C38 



On its inner wall, at its fore part, is a small circular depression, fovea hemisplie- 

 rica; it is perforated, at its anterior and inferior part (macula cribrosa), by seve- 

 ral minute holes for the passage of filaments of the auditory nerve ; and behind 

 it is a vertical ridge, the pyramidal eminence. At the hinder part of the inner 

 wall is the orifice of the aquseductus vestibuli, which extends to the posterior sur- 

 face of the petrous portion of the temporal bone. It transmits a small vein, and, 



Fig. 316. The Osseous Labyrinth laid open. (Enlarged.) 



,,1* 



according to some, contains a tubular prolongation of the lining membrane of the 

 vestibule, which ends in a cul-de-sac, between the layers of the dura mater within 

 the cranial cavity. On the upper wall or roof is a transversely-oval depression, 

 fovea semi-elliptica, separated from the fovea hemispherica by the pyramidal 

 eminence, already mentioned. Behind, the semicircular canals open into the 

 vestibule by five orifices. In front is a large oval opening which communicates 

 with the scala vestibuli of the cochlea by a single orifice, apertura scales vestibuli 

 cochleae. 



The Semicircular canals are three bony canals, situated above and behind the 

 vestibule. They are of unequal length, compressed from side to side, and describe 

 the greater part of a circle. They measure about one-twentieth of an inch in 

 diameter, and each presents a dilatation at one end, called the ampulla, which 

 measures more than twice the diameter of the tube. These canals open into the 

 vestibule by five orifices, one of the apertures being common to two of the 

 canals. 



The superior semicircular canal is vertical in direction, its arch forming a 

 round projection on the anterior surface of the petrous bone. It forms about two- 

 thirds of a circle. Its outer extremity, which is ampullated, commences by a 

 distinct orifice in the upper part of the vestibule ; the opposite end of the canal, 

 which is not dilated, joins with the corresponding part of the posterior canal, and 

 opens by a common orifice with it in the back part of the vestibule. 



The posterior semicircular canal, also vertical in direction, is directed back- 

 wards to the posterior surface of the petrous bone ; it is the longest of the three, 

 its ampullated end commencing at the lower and back part of the vestibule, its 

 opposite end joining the common canal already mentioned. 



The external or horizontal canal is the shortest of the three, its arch being 



