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 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 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 
