956. The LABYRINTH consists of a membranous and a bony portion. The bony labyrinth presents a series of cavities which are channelled through the substance of the petrous bone. It is situated between the cavity of the tympanum and the Aud´it-o-ry Nerve. The labyrinth is divided into the Ves´ti-bule, Sem-i-cir´cu-lar Canals, and Coch´le-a.
957. The VESTIBULE is a small, three-cornered cavity, situated immediately within the inner wall of the tympanum.
958. The SEMICIRCULAR CANALS are three bony passages which communicate with the vestibule, into which two of them open at both extremities, and the third at one extremity.
959. The COCHLEA forms the anterior portion of the labyrinth. It consists of a bony and gradually tapering canal, about one and a half inches in length, which makes two turns and a half, spirally, around a central axis, called the mo-di´o-lus. The modiolus is large near its base, where it corresponds with the first turn of the cochlea, and diminishes in diameter toward its extremity.
960. The interior of the canal of the cochlea is partially divided into two passages, by means of a bony and membranous plate. At the extremity of the modiolus, the two passages communicate with each other. At the other extremity, one opens into the vestibule; the other into the tympanum, by 419 the foramen rotundum. The internal surface of the bony labyrinth is lined by a fibro-serous membrane.
955. What passes through the small openings of the middle ear? 956. Of what does the labyrinth consist? Give the parts of the internal ear. 957. Describe the vestibule. 958. What is said of the semicircular canals? 959. Why is the cochlea so called? Of what does it consist? 960. How is the interior of the canal of the cochlea divided? Where do they communicate with each other?
961. The membranous labyrinth is smaller in size, but a perfect counterpart, with respect to form, of the bony vestibule, cochlea, and semicircular canals. Within this labyrinth are two small, elongated sacs, which are filled with a fluid.
Fig. 147.
