THE BONES OF THE SKULL 181 



the sphenooccipital synchondrosis (synchondrosis sphenooccipi- 

 taHs). Posteriorly its dorsal and ventral surfaces come together 

 in a thin concave edge which forms the ventral boundary of the 

 foramen magnum. Laterally it is bounded by the petrotympanic 

 bone and by the lateral portion of the occipital. The dorsal surface 

 bears a median groove, deeper in its middle portion, where the 

 lateral margins of the bone are raised to form a pair of rounded 

 bosses for articulation with the petrotympanic. The groove 

 represents the sloping portion or clivus of the occipital, and lodges 

 in the natural condition, as described above, the ventral portion 

 of the medulla oblongata. The ventral surface presents a similar 

 groove, in the posterior portion of which there is a small ridge-like 

 elevation, the pharyngeal tubercle (tuberculum pharyngeum). 



The exocclpltal is directed dorsad from the basloccipital in such 

 a way that it falls in the plane of the nuchal surface. It is applied 

 to the posterior surface of the petrotympanic bone, and also extends 

 downward beyond the latter as the jugular process. The occipital 

 condyle is borne on the exocclpltal, with the exception, however, 

 of its ventral tip, which belongs to the basloccipital. The portion 

 of the occipital bone connecting the basioccipital and exocclpltal 

 contains the jugular fossa and the apertures representing the 

 hypoglossal canal. Its anterior margin bears a jugular incisure 

 (Incisura jugularis), forming the occipital boundary of the jugular 

 foramen, the remaining portion of the latter being formed by the 

 petrotympanic. 



The supraoccipital Is the dorsal portion of the bone. Its dorsal 

 margin is bent sharply forward, so that it tends to fall, like the 

 basloccipital, in a horizontal plane. Its external surface bears the 

 nuchal crest and the external occipital protuberance. Paired 

 lateral wing-like expansions rest upon, and partly overlap, the 

 dorsal margins of the petrotympanic bones. The anterior boundary 

 Is formed by the interparietal, parietal, and squamosal bones, but 

 In young skulls the squamosal connection is represented by a 

 vacuity. The Internal surface bears-a median longitudinal groove, 

 lodging in the natural condition the vermis of the cerebellum. It 

 Is crossed at Its anterior end by a shallow transverse groove (sulcus 

 transversus), which marks the position of the transverse sinus of 

 the dura mater. 



