308 ROBERT J. TERRY 



pulla opens posteriorly into the canalis semicircularis lateralis. 

 The posterior wall is formed by the mass of cartilage already re- 

 ferred to which projects medialward from the lateral wall of the 

 otic capsule (fig. 11). This crista intervestibularis, as it may be 

 termed, intervenes between the anterior and posterior vestibular ' 

 caves, but fails to meet the medial vestibular wall ; there is left a 

 communicating space between the two divisions occupied by a 

 part of the utricle. Ventrally, the crista inter vestibularis enters 

 into the roof of the lateral semicircular canal ; dorsally it becomes 

 continuous with the massa angularis, the great body of cartilage 

 encompassed by the semicircular canals. The dorsal wall of the 

 cavum vestibulare anterius is made by that portion of the angu- 

 lar mass which fills the concavity of the canalis semicircularis 

 anterior. 



Cavum vestibulare posterius. In form, the posterior division of 

 the vestibule is oval dorsally, narrow and tunnel-shaped ven- 

 trally, expanding into a roomy space in the region of the fenestra 

 cochleae. The posterior wall bulges in adaptation to the adja- 

 cent recessus ampullaris posterior, forming on the surface of the 

 ear capsule the prominentia utriculo-ampullaris inferior. The 

 ventral part of the posterior wall presents the posterior orifice 

 of the lateral semicircular canal. The dorsal part, at its junc- 

 tion with the roof of the cavum vestibuH posterius, shows the 

 orifice of the posterior semicircular canal. The medial wall, con- 

 tinuous with the medial wall of the anterior cave, corresponds 

 to that part of the otic capsule marked on the surface by the 

 beginning of the prominentia cruris communis and the incisura 

 singularis. It presents a ventral free edge looking into the in- 

 ferior acustic foramen. The dorsal wall consists of that part of 

 the massa angularis intervening between the canalis semicircu- 

 laris posterior and the vestibule. In the highest part of the 

 roof are two openings: — the great circular orifice of the cavum 

 cruris communis, containing the crus commune of the anterior 

 and posterior canals; the small irregular hole, foramen endolym- 

 phaticum, looking medialwards and transmitting the ductus en- 

 dolymphaticus. A floor is present in the caudal half only, be- 

 neath the ampullary end of the posterior semicircular duct; 



