THE SKULL THE CRANIUM 



175 



FIG. 114. 



The occipital condyles are flatter than those of the cat, and, owing 

 to the position in which the head is held upon the vertebral column, 

 their articular surfaces are more nearly parallel to the plane of the 

 base of the skull. 



The occipital foramen (foramen magnum) is oval, with the major 

 'axis in the long axis of the skull. When the posterior condyloid 

 foramen is present its posterior opening lies in a fossa above the 

 condyle, and its anterior opening appears near the jugular notch in 

 the groove for the lateral sinus. 



The internal surface (Fig. 114) of the occipital plate is divided 

 into an upper and a lower part by a transverse groove, bordered by 

 prominent ridges and elevated at the 

 median point into an internal occipital 

 protuberance. The part above the 

 groove appears to correspond to the pos- 

 terior part of the intracranial surface of 

 the parietals in the cat. 



The groove lodges the lateral sinus, 

 and the ridges afford attachment to the 

 membranous tentorium cerebelli. In the 

 cat the tentorium is a bony plate ; hence 

 its base, or attached edge, is tunnelled by 

 the sinus. From the internal protuber- 

 ance a vertical groove passes upward to 

 the superior angle and lodges the supe- 

 rior longitudinal sinus in the falx cer- 

 ebri. At the point where the vertical 

 groove meets the transverse groove, in 

 the middle line, or more frequently to 

 the right of it, there is occasionally a 

 circular fossa, which receives a dilata- 

 tion of the sinuses, known as the torcular 1 Heroph'ili. 2 A vertical 

 ridge, the internal occipital crest, passes downward from the pro- 

 tuberance and affords attachment to the falx cerebelli. Its lower 

 part may be excavated to receive the vermiform process of the cere- 

 bellum. Through this cross-shaped arrangement of grooves and crests 

 the internal surface is divided into four concave areas, whereof the 



1 A wine-press. 2 Herophilus (about 300 B.C.). 



OCCIPITAL BONE. INTERNAL VIEW. 

 1, fossa for the cerebrum ; 2, fossa for the 

 cerebellum ; 3, groove for the longitudinal 

 sinus; 4, internal occipital crest for the at- 

 tachment of the cerebellar falx; 5, groove 

 for the lateral sinus; 6, internal occipital 

 protuberance ; 7, occipital foramen ; 8, basilar 

 process; 9, 10, groove for the lateral sinus, 

 descending on the jugular process to the 

 jugular notch; 11, border for the parietal 

 bone ; 12, border for the temporal bone ; 13, 

 position at which the ninth, tenth, and 

 eleventh cerebral nerves pass through the 

 jugular foramen ; 14, anterior condyloid 

 foramen. 



