952. The FENESTRA ROTUNDA serves to establish a communication 417 between the tympanum and the cochlea. it is closed by a proper membrane, as well as by the lining of both cavities.
What does this cavity contain? 950. How many openings in the tympanum? Explain fig. 145. 951. Describe the fenestra ovalis. 952. The fenestra rotunda.
953. The MASTOID CELLS are very numerous, and occupy the whole of the interior of the mastoid process of the temporal bone, and part of the petrous bone. They communicate, by a large, irregular opening, with the upper and posterior circumference of the tympanum.
Fig. 146.
Fig. 146. A view of the labyrinth laid open. This figure is highly magnified. 1, 1, The cochlea. 2, 3, Two channels, that wind two and a half turns around a central point, (5.) 7, The central portion of the labyrinth, (vestibule.) 8, The foramen rotundum. 9, The fenestra ovalis. 11, 12, 13, 14, 15, 16, 17, 18, The semicircular canals. The cochlea and semicircular canals open into the vestibule.
954. The EUSTACHIAN TUBE is a canal of communication, extending obliquely between the pharynx and the anterior circumference of the tympanum. In structure it is partly 418 fibro-cartilaginous and partly bony. It is broad and expanded at its pharyngeal extremity, and narrow and compressed at the tympanum.
953. Where are the mastoid cells? Explain fig. 146. 954. Describe the Eustachian tube.
955. The small openings of the middle ear are for the entrance and exit of the chorda tympani, (a small nerve that crosses the tympanum,) and for the exit of the muscles that act upon the membrana tympani and bones of the ear.