The Ear.—The special organ of hearing is the ear, to which there are three parts, the external, the middle, and the internal ear.
The external ear consists of the pinna or expanded cartilaginous portion, for the concentration and direction of sound waves, and the external auditory canal, partly cartilage, partly bone, which is directed forward, inward, and downward and conveys sound to the middle ear.
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Fig. 21.—The small bones of the ear; external view (enlarged). (After Gray.)
The middle ear or tympanum is an irregular cavity in the petrous portion of the temporal bone. Its outer wall is formed by the membrana tympani or drum, an oval translucent membrane placed obliquely at the bottom of the external auditory canal. The middle ear communicates with the inner ear through the fenestra ovalis or oval window and contains the ossicles, the malleus or hammer, the incus or anvil, and the stapes or stirrup, which are arranged in a movable chain from the drum to the oval window. The malleus, which is connected with the membrana tympani, articulates by its head with the body of the incus, while the stapes articulates with the incus by its head and is connected by its base with the margin of the oval window. Connection is made between the middle ear and the pharynx and the pressure of the air upon the drum made equal on either side by means of the Eustachian tubes. These tubes are about an inch and a half long, have cilia, and convey wax and
