(1) Bony labyrinth. The bony labyrinth (LABYRINTH = a maze) is a complex
cavity within the temporal bone. It has three semicircular canals, a vestibule (hallway),
and a snail-shaped cochlear portion.
(2) Membranous labyrinth. The membranous labyrinth is a hollow tubular
structure suspended within the bony labyrinth.
b. Fluids of the Internal Ear. The endolymph is a fluid filling the
space within the membranous labyrinth. The perilymph is a fluid filling the space between
the membranous labyrinth and the bony labyrinth. These fluids are continuously formed and
drained away.
ENDO = within
PERI = around
MD0006 11-33
c. The Cochlea. The cochlea is a spiral structure associated with
hearing. It has 2-1/2 turns. Its outer boundaries are formed by the snail- shaped portion
of the bony labyrinth.
(1) The central column or axis of the cochlea is called the modiolus.
Extending from this central column is a spiral shelf of bone called the spiral lamina. A
fibrous membrane called the basilar membrane (or basilar lamina) connects the spiral
lamina with the outer bony wall of the cochlea. The basilar membrane forms the floor of
the cochlear duct, the spiral portion of the membranous labyrinth. Within the cochlear
duct, there is a structure on the basilar membrane called the organ of Corti. The organ of
Corti has hairs which are the sensory receptors for the special sense of hearing.
LAMINA = thin plate
(2) Within the bony cochlea, the space above the cochlear duct is known as
the scala vestibuli and the space below is known as the scala tympani. (See figure 11-14.)
Since the scalae are joined at their apex, they form a continuous channel and the
connection between them is called the helicotrema.
(3) Between the scalae and the middle ear cavity are two windows.
(a) Fenestra vestibuli (oval window). Between the middle ear cavity and
the scala vestibuli is an oval window called the fenestra vestibuli. It is filled with the
foot plate of the stapes.
(b) Fenestra cochleae (round window). Between the middle ear cavity and
the scala tympani is a round window called the fenestra cochleae. It is covered or closed
by a membrane.
d. Transmission.
(1) The sound stimulus is transferred from the stapes to the perilymph of
the scala vestibuli. Here the stimulus is transmitted as a pressure pulse in the fluid.
(2) In response, the basilar membrane of the cochlea vibrates
(mechanically oscillates). Only selected portions of the basilar membrane vibrate at any
one time, depending on the frequency of the sound stimulus.
(3) The hair cells of the organ of Corti at that particular location are
mechanically stimulated. This stimulation is transferred to the neurons of the acoustic
nerve (cranial nerve VIIIa). The acoustic nerve passes out of the modiolus into the
internal auditory meatus of the temporal bone. From here, it enters into the cranial
cavity and goes to the brain.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
