub- 
divide, and piercing the lamina cribrosa, 
supply the several parts of the labyrinth. 
The superior or vestibular trunk (n. 
Fia. 465.—ScHEME OF THE ORIGIN AND DISTRIBUTION 
vestibul1) in the internal auditory meatus or THE AUDITORY NERVE. 
usually receives fibres from the pars Py, Pyramid; Ol, Olive; R.B, Restiform body; A.St, 
intermedia, and gives off a communicat- Strie acustice ; Do, Dorsal nucleus; G, Lateral 
cochlear nucleus; Ve, Ventral nucleus; L.R, 
ing branch to the geniculate ganglion of Lateral root, and M.R, Median root of anditory 
the facial nerve. It then separates into nerve; P.I, Pars intermedia; F, Facial nerve ; 
three terminal branches which pierce &-.%, Geniculate ganglion ; Sup, Superior (coch- 
ee culerati Fbrdcsnwandits aT (1 tl lear), and Inf, Inferior (vestibular) branches of 
16 Lamina CUA! ane pe : ) Le auditory nerve; Co, Cochlea; 8, Saccule; P, 
macula acustica of the utricle, and the Posterior semicirenlar canal; e, External semi- 
ampullee of (2) the superior and (3) ex- circular canal ; 8, Superior semicircular canal; U, 
Utricle. 
ternal semicircular canals. 
The inferior or cochlear trunk SS cochlee) gives off branches (1) to the 
macula acustica of the saccule, (2) to the ampulla of the posterior semicircular canal, 
and (3) 1s continued through the lamina cribrosa to the labyrinth as the cochlear 
nerve, which is distributed through the modiolus and osseous spiral lamina to the 
organ of Corti in the cochlea. 
Both the vestibular and cochlear nerves contain among their fibres collections of 
nerve cells, forming in each nerve a distinct ganglion—the vestibular ganglion (¢. vesti- 
bulare) on the vestibular trunk, and the spiral ganglion of the cochlea (g. spirale) on the 
cochlear trunk. 
THE NINTH OR GLOSSO-PHARYNGEAL NERVE. 
The glosso-pharyngeal nerve (n. glosso-pharyngeus) (Fig. 452, p. 633) arises from 
the brain r by five or six fine radicles which emerge from the medulla oblongata between 
the olive and the restiform body, close to the facial nerve above, and in series with 
the roots of the pneumogastric nerve below (for deep connexions, see p. 478). The 
rootlets combine to form a nerve which extends outwards to the jugular foramen, 
through which it passes, along with the pneumogastric and spinal accessory nerves, 
but. enveloped j In a separate sheath of dura mater (Fig. 457, p. 636). Reaching the 
neck, the nerve arches downwards and forwards to the interval between the hyoid 
bone and the lower jaw. It lies at first between the internal carotid artery and 
the internal jugular vein, and then between the internal and external carotid 
a 
