which it is constructed may be reduced to a few. Mechanical or
physical principles, as well as physiological ones, are involved.

The entire apparatus has for its purpose the conversion of the
vibrations of the air into the vibrations of a fluid, which thus
stimulates the end-organ, and brings about those changes in the nerve
which result in corresponding changes in the brain, that are
associated, in some way we cannot explain, to that state of
consciousness we term hearing. Complicated as is the auditory
apparatus, it can be readily enough comprehended, if the reader
accompany the perusal of the text by an examination of the figures
introduced.

[Illustration: FIG. 60. (Beaunis). In this illustration parts are
exposed to view by the removal of others. The whole of the inner ear
lies within bone, which in this figure is cut away. The drum-head
(membrana tympani); the Eustachian tube, extending from the back of
the throat, and opening into the middle ear; the semicircular canals
(which are not concerned with hearing, but with the maintenance of
equilibrium); the cochlea, (snail-shell), which contains the various
parts most essential to hearing, as the "hair-cells," the terminals of
the auditory nerve, the latter nerve itself, and several other
parts--are well shown. Should the Eustachian tube be closed owing to
swelling of its lining mucous membrane, a certain amount of temporary
deafness may result, because, the air within the middle ear (drum)
being absorbed, and fresh air not being admitted, the outer air
presses against the drum-head uncounteracted, and renders the
conducting mechanism too rigid.]

Anatomists speak of (1) an outer or external ear, (2) a middle ear,
drum, or tympanum, and (3) an inner ear, or labyrinth.

[Illustration: FIG. 61 (Beaunis). Diagrammatic representation of the
auditory apparatus. The external, middle, and internal ear are
separated by dotted lines. A, the external; B, the middle; C, the
internal ear; 1, auricle; 2, external auditory meatus; 3, tympanum
(middle ear), with its chain of bones, 7, 8, 9. Into it opens 5,
Eustachian tube, leading from back of throat; 4, membrana tympani or
drum-head, closing the middle ear off from the external ear. The most
important part of the inner ear is 13, the cochlear canal, in which
the "hair-cells" are found, around which latter the final branches of
the auditory nerve end. Above it is the scala vestibuli and below it
the scala tympani, passages