The Bones of the Skull. 141 



The basioccipital is that portion lying below and in front of 

 the foramen magnum. Its main surfaces are respectively dorsal 

 and ventral. Its anterior margin is United with the posterior 

 margin of the basisphenoid by a thin, transverse cartilage union, 

 the sphenooccipital synchondrosis (synchondrosis spheno- 

 occipitalis) . 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 petro- 

 tympanic 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 cUvuS 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 exoccipital is directed dorsad from the basioccipital 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 exoccipital, with the exception , 

 however, of its ventral, tip, which belongs to the basioccipital. 

 The portion of the occipital bone connecting the basioccipital and 

 exoccipital contains the jugular fossa and the apertures repre- 

 senting 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 basioccipital, in a horizontal plane. Its external surface bears 

 the nuchal crest and the external occipital protuberance. 

 A pair of lateral wing-like expansions rest upon, and partly over- 

 lap, 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 



