182 ANATOMY OF THE RABBIT 



The Posterior Sphenoid 



The sphenoid bone, as identified from the human condition, is 

 a complex of elements belonging to two segments, namely, the 

 posterior sphenoid (os sphenoidale posterius) and the anterior 

 sphenoid (os sphenoidale an terius). In the rabbit, as in mammals 

 generalh', 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 

 basioccipital 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 naso- 

 pharynx. It is perforated in its middle by a round aperture, the 

 foramen cavernosum, w^hich leads into the interior of the bone. 

 The dorsal surface is occupied by the hypophyseal 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 wath both the foramen caver- 

 nosum and the hypophyseal fossa. 



The alisphenoid extends laterad at first, but soon changes its 

 direction so that its axis becomes dorsoventral. 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 along with the basisphenoid, and 

 to a certain extent with the orbitosphenoid, encloses 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 orbital and toward the ventral .surface of the skull. In the 

 posterior portion of the orbit this surface bears a jagged elevation, 

 the crista alae magnae. The internal surface forms a portion of 



