ivide it into three parts: the
sound-conducting part, known as the external ear, the middle
ear, and the deeply placed nerve portion, the inner ear.

343. The External Ear. The external ear consists of an expanded
portion known as the pinna or _auricle_, and of a passage, the
auditory canal or _meatus_, leading inwards from it. The surface of
the auricle is convoluted to collect and transmit the vibrations of air by
which sound is produced the auditory canal conducts these vibrations to
the tympanic membrane. Many animals move the auricle in the direction of
the sound. Thus the horse pricks up its ears when it hears a noise, the
better to judge of the direction of sounds.[48]

The external auditory meatus, the passage to the middle ear, is curved
and is about an inch and a quarter long. Near its outer portion are a
number of fine hairs slanting outwards to prevent the entrance of insects.
Embedded in the deeper parts of the canal are glands which secrete the
_cerumen_, or ear-wax, which keeps the canal moist, and helps to protect
it against foreign bodies and insects. As the result of a cold, this wax
may collect in sufficient quantities to block the passage, and to diminish
to a considerable extent the power of hearing.

344. The Middle Ear. At the inner end of the outer ear passage is the
tympanum, known as "the drum of the ear." It is a thin, oval membrane,
stretched at an angle across the deep end of the passage, which it
completely closes. The tympanum is thus a partition between the
passage of the outer ear and the cavity of the middle ear. On its inner
side is a small air chamber in the petrous portion of the temporal bone,
called the cavity of the tympanum. Its bony walls are lined with
mucous membrane similar to that lining the nose, mouth, and throat. On the
inner wall of the tympanum are two openings, the round window, or _foramen
rotundum_, and the oval window, or _foramen ovale_.

The tympanic cavity communicates with the back part of the throat, by the
Eustachian tube. This tube is about one and a half inches long and
lined with mucous membrane similar to that of the tympanic chamber and the
throat. This passage is usually closed, but is opened in the act of
swallowing. In health there is no communication between the chamber of the
middle ear and the outside, except by the Eustachian tube. Thus a throat
cold, with redness and swelling of the mucous membrane, is usually
accompanied with some degree o