462 VESTIBULE. 



osseous labyrinth presents a series of cavities, which are channelled through 

 the substance of the petrous bone, and is situated between the cavity of 

 the tympanum and the meatus auditorius internus. It is divisible into 

 the 



Vestibule, 



Semicircular canals, 



Cochlea. 



The VESTIBULE is a small three-cornered cavity, compressed from with- 

 out inwards, and situated immediately within the inner wall of the tym- 

 panum. The three corners, which are named ventricles or cornua, are 

 placed, one anteriorly, one superiorly, and one posteriorly. 



The anterior ventricle receives the oval aperture of the scala vestibuli ; 

 the superior, the ampullary openings of the superior and horizontal semi- 

 circular canals ; the posterior, the ampullary opening of the oblique semi- 

 circular canal, the common aperture of the oblique and perpendicular 

 canals, the termination of the horizontal canal, and the aperture of the 

 aquseductus vestibuli. In the anterior ventricle is a small depression, 

 which corresponds with the posterior segment of the cul de sac of the 

 meatus auditorius internus ; it is called the fovea hemispherica, and is 

 pierced by a cluster of small openings, the macula cribrosa. In the supe- 

 rior ventricle of the vestibule is another small depression, the fovea ellip- 

 tica, which is separated from the fovea hemispherica by a projecting crest, 

 the eminentia pyramidalis. 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 up- 

 wards to the ostium aquaeductus vestibuli. The internal wall of the ves- 

 tibule corresponds with the bottom of the cul de sac of the meatus audito- 

 rius internus, and is pierced by numerous small openings for the transmis- 

 sion 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 prominent rim towards the cavity of the vestibule. 



The openings of the vestibule may be arranged, like those of the tym- 

 panum, 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 



Aquseduetus vestibuli, 



Openings for small arteries, 



Openings for branches of the auditory nerve. 



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

 the tympanum. 



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

 ar canal of the cochlea. 



The Aquaductus vestibuli (canal of Cotunnius) is the commencement 

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

 surface of the petrous bone. It gives passage to a process of membrane 



