c. The Internal Ear. Because of its complicated series of canals, the internal
ear is also called the labyrinth. There are two main divisions: a body labyrinth and a
membranous labyrinth that fits in the body labyrinth.
(a) 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 portion.
(b) Membranous labyrinth. The membranous labyrinth is a hollow
tubular structure suspended within the bony labyrinth.
(2) 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.
(3) 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
(a) 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 apiral
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.
(b) 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 scalae are joined at their apex, they form a continuous channel, and the
connection between them is called the helicotrema.
(c) Between the scalae and the middle ear cavity are two windows: the
oval window and the round window. The oval window, also called the fenestra vestibuli,
is between the middle ear cavity and the scala vestibuli. It is filled with the foot plate of
the stapes. The round window which is also known as the fenestra cochlea is located
between the inner ear cavity and the scala tympani. This window is covered or closed
by a membrane.