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
(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.
(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 mecha-
nically 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.