f deafness, because the swelling may block
the lumen of the tube, and thus prevent the free passage of air to and
fro.

[Illustration: Fig. 139.--General View of the Organ of Hearing.

A, pinna;
B, cavity of the concha, showing the orifices of a great number of
sebaceous glands;
C, external auditory meatus;
D, membrana tympani;
F, incus;
H, malleus;
K, handle of malleus applied to the internal surface of the membrana
tympani;
L, tensor tympani muscle;
between M and K is the tympanic cavity;
N, Eustachian tube;
O, P, semicircular canals;
R, internal auditory canal;
S, large nerve given off from the facial ganglion;
T, facial and auditory nerves.
]

A most curious feature of the ear is the chain of tiny movable bones which
stretch across the cavity of the middle ear. They connect the tympanic
membrane with the labyrinth, and serve to convey the vibrations
communicated to the membrane across the cavity of the tympanum to the
internal ear. These bones are three in number, and from their shape are
called the malleus, or _hammer_, incus, or _anvil_; and
stapes, or _stirrup_.

The hammer is attached by its long handle to the inner surface of the drum
of the ear. The round head is connected with the anvil by a movable joint,
while the long projection of the anvil is similarly connected with the
stirrup bone. The plate of the stirrup is fixed by a membrane into the
oval window of the inner wall of the tympanic chamber.

These little bones are connected with each other and the tympanum by
ligaments and moved by three tiny muscles. Two are attached to the hammer,
and tighten and relax the drum; the other is attached to the stirrup, and
prevents it from being pushed too deeply into the oval window.

[Illustration: Fig. 140.--Ear-Bones. (Anterior View.)

1, malleus, or hammer;
2, incus, or anvil;
3, stapes, or stirrup.
]

345. The Internal Ear. This forms one of the most delicate and
complex pieces of mechanism in the whole body. It is that portion of the
organ which receives the impression of sound, and carries it directly to
the seat of consciousness in the brain. We are then able to say that we
hear.

The internal ear, or bony labyrinth, consists of three distinct parts, or
variously shaped chambers, hollowed out in the temporal bone,--the
vestibule, the semicircular canals, and the cochlea, or snail's shell.

[Illustration: Fig. 141.--A Cast of the External Audit