142 Anatomy of the Rabbit. 



by a vacuity. The internal surface 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 complex 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: (i) 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 

 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 by the hypophyseal fossa 

 and related structures, namely, the dorsum sellae and the pos- 

 terior 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 hypophyseal fossa. 



The alisphenoid extends at first laterad, but soon changes its 

 direction so that its axis beomes 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 orbitpsphenoid, 

 dorsally by the squamosal, and posteriorly by the petrotympanic. 

 The anterior margin of its root encloses with the basisphenoid, 



