306 ROBERT J. TERRY 



acustic meatus (figs. 10, 11, 19), makes a steep descent toward 

 the floor of the cochlear cavity, and continues as a low ridge 

 to the extent of somewhat more than half a turn from the be- 

 ginning of the septum. Its position externally, is indicated on 

 the ventral surface of the cochlear capsule by the sulcus septahs 

 (p. 302). 



The oval fenestra vestibuli (figs. 3, 20, 21) appears externally 

 at the bottom of a deep depression of the surface of the capsular 

 wall, standing in a coronal plane, above and lateral to the peri- 

 lymphatic foramen. Its superior and medial boundaries are very 

 prominent; its inferior limit obscure. Within this depression 

 is the foot-plate of the cartilaginous stapes, embedded in mesen- 

 chyma which completely fills the window. Within the cavum 

 cochleae, the fenestra appears on an eminence of the lateral 

 wall which projects posteriorly toward the vestibule. 



The large foramen perilymphaticum (figs. 2, 12, 20) extends 

 upon both the caudal and dorsal surfaces of the pars cochlearis 

 and is incompletely divided by a process of the cochlear capsule 

 into two parts, the fenestra cochleae and the aquaeductus 

 cochleae. The former is located at the caudal pole of the coch- 

 lear prominence, stands in a transverse plane and is occupied by 

 mesenchyma. In figure 11 it appeiars completely separated 

 from the aquaeductus cochleae, but the septum between these 

 openings is chondrified only in its medial part; laterally it is 

 composed of precartilage. The aquaeductus cochleae, lying in a 

 frontal plane, opens into the fossa occipito-canahcularis above 

 (p. 291), and communicates with the cavum cochleae below. It is 

 filled with mesenchyma, through which small veins pass to the 

 vena jugularis interria. The aqueduct of the cochlea at the 

 present stage is an extension of the jugular foramen forward in 

 the form of a deep notch between the commissura basivestibu- 

 laris and the prominentia utriculo-ampullaris inferior. It con- 

 stitutes a vacuity in the roof of the cochlear capsule. The 

 aquaeductus cochleae is separated from the jugular foramen 

 proper (i.e. the posterior part transmitting the vein and nerves) 

 by the free angular projection of the cochlear wall above men- 

 tioned (indicated by * in figs. 2, 12, 19). 



