530 THE SENSE OF HEARING. 



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

 osseous, labyrinth, is a fluid called perilymph (see p. 533). 



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 (5, Fig. 198), an opening filled by the base of the stapes, 



FIG. 198. 



FIG. 198a. 



FIG. 198. A view of the labyrinth of the left ear of a foetus of 8 months, as seen 

 from above. Magnified 4 diameters. 1,2, 3. The cochlea. 1, 1. Its first turn. 2, 2. Its 

 second turn. 3, 3. Its third or half turn, and apex or cupola. 4. The fenestra ro- 

 tunda. 5. The fenestra ovalis. 6. The groove around it. 7, 7. The vestibule. 8, 9, 10. 

 The posterior semicircular canal, with its ampulla at 8. 11, 11. The superior semi- 

 circular canal. 12. The external semicircular canal. (S. & H.) 



FIG. 198a. An outline, of the natural size, of figure 198. 



one of the small bones of the ear ; in its posterior and superior 

 walls, five openings by which the semicircular canals communi- 

 cate 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 unknown purpose. 



