filled with fluid. The openings to these
canals are closed with membrane. Attached to the membrane of the oval
opening is the stapes (stirrup). It is thus seen that vibrations
communicated to the chain of bones from the tympanic membrane are
passed on to the fluid filling the passages (scalae) of the cochlea,
and thus affect the hair-cells, and so the nerve of hearing, and
through it the brain. The parts indicated by 12 and 16 are important
in the maintenance of equilibrium, but are not concerned in hearing.]

The purpose of the _outer ear_ is to collect the air vibrations and
convey them to the middle ear, which passes them on to the inner ear,
where they produce the vibrations in the fluid therein contained and
which affect the end-organ and nerve-endings, and thus initiate the
essential physiological processes in the nerve of hearing. It follows
that we have an instance of the conversion of one kind of vibrations,
those of the air, into another kind, those of fluid, which latter
furnish a sufficiently delicate stimulus or excitation of the fine
hair-like extensions (_processes_) of the cells known as
_hair-cells_, about which the nerves in their final smallest branches
wrap themselves.

[Illustration: FIG. 62 (Beaunis). Two of the bones of the ear (the
malleus or hammer and the incus or anvil) enlarged. These small
ear-bones have joints like larger ones. The line of conveyance of
vibrations is indicated by B A.]

When we ourselves hear sounds when under water, we are affected
directly by the vibrations of that water; in this case we, in our
whole body, represent the hair-cells which are stimulated by the fluid
(_endolymph_) which surrounds them.

[Illustration: FIG. 63 (Beaunis). The complete chain of bones. The
arrows indicate in a general way the direction of the line of
transmission of vibrations from the tympanic membrane on to the fluid
within the passages of the inner ear.]

The external ear, well developed in many of the lower animals, being
often highly movable, is practically immovable in man, and is wholly
wanting in some animals, as the frog. The circular plate one sees
behind the eye of the frog is the drum-head of the middle ear.

From the _drum-head_, or _tympanic membrane_, the vibrations, which
are now those of a solid, are communicated by a series of very small
bones, most beautifully linked together by perfect joints, to another
membrane, which closes a small hole in the outer wall of the in