THE SKULL THE CRANIUM 227 



and superior ampullae. The lower non-ampullar end of the external 

 canal enters the vestibule so close to the ampulla of the posterior canal 

 that the two appear to have a common opening. The three canals 

 therefore communicate with the vestibule really by five openings, 

 though apparently by only four. 



The aquaeductus vestibuli is a fine canal which begins at the pos- 

 terior wall of the vestibule, immediately within and in front of the 

 opening of the united non-ampullar ends of the superior and posterior 

 semicircular canals, with which it runs parallel ; it then diverges down- 

 ward and inward and opens in the cleft at the posterior margin of the 

 posterior surface of the petrous. 



The vestibule is an irregular cavity, higher than it is wide or long, 

 which in the dried bone communicates in front laterally with the 

 tympanum by the fenestra ovalis and below with the cochlea ; it com- 

 municates by five openings with the semicircular canals, and, sometimes, 

 by the aqua3ductus vestibuli with the cranial cavity. The position of 

 the vestibule within the petrous is indicated, externally, on the posterior 

 surface by the posterior wall of the internal auditory meatus and the 

 narrow surface between the meatus and the appendicular fossa. It is 

 indicated on the inferior surface by the fenestra ovalis and the region 

 above and lateral to it ; also by the posterior wall of the fossa for the 

 tensor tympani muscle and by the swelling anterior to the fossa for 

 the stapedius muscle. 



The cochlea 1 is the largest part of the labyrinth. It lies in the 

 lower part of the petrous, in front and to the inner side of the vesti- 

 bule. Its inferior and outer walls produce the swelling on the inferior 

 surface of the bone known as the promontory. It lies in part in front 

 of the anterior wall of the internal auditory meatus. 



The cochlea encloses a tubular cavity, which is large behind and 

 small in front and is twisted like a snail's shell three times round a 

 central bony axis or column, the modiolus. 2 The general direction 

 of this axis is in a longitudinal vertical plane, from behind forward 

 and slightly downward ; it is, however, subject to variations. The 

 tubular cavity of the cochlea is divided lengthwise by a bony parti- 

 tion, the lamina spiralis, one edge of which is attached to the modiolus 

 along a spiral line ; the other edge does not quite reach the cochlear 

 wall. There are thus formed two spiral canals, termed scalse, 3 which 



1 A snail. 2 The nave of a wheel. 3 Ladders. 



