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. [Illustration: FIG. 19.--INTERPARIETAL BONE, OUTER SURFACE. FIG. 20.--SPHENOID BONE, DORSAL OR INTERNAL SURFACE. Fig. 19, showing the sagittal crest running craniocaudad across its middle. Fig. 20.--_a_, body; _b_, wings; _c_, pterygoid process; _d_, tuberculum sellæ; _e_, dorsum sellæ; _f_, sella turcica; _g_, notch which aids in forming the foramen lacerum; _h_, longitudinal groove of alisphenoid; _i_, notch which aids in forming the orbital fissure; _j_, foramen rotundum; _k_, foramen ovale.] =Sphenoid Bone.= =Os sphenoidale= (Fig. 20).--The sphenoid 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 =alisphenoids=. 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. The sphenoid may thus be described as composed of a central portion, the body (_a_) (basisphenoid), and of two thin expanded wings (_b_) (alisphenoids, alæ magnæ of the human sphenoid); each of which has arising from it a thin curved process, the =pterygoid process= (_c_), directed craniad and largely made up of the pterygoid bone. The body of the sphenoid (_a_) lies in the middle line of the base of the skull. It is wedge-shaped, with the converging sides of the wedge directed laterad and its apex pointed craniad. It has six surfaces, of which the dorsal and a part of the laterals look into the cranial cavity. The cranial end articulates with the body of the presphenoid, and the caudal with the body of the occipital.