t, known as the _vestibule_, the _semicircular canals_, and the
_cochlea_ (Fig. 152).

[Fig. 152]

Fig. 152--*General form, of internal ear.* The illustration represents the
structures of the internal ear surrounded by a thin layer of bone. 1.
Vestibule. 2. Cochlea. 3. Semicircular canals. 4. Fenestra ovalis. 5.
Fenestra rotunda.

*The Vestibule* forms the central portion of the internal ear and is
somewhat oval in shape. It is in communication with the middle ear through
a small opening in the bone, called the _fenestra ovalis_, at which place
it is separated from the middle ear only by a thin membrane. Sound waves
enter the liquids of the internal ear at this point, the foot of the
stapes being attached to the membrane. Six other openings lead off from
the vestibule at different places. One of these enters the cochlea. The
other five open into

*The Semicircular Canals.*--These canals, three in number, pass through the
bone in three different planes. One extends in a horizontal direction and
the other two vertically, but each plane is at right angles to the other
two. Both ends of each canal connect with the vestibule, though two of
them join by a common opening. The inner membranous labyrinth is
continuous through each canal, and is held in position by small strips of
connective tissue.

The purpose of the semicircular canals is not understood. It is known,
however, that they are not used in hearing. On the other hand, there is
evidence to the effect that they act as equilibrium sense organs, exciting
sensations necessary for balancing the body. Their removal or injury,
while having no effect upon the hearing, does interfere with the ability
to keep the body in an upright position.

[Fig. 153]

Fig. 153--Diagram showing the divisions of cochlear canal.

*The Cochlea* is the part of the internal ear directly concerned in
hearing. It consists of a coiled tube which makes two and one half turns
around a central axis and bears a close resemblance to a snail shell
(Figs. 151 and 152). It differs in plan from a snail shell, however, in
that its interior space is divided into three distinct channels, or
canals. These lie side by side and are named, from their relations to
other parts, the _scala vestibula_, the _scala tympani_, and the _scala
media_. Any vertical section of the cochlea shows all three of t