704 ORGANS OF SENSE. 



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 communi- 

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

 vestikuli cochleae. 



The Semilunar 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 one-twentieth 

 of an inch in diameter, and each presents a dilatation at one end, called the 



Fig. 381. The Osseous Labyrinth laid open. (Enlarged.) 



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 com- 

 mon to two of the canals. 



The superior semicircular canal is vertical in direction, and stretches across 

 the petrous portion of the temporal bone, at right angles to its posterior sur- 

 face; its arch forms a round projection on the anterior surface of the petrous 

 bone. It describes 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 correspond- 

 ing 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, nearly parallel 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 to form the common canal already 

 mentioned. 



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

 directed outwards and backwards ; thus each semicircular canal stands at right 

 angles to the other two. Its ampullated end corresponds to the upper and 

 outer angle of the vestibule, just above the fenestra ovalis ; its opposite end 

 opens by a distinct orifice at the upper and back part of the vestibule. 



