OPENINGS OP THE VESTIBtLE. 457 



the horizontal canal, and the aperture of the aquaeductus vestibulL 

 In the anterior ventricle is a small depression, which corresponds 

 with the posterior segment of the cul de sac of the meatus audi- 

 torius internus ; it is called the fovea hemispherica, and is pierced 

 by a cluster of small openings, the macula cribrosa. In the superior 

 ventricle of the vestibule is another small depression, the fovea 

 elliptica, which is separated from the fovea hemispherica by a pro- 

 jecting crest, the eminentia pyramid alis. The latter is pierced by 

 numerous minute openings for the passage of nervous filaments. 

 The posterior ventricle presents a third small depression, the fovea 

 sulciformis, which leads upwards to the ostium aquseductus vestibuli. 

 The internal wall of the vestibule corresponds with the bottom of 

 the cul de sac of the meatus auditorius internus, and is pierced by 

 numerous small openings for the transmission of nervous filaments. 

 In the external or tympanic wall is the reniform opening of the 

 fenestra ovalis (fenestra vestibuli), the margin of which presents a 

 projecting rim towards the cavity of the vestibule. 



The openings of the vestibule may be arranged, 'like those of the 

 tympanum, into large and small. 



The Large openings are seven in number, viz. the 



Fenestra ovalis, 



Scala vestibuli, 



Five openings of the three semicircular canals. 



The Small openings are the 



Aquasductus vestibuli, 



Openings for small arteries, 



Openings for branches of the auditory nerve. 



The Fenestra ovalis has already been described ; it is the opening 

 into the tympanum. 



The opening of the scala vestibuli is the oval termination of the 

 vestibular canal of the cochlea. 



The Aquaductus vestibuli (canal of Cotunnius) is the commence- 

 ment of the small canal which opens under the osseous scale upon 

 the posterior surface of the petrous bone. > It gives a passage to a 

 process of membrane which is continuous internally with the lining 

 membrane of the vestibule, and externally with the dura mater, and 

 to a small vein. 



The Openings for the arteries and nerves are situated in the in- 

 ternal wall of the vestibule, and correspond with the termination of 

 the meatus auditorius internus. 



The SEMICIRCULAR CANALS are three bony passages which com- 

 municate with the vestibule, into which they open by both extre- 

 mities. Near one extremity of each of the canals is a remarkable 

 dilatation of its cavity, which is called the ampulla (sinus ampullaceus). 

 The superior or perpendicular canals (canalis semicircularis verticalis 



58 



