es the labyrinth consist? Give the parts of the internal ear.
957. Describe the vestibule. 958. What is said of the semicircular
canals? 959. Why is the cochlea so called? Of what does it consist?
960. How is the interior of the canal of the cochlea divided? Where do
they communicate with each other?

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961. The membranous labyrinth is smaller in size, but a perfect
counterpart, with respect to form, of the bony vestibule, cochlea, and
semicircular canals. Within this labyrinth are two small, elongated
sacs, which are filled with a fluid.

[Illustration: Fig. 147. A view of the auditory nerve. 1, The spinal
cord. 2, The medulla oblongata. 3, The lower part of the brain. 4, The
auditory nerve. 5, A branch to the semicircular canals. 6, A branch to
the cochlea.]

962. The AUDITORY NERVE enters the temporal bone upon its internal
surface, and divides into two branches, at the bottom of the cavity of
the internal ear. These branches enter the structure of the elongated
sacs and membranous labyrinth, radiating in all directions, and
finally, they terminate upon the inner surface of the membrane, in
minute papillae, resembling those of the retina.

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By what is the internal labyrinth lined? 961. Describe the membranous
labyrinth. What does fig. 147 represent? 962. Where does the auditory
nerve enter and divide? Where do the branches of the auditory nerve
enter and terminate?

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CHAPTER XLVII.

PHYSIOLOGY OF THE ORGANS OF HEARING.

963. HEARING is that function by which we obtain a knowledge of the
vibratory motions of bodies, which constitute sounds. The precise
function of all the different parts of the ear is not known.

964. The function of that part of the external ear which projects from
the head is to collect sounds and reflect them into the meatus.

965. The membrana tympani serves to facilitate the transmission of
sounds, and also to moderate their intensity. It is so arranged that
it can be relaxed or tightened.

_Observation._ This membrane, when healthy, has no opening; and it
must be apparent that the apprehension which is often expressed, that
insects will penetrate further, is groundless. The pain is owing to
the extreme sensibility of the membrana tympani.

966. The supposed office of the tympanum is to transmit the vibrations
made on the membrana tympani to the internal ear. This is effected by
the air whic