16 ANATOMY OF THE RAT 



and lowered vertically without a corresponding movement 

 of the neck. The hypoglossal canal is anterior to the con- 

 dyle. The exoccipitals are fused dorsally with the 

 supraoccipital, ventrally with the basioccipital, and are in 

 contact anteriorly with the mastoid process of the petrosal 

 bone. 



Viewed from above the dorsal edge of the supraoccipital 

 region resembles the segment of a circle with its center 

 anterior to the bone. The ends of the segment terminate 

 in a dorsoventral ridge where the supraoccipital connects 

 with the squamosal bone. This ridge is continuous with the 

 post-tympanic hook. A median vertical ridge (the poste- 

 rior occipiital proiuberance) bisects the nuchal surface. 



Sphenoid Bone (Os Sphenoidale) 



The sphenoid is a complicated structure consisting of two 

 distinct parts, the anterior sphenoid and posterior sphenoid. 

 Each of these, in turn, consists of three elements, separ- 

 ated in some of the lower vertebrates, but fused in the 

 higher. 



The three fused elements forming the posterior sphenoid 

 are the single median hasisphenoid (post-sphenoid) and 

 the two lateral alisphen^ids. The hasisphenoid is bounded 

 caudally by the basioccipital, anteriorly by the anterior 

 sphenoid, and laterally is fused with the alisphenoid bones. 

 Anterior to its union with the alisphenoids, the hasisphenoid 

 increases slightly ii> width up to its attachment with the 

 presphenoid. The pterygoid processes (one pair on each 

 side), described in the discussion of the ventral view of 

 the skull, are attached to the ventral surface of each 

 alisphenoid bone. Dorsal to the lateral pterygoid process 

 is a depression into which the foramen ovale and two 

 other foramina open. The alisphenoid expands, dorsal 



