ates them upon a suitable exposure of nerve cells. It
includes three parts--the _external ear_, the _middle ear_, and the
_internal ear_.

*External Ear.*--The external ear consists of the part on the outside of
the head called the _pinna_, or auricle, and the tube leading into the
middle ear, called the _auditory canal_ (Fig. 151). The pinna by its
peculiar shape aids to some extent the entrance of sound waves into the
auditory canal.(119) It consists chiefly of cartilage. The auditory canal
is a little more than an inch in length and one fourth of an inch in
diameter, and is closed at its inner end by a thin, but important
membrane, called

*The Membrana Tympani.*--This membrane consists of three thin layers. The
outer layer is continuous with the lining of the auditory canal; the inner
is a part of the lining of the middle ear; and the middle is a fine layer
of connective tissue. Being thin and delicately poised, the membrana
tympani is easily made to vibrate by the sound waves that enter the
auditory canal. In this way it serves as a receiver of sound waves from
the air. It also protects

[Fig. 151]

Fig. 151--*Diagram of section through the ear*, showing relations of its
various parts. (See text.)

*The Middle Ear.*--The middle ear, or tympanum,(120) consists of an
irregular cavity in the temporal bone which is lined with mucous membrane
and filled with air. It is connected with the pharynx by a slender canal
called the _Eustachian tube_. Extending across the middle ear and
connecting with the membrana tympani on one side, and with a membrane
closing a small passage to the internal ear on the other, is a tiny bridge
formed of three small bones. These bones, named in their order from the
membrana tympani, are the _malleus_, the _incus_, and the _stapes_ (Fig.
151). Where the malleus joins the membrane is a small muscle whose
contraction has the effect of tightening the membrane. The Eustachian tube
admits air freely to the middle ear, providing in this way for an equality
of atmospheric pressure on the two sides of the drum membrane. The bridge
of bones and the air in the middle ear receive vibrations from the
membrana tympani and communicate them to the membrane of the internal ear.

*Purposes of the Middle Ear. *--The middle ear serves two important
purposes. In the first place, it makes it possible for sound waves to set
the membrana tympani in vi