CH. LIII.J 



THE OSSEOUS LABYRINTH. 



717 



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 mem- 

 branous labyrinth. The membranous labyrinth contains a fluid 

 called endolymph ; while outside it, between it and the osseous 



Fig. 541. Right bony labyrinth, viewed 

 from the outer Bide. The specimen 

 here represented is prepared by sepa- 

 rating piecemeal the looser substance 

 of the petrous bone from the dense 

 walls which immediately enclose the 

 labyrinth, i, the vestibule; 2, fen- 

 estra ovalis ; 3, superior semicircular 

 canal; 4, horizontal or external canal; 

 5, posterior canal ; , ampullae of the 

 semicircular 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. 2 *. (Sommering.) 



Fig. 542. View of the interior of the left 

 labyrinth. The bony wall of the laby- 

 rinth is removed superiorly and exter- 

 nally, i, fovea hemielliptica ; 2, fovea 

 hemispherica ; 5, 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 semicir- 

 cular 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 scam ves- 



tibuli. 2 -i (Sommering.) 



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, several openings for the entrance of the divisions of the 

 auditory nerve ; in its outer wall, the fenestra ovalis (2, fig. 541), 

 an opening filled by membrane in which is inserted the base of 

 the stapes ; in its posterior and superior walls, five openings by 

 whirh the semicircular canals communicate with it : in its anterior 

 wall, an opening leading into the cochlea. The structure of the 

 semicircular canals is described in Chapter XLIX. 



The cochlea (6, 7, 8, fig. 541, and 8, fig. 542), a small organ, 



