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.
ENDO = within
PERI = around
These fluids are continuously formed and drained away.
c. The Cochlea. The cochlea is a spiral structure associated with hearing. It
has 2 1/2 turns. The snail- shaped portion of the bony labyrinth forms its outer
(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 mebranous labyrinth. Within the cochlear
duct, there is a structure on the basilar membrane called the organ of Corti. The organ
of Corti has hairs that 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. Since the scala
are joined at their apex, they form a continuous channel and the connection between
them is called the helicotrema.
(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. The acoustic nerve passes out of the modiolus into the cranial cavity and goes
to the brain.