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. 
a é UAT 
Fic. 19. —INTERPARIETAL FIG. 20.-- SPHENOID Bong, DorsaL oR 
Bone, OUTER SURFACE. INTERNAL SURFACE. 
Fig. 19, showing the sagittal crest eae craniocaudad across its middle. 
Fig. 20.—a, body; 4, wings; ¢, pterygoid process; @, tuberculum sellz; e¢, dor- 
sum sell; /, sella turcica; g, notch which aids in forming the foramen lacerum; 
A, longitudinal groove of alisphenoid; 7, notch which aids in forming the orbital 
fissure; 7, foramen rotundum; 4, 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). 
an 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. 
