EAK. 579 EAB. a separate bone. The entire r;iiial, as well as the outer surface of the drumuienibrauc, is liued with skill. The -Middle E.r. This is a small, irregular lavity in the temporal lione. lined with a mucous nu-nilirane. the epithelium of wliiili is ciliated like that of the Eustachian tube and pharyn., with which it is continuous. It contains tliree ossicles — the malleus (hanuner), incus (anvil), and stapes (stirrup) — joined' by lijiaments to form a small chain at- tached to its outer end, by the handle iif tile malleus, to the drum-membrane, and at its inner end, by the base of the stapes, to the membrane coverinfi the fenestra ovalis. The muscles of the middle ear are the tensor tymiiaiii and the stapedius. The former arises from the under surface of the petrous por- tion of the temporal bone and is inserted into the handle of the malleus. The latter takes its origin from the hollow conical eminence, termed the pyramid, which lies behind the fenestra ocalis, and is inserted into the neck of the stapes: by pulling the neck backward it probably compresses the contents of the vestil)ule. The Eustachian tube, into which the tympanic cavity opens anteriorly, is about an inch and a half in length, and passes downward, forward, and inward to its opening in the pharynx. It is partly bony, but chiefly cartilaginous. It serves for the escape of mucus from the middle ear into the phar.iTix: also by allowing free passage of air in and out of the tympanum it renders the pressure upon the inner surface of the drum- membrane equal to the atmospheric pressure upon its outer surface. Upon the inner wall of the tympanum there are two openings: the fenestra oralis, mentioned above, leading into the vestibule: and the fenestra rotunda, covered by a membrane, leading into the cochlea. The IXTERX.A.L E.vk. or L.^btrixth. This con- sists of three parts: the vestibule, the cochlea, and the semicircular canals. The vestibule is a AMPVI.LA Vr^itnQ o« liQuEDVCnj^ t^-Mit* Flo. i. THE OSSEOC8 LABVKINTH, I,AH> OPEN AND E.VLARGED. single opening, the beginning of the scala vesti- huli described below, (hi its outer wall is the fenestra oralis, and on its inner there are a num- ber of minute orifices for the entrance of the lilaments of the auditory nerve. The cochlea (Lat.. snail-shell) is situated in front of the vestibule. It is a spiral canal wound around a bony axis, termed the modiolus, which is per- forated at its base for the entrance of the filaments of the cochlear portion of the auditory ,pOLA_ SCUA TVMPAMI / small central lavity communicating by openings in its posterior wall with the semicircular canals; anteriorly, the cochlea enters it by a AUDITORY NK^^^ FlG, 5. THE COCHLEA, EN'LAltCED. nerve. The interior of the cochlea is subdivided by an osseo-membranous partition, the lamina spi- ralis, into two passages: the s<:ula restibuli. open- ing into the vestibule, and the seala tympuni, the communication of which with the tympanic cav- ity — the fenestra rotunda — is covered by a mem- brane as described above ; the two scalae com- municate at the apex of the cochlea by an open- ing called the helieotrcma. Between the two scahe there is a third space, termed the seiihi intermedia, which contains the terminations of the filaments of the auditory nerve in a com- plicated arrangement of peculiarly formed neuro- epithelial cells termed collectively the organ of Corti. (See Auditory Xekve.) The semicircu- lar canals are three in number, and open at both ends into the vestibule. They vary in length, and, notwithstanding their name, each is con- siderably more than a semicircle, the superior vertical canal being the longest. Their aver- age diameter is about a twentieth of an inch, the extremities of each canal exhibiting a slight dilatation called an ampulla. Each canal lies in a different plane, very nearly at right angles to the planes of the other two, hence their names of the superior vertical, the inferior vertical, and the horizontal canals. We now turn to the membranous labyrinth. The membranous and osseous labyrinths have the same shape, but the former is considerably smaller than the latter, a fluid, termed the perilymph, intervening in some quantity between them. At certain points the membranous is firmly adherent to the osseous labyrinth. The vestibular portion consists of two sacs, an upper and larger one, of an oval sliape, termed the utriculus, and a lower and smaller one of a more globular shape, called the sacculus. The membranous semicircular canals resemble in form and arrangement the osseous canals which inclose them, but are only one-third of the diameter of the latter. The membranous laby- rinth is filled by a fluid which is termed the endolymph ; and in certain spots, especially at the terminations of the vestibular nerves, there are, suspended in the somewhat gclatiiinus endo- lymph, crystals of calcium carbonate known as otoliths, i,e, ear-stones, which are supposed to
