490 SCHULTE, SEI WHAIJv 



such as Voit found in the rabbit. The rest of the surface shows a vertical ridge which renders 

 it convex latero-mesad. 



The lateral surface has as its boundaries, dorsally the arch formed by the vertical semi- 

 circular canals, and ventrally the line of the external canal. The dorsal boundary is thus cres- 

 centic and meets the ventral edge at either extremity. The ventral edge, rostrally begins at 

 the caudal extremity of the prefacial commissure, and caudally ends at the tip of the processus 

 mastoideus, at the origin of the hyoid bar. This marks the caudal' extremity of the crista paro- 

 tica, which is the surface relief of the external semicircular canal. Rostrad to the canal the 

 border serves for the attachment of the tegmen tympani. 



The lateral surface, having the boundaries described above is roughly triangular, each side 

 of the triangle being formed by a semicircular canal. As a whole it is convex, due to the pro- 

 jection of that mass of cartilage known as the "massa angularis." 1 



The caudal portion of the surface, which shows in relief the underlying posterior semicir- 

 cular canal appears upon the ectal aspect of the skull between exoccipital and squamosum. 

 This area corresponds in its position to the adult mastoid, though there is as yet no indication 

 of the definitive mastoid. 



The ventral surface of the pars canalicularis is triangular, the apex pointing mesad. The 

 inner half of the surface rests on the pars cochlearis, and there is an intimate union of their 

 substance. The lateral portion of the surface forms the roof of the middle ear and is in relation 

 to the structures contained in it. The base of the triangle, which is placed laterad, corresponds 

 with the inferior border of the lateral surface, already described. 



The surfaces narrow at either extremity as do those of a triangular pyramid. Each apex 

 thus formed has attached to it a cartilaginous process. The caudal of these is the hyoid bar. 

 From this region is eventually developed the mastoid process. The rostral prolongation extends 

 to the pars cochlearis, roofing over the primitive facial canal, and thus forming the commissura 

 prsefacialis. 



Canalis facialis: - - The facial nerve passes laterad, reaching the ental opening of the facial 

 canal by crossing over a prominent ridge of cartilage which divides it from the foramen acusticum. 

 The two apertures for the facial and acoustic nerves are surrounded by a fairly well marked 

 cartilaginous rim. Thus is delimited the porus acusticus internus. At the ectal end of the 

 primitive facial canal, the nerve lies under the tegmen tympani entirely concealed from view. 

 The hiatus Fallopii is represented by a slit in the line of apposition of the tegmen tympani to 

 the ectal surface of the pars cochlearis. Beneath the cover of the tegmen tympani the nerve 

 turns sharply caudad. In its course above the fenestra ovalis it lies in a closed canal below the 

 crista parotica. Turning ventrad in this canal it appears at the caudal apex of the pars canali- 

 cularis, mesal to the hyoid cartilage. 



Tegmen tympani: - - The tegmen tympani is a quadrangular plate of cartilage, taking 

 origin by its caudo-dorsal border from the pars canalicularis. At its point of origin it covers 

 the angle formed by the meeting of the anterior and external semicircular canals. Its convex 

 ectal surface is continuous with the lateral surface of the pars canalicularis. The ental surface 

 roofs in the tympanic cavity, and conceals from view the outer end of the primitive facial canal 

 and its continuation to the point where it meets the crista parotica. To the rostro-dorsal 



1 Macklin, C. C. The skull of a human foetus of 40 mm. Am. Jour. Anat, Vol. XVI, 1914. 



