947. The MEATUS AUDITORIUS is a canal partly cartilaginous, and partly bony, about an inch in length, which extends inward from the pinna to the _Mem´bra-na Tym´pan-i_, (drum of the ear.) It is narrower in the middle than at the extremities. It is lined by an extremely thin pouch of cuticle, which, when withdrawn, after maceration, preserves the form of the canal. Some stiff, short hairs are also found in the interior of the channel, which stretch across the tube, and prevent the ingress of insects. Beneath the cuticle are a number of small follicles, which secrete the wax of the ear. [Illustration: Fig. 144. A representation of the four bones of the ear. The smallest is highly magnified. This bone is early matured, and in the adult it becomes united with the incus. These bones are retained in their places and moved by three ligaments and four muscles.] 948. The MEMBRANA TYMPANI is a thin, semi-transparent membrane, of an oval shape. It is about three eighths of an inch in diameter, and is inserted into a groove around the circumference of the meatus, near its termination. This membrane is placed obliquely across the area of that tube. It is concave toward the meatus, and convex toward the tympanum. 949. The TYMPANUM consists of an irregular bony cavity, situated within the temporal bone. It is bounded externally by the membrana tympani; internally by its inner wall; and in its circumference by the petrous portion of the temporal bone and mastoid cells. The tympanum contains four small bones, called the _os-sic´u-la au-di´tus_. These are named separately, the _mal´le-us_, _in´cus_, _sta´pes_, and _or-bic´u-lar_. -=-=-=-=-=-=-=-=-=-=-=-= 947. What is the meatus auditorius? What is found in this canal? What is their use? Where is the wax of the ear secreted? 948. Describe the membrana tympani. 949. Where is the tympanum situated? -=-=-=-=-=-=-=-=-=-=-=-= 950. There are ten openings in the middle ear; five large and five small. The larger openings are, the _Me-a´tus Aud-it-o´ri-us Ex-ter´nus_, _Fe-nes´tra O-va´lis_, (oval window,) _Fe-nes´tra Ro-tun´da_, (round window,) _Mas´toid Cells_, and _Eu-sta´chi-an Tube_. [Illustration: Fig. 145. A representation of the pinna, meatus, membrana tympani, bones of the ear, and semicircular canals. _a_, The pinna. _c_, The meatus auditorius externus. _g_, The membrana tympani. _k_, The tympanum. _e_, The bones of the ear. _b_, The semicircular canals. _f_, The cochlea. _h_, The vestibule. _i_, The Eustachian tube. _d_, The auditory nerve.] 951. The FENESTRA OVALIS is the opening of communication between the tympanum and the vestibule. It is closed by the foot of the stapes, or bone of the ear, and by the lining membrane of both cavities. 952. The FENESTRA ROTUNDA serves to establish a communication between the tympanum and the cochlea. it is closed by a proper membrane, as well as by the lining of both cavities. -=-=-=-=-=-=-=-=-=-=-=-=