h it contains, and by the chain of small bones that are
enclosed in this cavity.

967. The use of the Eustachian tube is to admit air into the tympanum,
which renders the pressure on both sides equal, and thus its membrane
is kept in a proper state of tension.

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963-971. _Give the physiology of the organs of hearing._ 963. What is
hearing? Are the precise functions of the different parts of the ear
known? 964. What is the function of the external ear? 965. Of the
membrana tympani? What observation in reference to this membrane? 966.
What is the supposed office of the middle ear? 967. What is the use of
the Eustachian tube?

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_Observation._ When near a cannon, or a field-piece, about being
discharged, by opening the mouth the impression upon the auditory
nerve will be diminished, and the unpleasant sensation lessened. This
is the result of the air in the middle ear escaping through the
Eustachian tube, when the vibrations of the membrana tympani are
violent.

[Illustration: Fig. 148. A view of all the parts of the ear. 1, The tube
that leads to the internal ear. 2, The membrana tympani. 3, 4, 5, The
bones of the ear. 7, The central part of the labyrinth, (vestibule.) 8,
9, 10, The semicircular canals. 11, 12, The channels of the cochlea. 13,
The auditory nerve. 14, The channel from the middle ear to the throat,
(Eustachian tube.)]

968. But little is known of the functions of the internal ear; its
parts are filled with a watery fluid, in which the filaments of the
auditory nerve terminate.

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What observation in this connection? 968. What is the function of the
internal ear?

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969. Many of the parts just enumerated aid in hearing, but are not
absolutely essential to this sense. But if the vestibule and auditory
nerve are diseased or destroyed, no sound is then perceived. If this
sense is destroyed in early life, the person also loses the power of
articulating words. Hence a man born deaf is always dumb.

970. The transmission of sound through the different parts of the ear
will now be explained by aid of fig. 148. The vibrations of air are
collected by the external ear, and conducted through the tube (1) to
the membrana tympani, (2.) From the membrane vibrations pass along the
chain of bones, (3, 4, 5.) The bone 5 communicates with the internal
ear, (7, 8, 9, 10, 11, 11, 11, 12, 12, 12.) From the in