THE OSSEOUS LABYRINTH. 



81 



than the back of the cavity, is a vertical ridge or crest (6). In crest on 

 [front of the crest is a circular depression, fovea hemispherica (a), ^th'fossa 

 whii-h is pierced by minute apertures for nerves and vessels, and in front ' 

 j corresponds with the bottom of the internal auditory meatus. 

 Behind the crest of bone, near the common opening of two of the 

 semicircular canals, is the aperture of the aqueduct of the vestibule (c), and aque- 

 a narrow canal which ends on the posterior surface of the petrous dnct behind > 

 portion of the temporal bone : it contains a process of the mem- 

 branous labyrinth called the ductus endolympliaticus, and a small 



The roof is occupied by a slight transversely oval depression, fossa in roof. 



FIG. 301. VIEW OF THE VESTIBULE OF THE RIGHT SIDE, OBTAINED BY CUTTING 

 AWAY THE OUTER BOUNDARY IN A FOETUS, ENLARGED THBEE TIMES. 



a. Fovea hemispherica. 



b. Crest of the vestibule. 



c. Aperture of aqueduct of the 

 vestibule. 



d. Common opening of two semi- 

 circular canals. 



e. Upper semicircular canal, partly 

 laid open. 



/. Horizontal semicircular canal, 

 partly opened. 



g. Opening of the scala vestibuli of 

 the cochlea. 



fovea hemielliptica ; this is separated from the fovea hemispherica by 

 a prolongation of the crest (6) on the inner wall. 



The SEMICIRCULAR CANALS (fig. 302) are three osseous tubes, Three semi- 

 which are situate behind the vestibule, and are named from their SSaisj" 

 form. 



Dissection. These small canals will be brought into view by preparation 

 the removal of the surrounding bone by means of a file or bone 

 forceps. Two may be seen opening near the aperture made in the 

 vestibule, and may be followed thence ; but the third is altogether 

 towards the posterior aspect of the petrous portion of the temporal 

 bone. 



The canals are unequal in length, and each forms more than half length; 

 an ellipse. They communicate at each end with the vestibule, but 



