This is a complicated structure of bony passages curled on themselves,
roughly as in a snail shell, and lined with a delicate membrane. This
membrane is, so to speak, floating in fluid. The layer of fluid between
it and the bone is called the _perilymph_, while the two layers of the
membrane enclose a similar fluid termed the _endolymph_. The internal
ear or membranous labyrinth may be divided roughly into three chief
parts: (1) the cochlea, the true organ of hearing; (2) the semi-circular
canals, which control the act of balancing; and (3) the vestibule, or
introductory chamber to the semi-circular canals.
The cochlea is a collection of three tubes curled up on themselves in
snail-shell fashion.
The central canal of these three is the connecting link by which the
sound waves, passed along over the three tiny bones--the malleus, incus,
and stapes--finally reach the endings of the main nerve of hearing, the
auditory nerve. (See Plate.)
[Illustration]