182 HAND-BOOK OF PHYSIOLOGY. 



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 endolymph; while outside it, between it and the 

 osseous labyrinth, is a fluid called perilymph. 



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. 359), an opening filled by 

 the base of the stapes, one of the small bones of the ear; in its posterior 



FIG. 359. FIG. 360. 



FIG. 359. Right bony labyrinth, viewed from the outer side. The specimen here represented is 

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

 which immediately enclose the labyrinth. 1, the vestibule; 2, fenestra ovalis; 3, superior semicircu- 

 lar 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. 360. View of the interior of the left labyrinth. The bony wall of the labyrinth is removed 

 superiorly and externally. 1, foveahemielliptica; 2, fovea hemispherica: 3, common opening of the 

 superior and posterior semicircular canals; 4, opening of the aqueduct of the vestibule; 5, the supe- 

 rior, 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. 2& (Sommering.) 

 1 



and superior walls, five openings by which the semicircular canals com- 

 municate with it: in its anterior wall, an opening leading into the 

 cochlea. The hinder part of the inner wall of the vestibule also presents 

 an opening, the orifice of the aquceductus vestibuli, a canal leading to the 

 posterior margin of the petrous bone, with uncertain contents and un- 

 known purpose. 



The semicircular canals (Figs. 359, 360), are three arched cylin- 

 driform bony canals, set in the substance of the petrous bone. They 

 all open at both ends into the vestibule (two of them first coalescing). 

 The ends of each are dilated just before opening into the vestibule; 

 and one end of each being more dilated than the other is called an 

 ampulla. Two of the canals form nearly vertical arches; of these the 

 superior is also anterior; the posterior is inferior; the third canal is hori- 

 zontal, and lower and shorter than the others. 



