178 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Fig. 112. 



-Section of the cochlea showing the 

 winding cavity. 



Bony Labyrinth. — The vestibule is a hollow space 

 about the size of a wheat grain. It is called the vesti- 

 bule because it is 

 the general hall into 

 which open all the 

 winding passages. 

 It is separated from 

 the tympanic cavity 

 by a wall in which 

 is the membrane- 

 closed oval open- 

 ing. Out of this 

 hall go out and 

 return again three 

 slender tubes — the 

 semicircular canals. 

 Two of these unite 

 at one end and enter by a common opening, so that 

 there are five openings into the vestibule instead of 

 six. At one end of each canal there is a flasklike en- 

 largement. 



The bony cochlea is a spiral cavity like a spiral stair- 

 way, winding about a central pillar two and a half 

 times and growing smaller to the end (Fig. 112). The 

 name is taken from its resemblance to the shell of a 

 snail. 



Membranous Labyrinth. — In the bony labyrin- 

 thine cavity just described is lodged the membranous 

 parts of the same names. The membranous vestibule, 

 or vestibular sac, within the cavity of the bony vestibule 

 is a small sac from which go and return the three mem- 

 branous semicircular canals, each with its flask-shaped 

 enlargement at one end, called the ampulla. The audi- 

 tory nerves are distributed on the vestibular sac and on 

 the ampullas, the fibers terminating directly on the inte- 



