86 Anatomy of the Rabbit. 



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 basi- 

 occipital, in a horizontal plane. Its external surface bears the superior 

 nuchal lines and the external occipital protuberance. A pair of 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 sur- 

 face bears a longitudinal groove, lodging in the natural condition the 

 median 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. 



2. THE POSTERIOR SPHENOID. 



The sphenoid bone, as identified from the human condition, is a com- 

 plex of elements belonging to two segments, namely, the posterior 

 sphenoid (os sphenoidale posterius) and the anterior sphenoid (os 

 sphenoidale anterius). In the rabbit, as in mammals generally, these 

 segments are separate throughout life. 



The posterior sphenoid comprises: (1) a median portion, the body, 

 or basisphenoid ; (2) paired dorsolateral expansions, the greater wings 

 (alae magnae), or alisphenoids ; and (3) paired ventral projections, the 

 pterygoid processes. 



The basisphenoid continues the basis cranii forward from the basi- 

 occipital to the body of the anterior sphenoid. It is united with the 

 latter by the intersphenoidal synchondrosis. Its surfaces correspond for 

 the most part to those of the basioccipital. The ventral surface forms 

 the chief part of the bony roof of the nasopharynx. It is perforated in 

 its middle by a round aperture, the foramen cavernosum, which leads 

 into the interior of the bone. The dorsal surface is occupied, as described 

 above, by the hypophysial fossa and related structures, namely, the 

 dorsum sellae and the posterior clinoid processes. On the lateral surface 

 of the base of the posterior clinoid process a faint groove, the sulcus 

 caroticus, marks the course of the internal carotid artery. The interior 

 of the bone contains a cavity of considerable size, the sphenoidal sinus 

 (sinus sphenoidalis), which communicates both with the foramen 

 cavernosum and the hypophysial fossa. 



The alisphenoid extends at first laterad, but soon changes its direction 

 so that its axis becomes dorso ventral. At the same time the bone is 

 rotated in such a way that its surfaces tend to fall in a transverse plane. 

 It is bounded anteriorly by the orbitosphenoid, dorsally by the squamosal, 

 and posteriorly by the petrotympanic. The anterior margin of its root 

 encloses with the basisphenoid, and to a certain extent with the orbito- 

 sphenoid, the superior orbital fissure. The foramen lacerum is formed 

 ' by the posterior margin of its root in association with the petrotympanic. 



The external surface of the alisphenoid is convex, both toward the 



