356 CHARLES CLIFFORD MACKLIN 



mune, and the upper part of the posterior canal (fig. 8). The 

 two lateral otic eminences mark the extremities of the greatest 

 transverse diameter of the primitive skull. 



The dorsal extremity of the posterior semicircular prominence 

 forms a gentle rise in the caudal area of the lateral surface, and 

 then passes over the root of the mastoid process, as we have seen, 

 to become prominent in its ventralmost portion, the prominentia 

 utriculoampullaris inferior (figs. 6 and 7), which forms the con- 

 spicuous border between the ventral and medial surfaces and 

 acts as the upper border of the lateral part of the jugular 

 foramen. 



The prominentia semicircularis lateralis is an indistinct swell- 

 ing passing downward and backward from the crista parotica 

 in the region of the incus to the dorsal extremity of the posterior 

 semicircular prominence. 



The medial surface (fig. 7) is more extensive than the lateral. 

 Its cranial and dorsal borders are the same as those of the lateral 

 surface; its ventral border is marked above by a rounded ridge 

 passing downwards from the superior utriculoampullary prom- 

 inence to the superior otic notch, and below by the posterior 

 semicircular prominence, which, as has been seen, terminates 

 ventro-medially in the inferior utriculoampullary prominence, 

 the latter bearing a ventrally-projecting process, the processus 

 interperilymphatica (Voit). The middle portion of the ventral 

 boundary is formed by the transition of the medial surface of 

 the pars canalicularis into that of the vestibular portion 

 of the pars cochlearis. As has been mentioned the medial 

 surface is convex, and presents in its central area as its most 

 prominent object the prominentia cruris communis (Voit), formed 

 by the crus conmiune within (fig. 9). Upon the dorsal part of 

 this prominence is seen the long, almost horizontal, slit-like 

 foramen endolymphaticum, for the outlet of the ductus endo- 

 lymphaticus. Both lips of this foramen are formed of a young 

 type of cartilage and it may be noted that the upper lip projects 

 medially in its dorsal part to ovei^hq-ng the duct, and is continued 

 dorsally past the foramen to form a groove, in which the duct 

 lies (fig. 7). The dorsal extremity of this upper lip appears as a 

 short, free process, overlying the duct. 



