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, vestibular 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 distributed to the
utriculus, the sacculus, and to the ampullæ of the semicircular canals,
of the internal ear. In the internal 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. cochleæ) detaches a filament to the
sacculus, passes through the lamina cribrosa to the labyrinth, and is
distributed 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 the hair cells of the maculæ and cristæ
acusticæ of the utriculus, sacculus, and semicircular canals. The
fibers enter the medulla, pass between the restiform body and the
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
opposite 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 medulla 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 striæ acusticæ) 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 lobe of the hemisphere.