_1_, _2_, _3_, Superior, external, and inferior semicircular canals;
_4_, utricle; _5_, saccule; _6_, cochlea; _7_, auditory nerve.
(After Ellenberger, in Leisering’s Atlas.)
]
=Structure.=[223]—The membranous labyrinth consists in general of an
outer thin fibrous layer, a middle transparent tunic, and an internal
epithelium, composed of flattened cells. But in certain situations
special and remarkable structures occur, among which are the following:
(1) The =maculæ acusticæ= appear as small whitish thickenings of the
inner walls of the saccule and utricle. The epithelium here consists of
two kinds of cells, viz., supporting cells and hair cells. The latter
are flask-shaped and are surrounded by the fusiform supporting cells.
The free end of each hair cell bears a stiff, hair-like process composed
of a bundle of cilia. Fibers of the saccular and utricular branches of
the vestibular nerve form arborizations about the basal parts of the
hair cells. Adherent to the surface of the maculæ are fine crystals of
lime salts, embedded in a mucoid substance, and termed otoconia. (2) The
=cristæ acusticæ= are linear thickenings of the wall of each ampulla of
the membranous semicircular canals. Their structure is similar to that
of the maculæ. (3) The =spiral organ of Corti= (Organon spirale) is an
epithelial elevation which is situated upon the inner part of the
membrana basilaris, and extends the entire length of the ductus
cochlearis. It is very complicated in structure, but consists
essentially of remarkable supporting cells and hair cells. Fibers of the
cochlear nerve ramify about the basal parts of the hair cells.
=Vessels and Nerves.=—The =artery= of the internal ear is the internal
auditory artery, a very small vessel which usually arises from the
posterior cerebellar artery, and enters the internal auditory meatus.
The =veins= go to the inferior petrosal sinus. The =vestibular nerve= is
distributed to the utricle, saccule, and membranous semicircular canals,
and mediates equilibration. The =cochlear nerve= gives a branch to the
saccule and enters the central canal of the modiolus. Along its course
it gives off fibers which radiate outward between the two plates of the
lamina spiralis ossea, and ramify about the hair cells of the organ of
Corti. The =ganglion spirale= or ganglion of Corti is situated in the
spiral canal of the modiolus near the fixed border of the lamina
spiralis. The cochlear nerve mediates the sense of hearing.