TOPOGRAPHICAL ANATOMY OF THE EAR. 165 



The general arrangement of the semicircular canals is 

 shown in Fig. 13 (6, 7, 8, 9, 10, 11, 12). 



The arrangement of the cochlea, the anterior division of 

 the labyrinth, is shown in Fig. 13 (1, 3, 4). This is a spiral 

 canal, about an inch and a half long, and one-tenth of an inch 

 wide at its commencement, gradually tapering to the apex, 

 and making, in its course, two and a half turns. Its interior 

 presents a central pillar, around which winds a spiral lamina 

 of bone. The fenestra rotunda (2, Fig. 13), closed in the 

 natural state by a membrane (the secondary membrana tym- 

 pani), lies between the lower portion of the cochlea and the 

 cavity of the tympanum. 



What is called the membranous labyrinth is contained 

 within the bony parts just described. Its structure, and 

 the ultimate distribution and connections of the auditory 

 nerve, which penetrates by the internal auditory meatus, 

 involve some of the most intricate and difficult points in the 

 whole range of minute anatomy. Some of these have direct 

 and important relations to the physiology of hearing, while 

 many are of purely anatomical interest. Such facts as bear 

 directly upon physiology will be considered fully in connec- 

 tion with the functions of the internal ear. 



