ory Canal.
(Posterior view)]

The vestibule is the common cavity with which all the other portions
of the labyrinth connect. It is an oval-shaped chamber, about 1/3 of an
inch in diameter, occupying the middle part of the internal ear. It is on
the inner side of the oval window, which was closed, as we have seen, by
the stirrup bone. From one side of this vestibule, or central hall, the
three semicircular canals pass off, and from the other side, the cochlea.

The three semicircular canals, so called from their shape, are
simply bony tubes about 1/20 of an inch in width, making a curve of about
1/4 of an inch in diameter. They pass out from the vestibule, and after
bending around somewhat like a hoop, they return again to the vestibule.
Each bony canal contains within it a membranous canal, at the end of which
it is dilated to form an _ampulla_.

Experiment 157. _To vibrate the tympanic membrane and the little
ear-bones._ Shut the mouth, and pinch the nose tightly. Try to force air
through the nose. The air dilates the Eustachian tube, and is forced
into the ear-drum. The distinct crackle, or clicking sound, is due to
the movement of the ear-bones and the tympanic membrane.

The cochlea, or snail's shell, is another chamber hollowed out in the
solid bone. It is coiled on itself somewhat like a snail's shell. There is
a central pillar, around which winds a long spiral canal. One passage from
the cochlea opens directly into the vestibule; the other leads to the
chamber of the middle ear, and is separated from it by the little round
window already described.

The cochlea contains thousands of the most minute cords, known as the
fibers or _organ of Corti_.[49] Under the microscope they present the
appearance of the keyboard of a piano. These fibers appear to vibrate in
sympathy with the countless shades of sounds which daily penetrate the
ear. From the hair-like processes on these tightly stretched fibers,
auditory impulses appear to be transmitted to the brain.

The tubes and chambers of the inner ear enclose and protect a delicate
membranous sac of exactly the same shape as themselves. Between the bony
walls of the passages and the membranous bag inside is a thin, clear
fluid, the _perilymph_. The membranous bag itself contains a similar
fluid, the _endolymph_. In this fluid are found some minute crystals of
lime like tiny particles of sand, called _otoliths_, or ear-stones. Every
movement of the fluid its