790 HEARING [CH. LVI. 



layers of the membrana tympani. The head of the malleus is 

 irregularly rounded ; its neck, or the line of boundary between the 

 head and the handle, supports two processes: a short conical one, 

 and a slender one, processus gracilis, which extends forwards, and is 

 attached to the wall of the cavity at the G-laserian fissure. The 

 incus, or anvil-bone, shaped like a bicuspid molar tooth, is articulated 

 by its broader part, corresponding with the surface of the crown of 

 the tooth, to the malleus. Of its two fang-like processes, one, 

 directed backwards, has a free end attached by ligament to a depres- 

 sion in the mastoid bone ; the other, curved downwards, longer and 

 more pointed, articulates by means of a roundish tubercle, formerly 



called os orbiculare, with the stapes, 

 a little bone shaped like a stirrup, 

 of which the base fits into the 

 membrane of the fenestra ovalis. 



The muscles of the tympanum 

 are two in number. The tensor 

 tympani arises from the cartila- 

 ginous end of the Eustachian tube 

 and the adjoining surface of the 

 sphenoid, and from the sides of 

 the canal in which the muscle lies ; 

 the tendon of the muscle bends at 

 FIG. 490. interior view of the tympanum, with nearly a right angle over the end 



membrana tympani and bones in natural posi- ,, it * ,, . 



tion. 1, Membrana tympani ; 2, Eustachian 01 the prOCCSSUS COCIllearilOrmiS, 



tube; 3, tensor tympani muscle ; 4,lig. mallei ~ nr ] _ i nQpr fp^ \r\tr\ f"hp> irmek-r -ravf 



exter.; 5, lig. mallei super.; 6, chorda-tympani ana 1S inserted into W16 inner part 



nerve; a, b, and c, sinuses about ossicles. O f tf&Q handle of the malleUS The 



(Schwalbe.) ,. . ' , ... . 



stapedius is concealed within a 



canal in the bone in front of the aqueductus Fallopii. Its tendon 

 is inserted into the neck of the stapes posteriorly. 



The Internal Bar. The proper organ of hearing is formed by the 

 distribution of the auditory nerve, within the internal ear, or laby- 

 rinth, a set of cavities within the petrous portion of the temporal 

 bone. The bone which forms the walls of these cavities is denser 

 than that around it, and forms the osseous labyrinth ; the membrane 

 within the cavities forms the membranous labyrinth. The mem- 

 branous labyrinth contains a fluid called endolymph ; while outside 

 it, between it and the osseous labyrinth, is a fluid called perilymph. 

 This fluid is not pure lymph, as it contains mucin. 



The Osseous Labyrinth consists of three principal parts, namely, 

 the vestibule, the cochlea, and the semicircular canals. 



The vestibule is the middle cavity of the labyrinth, and the 

 central chamber of the auditory apparatus. It presents, in its 

 inner wall, several openings for the entrance of the divisions of the 

 auditory nerve ; in its outer wall, the fenestra ovalis (2, fig. 49 la), 



