128 



CONSCIOUS NERVOUS OPERATIONS 



Posterior 

 Canal 



A ntertor 

 Canal 



Fenestra Ovalia 



Fenestra 

 Rotunda 



and 76). Within it lies a closed membranous sac called 

 the membranous labyrinth, which follows the windings of 

 the bony cavity, and whose parts receive names corre- 

 sponding to the names given to the parts of the cavity. 



The central part of the labyrinth is called the vestibule ; 

 it is about one eighth of an inch in diameter. In its walls 



are the round and oval Avin- 

 dows already mentioned. 

 The membranous vestibule 

 is composed of two bags, 

 called the utricle and the 

 saccule, connected by a 

 roundabout passage. 



171. From the utricle 

 arise three semicircular 

 canals, lying in the bony 

 passages of the same name. 

 One of these is horizontal 

 when a person stands up- 

 right; the others are vertical but at right angles to each 

 other. Two of these canals are united at one end, so that 

 there are but five openings from the canals into the vesti- 

 bule. Each canal has a swelling at one end called the 

 ampulla. At those swellings fibers from the eighth cranial 

 (auditory) nerve pass from the bony wall through the 

 membranes of the canals, firmly attaching one to the other. 

 For the rest of their course the membranous tubes are free, 

 or only loosely fastened by bands, of connective tissue to 

 the bony walls. 



172. The Cochlea is the third and most complex division 

 of the inner ear. It has much the appearance of a snail 

 shell of two and a half coils. A bony tube is coiled spirally 

 round a central tapering pillar of bone. Into this tube 



Fig. 76. The bony labyrinth of the 

 right ear. 



