488 MAMMALIAN ANATOMY 



back at each outer edge to form the anterior and greater portion 

 of the concave lateral wall, the posterior lower portion of which is 

 contributed by the parietal and the mastoid of the temporal. The 

 posterior wall is nearly vertical, and is the occipital plate of the 

 occipital bone. At its outer edges are seen the occipito-mastoid and 

 occipito-parietal sutures; above, it joins the interparietal. The pos- 

 terior wall is marked by the convolutions of the cerebellum. 



The arched roof is formed by the tentorium, the parietals proper, 

 and the interparietal ; it exhibits the dividing sutures seen on the 

 exterior of the skull. 



The floor is extensive, but filled on each side by the prominent 

 petrous portion of the temporal (Figs. 391, 395). The central part is 

 flat or gently concave, and is the basilar plate of the occipital which 

 supports the portion of the brain known as the pons. In front, a little 

 behind the dorsum sellse, is the transverse basilar or spheno-occipital 

 suture, at the outer end whereof, on each side, at the tip of the 

 petrous, is the minute middle lacerated foramen. In some specimens 

 this foramen is continued backward as a slit between the basioccipital 

 and the posterior border of the petrous. Along the inner edge of the 

 petrous is the groove which lodges the venous canal called the infe- 

 rior petrosal sinus ; it leads into the large jugular foramen. Another 

 groove passes upward and backward along the posterior edge of the 

 petrous and continues up the lateral wall to enter the parietal bone ; 

 it is the groove for the lateral sinus. Behind the jugular foramen 

 and nearer the median line is the anterior condyloid foramen, and 

 far behind this again the posterior condyloid foramen. The posterior 

 margin of the floor of the posterior fossa is deeply emarginate, and is 

 the lower margin of the foramen magnum. On each side, the large 

 petrous portion of the temporal occupies the anterior lateral angle. 

 Its anterior surface is almost entirely covered by the tentorium. Its 

 posterior surface faces inward, backward, and upward, and exhibits 

 the internal auditory meatus with its associated slit-like beginning 

 of the aquaeductus Fallopii. Above these is the deep pit for the 

 flocculus, or appendicular lobe, of the cerebellum, bounded behind 

 by the elevation for the superior and posterior semicircular canals. 

 Along the line where the posterior surface of the petrous joins the 

 tentorium is the groove for the superior petrosal sinus. The pos- 

 terior upward projection belongs to the mastoid. 



