Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/30

16 and lowered vertically without a corresponding movement of the neck. The hypoglossal canal is anterior to the condyle. 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 posterior occipital protuberance) bisects the nuchal surface.

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, separated in some of the lower vertebrates, but fused in the higher.

The three fused elements forming the posterior sphenoid are the single median basisphenoid (post-sphenoid) and the two lateral alisphenoids. The basisphenoid 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 basisphenoid increases slightly in 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