676 THE SENSE OF HEARING. 



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 (see p. 678). 



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 walls, several openings for the entrance of the 

 divisions of the auditory nerve; in its outer wall, ihefenestra 

 ovalis (2, fig. 198), an opening filled by the base of the stapes, 

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

 superior walls, five openings by which the semicircular 

 canals communicate with it : in its anterior wall, an open- 

 ing leading into the cochlea. The hinder part of the inner 

 wall of the vestibule also presents an opening, the orifice 

 of the aquatductus vestibuli, a canal leading to the posterior 

 margin of the petrous bone, with uncertain contents and 

 unknown purpose. 



The semicircular canals (figs. 198, 199) are three arched 

 cylindriform 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 horizontal, and lower and shorter than the others. 



The cochlea (6, 7, 8 , fig. 198, and fig. 200), a small 

 organ, shaped like a common snail-shell, is seated in 

 front of the vestibule, its base resting on the bottom 

 of the internal meatus, where some apertures transmit to 

 it the cochlear filaments of the auditory nerve. In its 

 axis, the cochlea is traversed by a conical column, the 

 modiolus, around which a spiral canal winds with about two 



