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.