514 SEMICIRCULAR CANALS. 



the passage of nervous filaments. The posterior ventricle presents a 

 third small depression, the fovea sulciformis, which leads upwards to 

 the ostium aquaeductus 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 trans- 

 mission 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 

 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 



Aquaeductus 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 ves- 

 tibular canal of the cochlea. 



The Aquaeductus vestibuli (canal of Cotunnius) is the commencement 

 of the small canal which opens under the osseous scale upon the pos- 

 terior surface of the petrous bone. It gives 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 internal 

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

 meatus auditorius internus. 



The SEMICIRCULAR CANALS are three bony passages communi- 

 cating with the vestibule, into which they open by both extremities. 

 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 canal (canalis semicircularis verticalis supe- 

 rior), is directed transversely across the petrous bone, forming a pro- 

 jection upon the anterior face of the latter. It commences by means 

 of an ampulla in the superior ventricle of the vestibule, and terminates 

 posteriorly by joining with the oblique, and forming a common canal, 

 which opens into the iipper part of the posterior ventricle. The 

 middle or oblique canal (canalis semicircularis verticalis posterior) cor- 

 responds with the posterior part of the petrous portion of the temporal 



