The vestibular nerve is purely sensory. With the peripheral processes of its cells of origin terminating in the neuro-epithelium of the semicircular canals and the vestibule, and their central processes conveying impulses which are distributed to the gray substance of the cerebellum and spinal cord, the nerve comprises a most important part of the apparatus for the equilibration of the body.
It has been customary to describe the vestibular [radix vestibularis] and the coch- lear [radix cochlearis] nerves combined as the acoustic (auditory) or eighth cranial nerve. While the two are blended in a common sheath from near the medulla to the bottom of the internal auditory meatus, they are likewise partty enclosed in the same sheath with the facial and glosso-palatine nerves and the internal audi- tory artery which accompany them in this meatus. At the bottom of the meatus the vestibular and the cochlear are separate; they are separate at their entrance into the lateral aspect of the medulla oblongata; and their central connections, peripheral distributions and functions are different.
The Left Membranous Labyrinth of a Human Foetus-of 10 Weeks (30 mm.), Lateral Aspect. Vestibular ganglion and nerve, red; cochlear nerve, yellow. (Streeter, American Journal of Anatomy.)
The vestibular nerve arises as processes of the cells of the vestibular ganglion (ganglion of Scarpa), situated upon and blended within the nerve at the bottom of the internal auditory meatus. Unlike the ordinary spinal ganglion, to which it corresponds, the cells of the vestibular ganglion retain an embryonal, "bipolar," form. The central processes course v/ith the cochlear nerve in the internal auditory meatus medialward, caudad and slightly downward, inferior to the accompanying' if acial and glosso-palatine nerves, and, arching ventrally around the restiform body, they enter the medulla at the inferior border of the pons, lat- eral to the glosso-palatine and facial and medial to the entrance of the cochlear nerve. They find their nucleus of termination spread in the floor of the fourth ventricle and grouped as the median, the lateral (Deiters'), the superior, and the nucleus of the spinal root of the vestibular nei've. In the internal auditory mea- tus, the vestibular nerve is connected by two small filaments of fibers with the glosso-palatine nerve. These are either visceral motor fibers for the vessels of the domain of the vestibular or are aberrant fibers which course only temporarily with the vestibular and return to the glosso-palatine.
The peripheral processes of the cells of the vestibular ganglion terminate in the specialised or neuro-epithelium comprising the maculm in the sacculus and the utriculus and the cristce in the ampullte of the three semicircular canals. Thus there are five terminal branches of the nerve. None of its fibers terminates in the cochlea. The vestibular ganglion has a lobar form, one lobe giving rise to a superior utriculo-ampuUar division which divides into three terminal branches; the other giving a sacculo-ampuUar division which gives two terminals.
The superior or utriculo-ampullar branch divides into the following terminal branches:
- The utricular branch passes through the superior macula cribrosa of the vestibule and terminates in the macula acustica of the utriculus.
- Accompanying the utricular branch through the superior macula cribrosa is a branch, the superior ampuUar, to the crista acustica of the ampulla of the superior semicircular canal, and
- A similar branch, the lateral ampullar, to the ampulla of the lateral semicircular canal.
The inferior or saccule-ampullar branch accompanies the cochlear nerve a short distance further than the superior, and divides into
- A branch, the posterior ampullar, which passes through the foramen singulare and the inferior macula cribrosa and tormiiiatcs in the ampulla of the posterior semicircular canal, and
- A branch, the saccular, which passes through the middle macula cribrosa and terminates in the macula acustica of the sacculus.
The large nucleus of termination of the vestibular nerve, spread through the area acustica in the floor of the fourth ventricle, and divided into four sub-nuclei, is associated with the nuclei fastigii, globosus, and emboliformis of the cerebellum, with the nuclei of the eye-moving nerves, with the spinal cord, and probably with the cerebral cortex.