elf throws these grains from side to side.

[Illustration: Fig. 142.--Bony internal Ear of Right Side. (Magnified; the
upper figure of the natural size.)

A, oval window (foramen ovale);
B, C, D, semicircular canals;
* represents the bulging part (ampulla) of each canal;
E, F, G cochlea, H, round window (foramen rotundum).
]

The auditory nerve, or nerve of hearing, passes to the inner ear,
through a passage in the solid bone of the skull. Its minute filaments
spread at last over the inner walls of the membranous labyrinth in two
branches,--one going to the vestibule and the ampullae at the ends of the
semicircular canals, the other leading to the cochlea.

346. Mechanism of Hearing. Waves of sound reach the ear, and are
directed by the concha to the external passage, at the end of which they
reach the tympanic membrane. When the sound-waves beat upon this thin
membrane, it is thrown into vibration, reproducing in its movements the
character of the air-vibrations that have fallen upon it.

Now the vibrations of the tympanic membrane are passed along the chain of
bones attached to its inner surface and reach the stirrup bone. The
stirrup now performs a to-and-fro movement at the oval window, passing the
auditory impulse inwards to the internal ear.

Every time the stirrup bone is pushed in and drawn out of the oval
window, the watery fluid (the perilymph) in the vestibule and inner ear is
set in motion more or less violently, according to the intensity of the
sound. The membranous labyrinth occupies the central portion of the
vestibule and the passages leading from it. When, therefore, the perilymph
is shaken it communicates the impulse to the fluid (endolymph) contained
in the inner membranous bag. The endolymph and the tiny grains of ear-sand
now perform their part in this marvelous and complex mechanism. They are
driven against the sides of the membranous bag, and so strike the ends of
the nerves of hearing, which transmit the auditory impulses to the seat of
sensation in the brain.

It is in the seat of sensation in the brain called the _sensorium_ that
the various auditory impulses received from different parts of the inner
ear are fused into one, and interpreted as sounds. It is the extent of the
vibrations that determines the loudness of the sound; the number of them
that determines the pitch.

Experiment 158. Hold a ticking watch between the teeth, or touch
the upper incisors with a vibrat