THE SKULL. 25 



The inner surface (Fig. 18) presents depressions for the 

 convolutions of the cerebellum. 



The dorsal border is thick and rough for articulation with 

 the parietals and interparietal. The ventral border abuts on 

 the foramen magnum and is thin and smooth. 



Interparietal Bone. Os interparietale (Fig. 19). This 

 is a small triangular bone lying between the parietals, with its 

 apex directed craniad, and its base in contact with the squamous 

 portion of the occipital. 



Its dorsal surface is arrow-shaped and has its posterior 

 border notched. It is marked by a median crest (part of the 

 sagittal crest) which is continued craniad from the middle of 

 the lambdoidal crest. 



The ventral surface is irregularly triangular, smooth, and 

 concave. The three borders are rough for articulation with 

 the parietals and occipital. 



FIG. 19. INTERPARIETAL FIG. 20. SPHENOID BONE^ DORSAL OR 



BONE, OUTER SURFACE. INTERNAL SURFACE. 



Fig. 19, showing the sagittal crest running craniocaudad across its middle. 



Fig. 20. a, body; b. wings; t, pterygoid process; d, tuberculum sellae; e, dor- 

 sum sellse;/, sella turcica; g; notch which aids in forming the foramen lacerum; 

 h, longitudinal groove of alisphenoid; /, notch which aids in forming the orbital 

 fissure ; /, foramen rotundum ; /, foramen ovale. 



Sphenoid Bone. Os sphenoidale (Fig. 20). The sphe- 

 noid bone of man is represented in the cat by two entirely 

 distinct bones, one cranial, the other caudal. The cranial 

 portion may be designated as the presphenoid (Fig. 21); the 

 caudal part will be described as the sphenoid (proper) (Fig. 

 20). 



The sphenoid bone in the kitten is in three parts : a central 

 portion, the basisphenoid, and two lateral portions, the ali- 

 sphenoids. In many lower vertebrates these three bones are 

 permanently distinct, but in the adult cat they are united to 

 form the sphenoid bone. To these there is added a fourth 

 element, separate in many vertebrates as the pterygoid bone. 



