XX 



I 111. KAK AND THE SENSE OF HEARING 



223 



similar peculiar shape situated in the petrous part of the 

 temporal lx>ne. This cavity is a completely closed one, the 

 two openings from it through the bone into the middle ear 

 being closed, as already mentioned, by membrane. Sur- 

 rounding the membraneous bag and its various parts, and 

 srp.irating it from the bony walls of the cavity, is a lymph- 

 like fluid, the perilymph. At certain parts, namely, where 

 fibres of the auditory nerve pass from canals in the bone to it, 

 the membraneous bag is firmly attached to the bony walls ; 



IfS.C 



FIG. 105. Diagram to show how the auditory nerve is distributed to the membraneous 



labyrinth and cochlea. 

 A. N, auditory nerve ; c/, utricle ; S, saccule ; A.S.C, I/.S.C, P.S.C, the dilated end 



of the three semicircular canals ; Cotk, cochlea . c, canal joining it to the saccule ; 



n f, the communicating tube between the utricle and saccule. 



otherwise the bag is free, or only loosely attached by bands 

 of connective tissue. 



The central part of the membraneous bag is an oval sac, 

 called the utricle. From this spring three hoop-like canals, 

 called the semicircular canals, which lie in corresponding 

 canals in the bone. In a man standing upright, one of these 

 lies horizontally and the other two lie vertically, but at right 

 angles to one another. Near the utricle, and in roundabout 

 communication with it, is another small sac, called the saccule. 

 Tin- two sacs together form the vestibule, and this, with the 

 semicircular canals, is spoken of as the membraneous 



