THE SENSES. G70 



it to the membrane closing the fenestra ovalis. . The malleus, also, 

 is movable in its articulation with the incus; and the membrana 

 tympani moving with it is altered in its degree of tension by the laxator 

 and tensor tympani muscles. The stapes is movable on the process of 

 the incus, when the stapedius muscle acting, draws it backward. The 

 axis round which the malleus and incus rotate is the line joining the pro- 

 cessus gracilis of the malleus and the posterior (short) process of the incus. 



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

 distribution of the auditory nerve within the internal ear, or labyrinth^ 

 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 membranous labyrinth contains a 

 fluid called endolympli; 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 

 organ of the whole auditory apparatus. It presents, in its inner wall, 



Fig. 400. Fig. 401. 



Fig. 400. Right bony labyrinth, viewed from the outer side. The specimen here represented 

 I? prepared by separating piecemeal the looser substance of the petrous bone from the dense 

 walls which immediately inclose the labyrinth. 1, the vestibule; 2, fenestra ovalis; 3, superior 

 semicircular canal; 4, horizontal or external canal; 5, posterior canal; *, ampullae of the semi- 

 circular canals ; 6, first turn of the cochlea ; 7, second turn ; 8, apex ; 9, fenestra rotunda. The 



smaller figure in outline below shows the natural size. ^- (Sommering.) 



Fig. 401. Vie~\ of the interior of the left labyrinth. The bony wall of the labyrinth is re- 

 moved superiorly and externally. 1, Fovea hemielliptica: 2, fovea hemispherica ; 3, common 

 opening of the superior and posterior semicircular canals ; 4, opening of the aqueduct of the 

 vestibule; 5, the superior, 6, the posterior, and 7, the external semicircular canals; 8, spiral 

 tube of the cochlea (scala tympani); 9, opening of the aqueduct of the cochlea; 10, placed on 



the lamina spiralis in the scala vestibuli. -^ (Sommering.) 



several openings for the entrance of the divisions of the auditory 

 nerve; in its outer wall, the fenestra ovalis (2, fig. 400), an open- 



