The membrana tympani (membrane of the drum), of which the name indicates the function, is a membranous partition stretched obliquely across the bottom of the auditory canal, which it separates from the middle ear or drum. This membrane is semi-transparent and very thin, although it is composed of three layers; it vibrates under the impression of the sonorous waves, and transmits the vibratory movement to the little bones of the ear. Between the membrana tympani and the internal ear is the drum or the tympanum, a cavity hollowed out, like all those of the middle and internal ear, in the petrous portion of the bone. Among the details of its form and organization, we remark the fenestra oralis, which communicates with the vestibule, and the fenestra rotunda, which leads to the cochlea. The drum also communicates with the mastoid cells, numerous sinuses which are found in the mastoid process of the temporal bone, containing air, and designed to multiply the vibratory surfaces; and lastly, it unites by a sort of funnel with the Eustachian tube, a canal about one inch and a third in length, which opens into the upper portion of the pharynx, and admits the air into the middle ear.