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 (scalæ) 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.
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.
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 inner ear.
The middle ear, it will be seen, is a drum with its stretched membrane like any other drum, and it too has a communication with the exterior air through a tube, the Eustachian tube, which leads from the drum into the back part of the throat. When one has a cold, the mucous membrane which lines this tube may become swollen or even catarrhal, and be so closed that no air can enter from the throat; the air already within the drum being absorbed, the outer air presses unduly against the drum-head, with the result that the whole conducting apparatus is put more or less out of condition, and a certain degree of deafness naturally results. The tension of the drum-head is regulated by a muscle attached to the bone which is connected with the inner part of this membrane.