THE INTERNAL EAR. 429 



communicating cavity between the cochlea and semicir- 

 cular canals, and is in contact with the inner wall of the 

 tympanum. It is ovoidal, and measures about one-fourth 

 inch in its long diameter. It presents the oval window, 

 to which the base of the stapes is applied. Opposite to 

 this is the fovea hemisphserica, a depression which lodges 

 the saccule. At the bottom of the fovea hemisphaBrica 

 are a number of foramina, forming the macula cribrosa, 

 which transmit filaments of the auditory nerve ; also, 

 on the inner wall, is an elevation, the pyramidal emi- 

 nence. The opening of the aquaeductus vestibuli is at 

 the lower posterior part. Above the fovea hemisphserica 

 is the fovea semi-elliptica, which 

 lodges the utricle. The three semi- 

 circular canals communicate with 

 the posterior portion of the vesti- 

 bule by five orifices. The cochlea 



FIG. 187. EXTERNAL Ap- 



opens in front by an elliptical ** C D F F"NES A TRA 

 opening into the scala vestibuli. ^f. h , ho M ^ 8 , ^ 



ml ji i 1 terior semicircular canals of the left 



1 he three semicircular canals de. The cochlea is seen to the left 



of the figure. 



constitute each five-sixths of a 



circle ; they are about one-twentieth inch in diameter, 

 slightly compressed, so that the section is an ellipse, and 

 at their origin each is dilated into an ampulla. They 

 are named the superior, posterior, and external, and each 

 canal is at right angles to the others. The superior and 

 posterior open by a common canal; the external opens 

 separately. 



The cochlea is a conical structure about one-quarter 

 inch at the base and one-quarter inch in height. The 

 apex is directed forward and outward and slightly down- 

 ward. It consists of a central, tapering, tubular axis, 

 the modiolus, around which is wound a spiral canal for 

 two and a half turns. The modiolus transmits branches 



