Organ of Hearing. 



235 



The three semicircular 

 canals are : a superior, 

 an inferior or posterior 

 and an external; each stands 

 at right angles to the other 

 two. There are three orifices 

 at one extremity, which are 

 dilated, the enlargement being 

 known as ampulla, but only 

 two orifices at the opposite 

 extremity, the superior and 

 posterior canals joining 

 together and opening by a 

 common aperture. The exter- 

 nal canal is the shortest, 

 the posterior the longest. 

 The cochlea consists of 

 a canal wound spirally around 

 a central axis for 2 ! / 2 turns; 

 this central axis is called, for 

 the first turn, Modiolus, 

 for the second Columella, 

 and for the uppermost half 

 turn Lamina modioli. The apex 

 of the cochlea is directed for- 

 wards towards the inner wall 

 of the tympanum, its base is 

 turned towards the internal 

 auditory meatus. The spiral 

 canal terminates in a cul-de- 

 sac, the Cupula; here the Scy- 

 plius Vieussenii or infundibu- 

 lum of the cochlea is formed 

 (Fig. 320). 



Lam. spiral 



Hi 



321. The Cochlea, laid open verti- 

 cally to the axis of the Petrous Portion 

 of the Temporal Bone. Magn. = 5. 



Jty'J 



ypirale 



322. Section of a Spiral 



Canal. (Diagrammatic.) 



30' 



