[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 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.]