The semicircular margin of the bone articulates with the squamous portion of the temporal. At the junction of its caudal and middle third there is sometimes a toothlike projection which underlies the root of the zygoma.
The whole of the cranial margin, except the lateral end, articulates with the wing of the presphenoid. At this end the angle formed by the junction of lateral and cranial borders is produced into a flat process, which passes dorsocaudad between the squamous portion of the temporal and the frontal, and articulates by the roughened internal surface of its free end with a similar process from the parietal.
The caudal margin laterad of the groove is bevelled and roughened at the expense of the dorsal surface and is overlaid by the ventral end of the tentorium. Mediad of the groove it projects caudad as a slender point, the lingula of the sphenoid. This is received into a narrow cleft between the apex of the petrous bone and the bulla tympani.
The pterygoid process (c) is a nearly square, thin plate of bone. The medial surface is smooth and concave, the lateral face is convex and marked by two parallel ridges. The medial one of these is continued craniad from the bony septum which separates the orbital fissure from the foramen rotundum, and the lateral one from the septum which separates the foramen rotundum from the foramen ovale. A sharp triangular spine projects laterad from near the caudal end of the lateral ridge.
The two ridges and that part of the lateral surface of the bone included between them form a part of the sphenoid bone known as the pterygoid process of the sphenoid bone, in those cases where the pterygoid is a separate bone.
The remainder of the process is equivalent to the pterygoid bone of other vertebrates.
Between the caudal margin of this bone and the lateral of the two ridges, i.e., between the pterygoid bone and the pterygoid process of the sphenoid, is a long deep fossa, the internal pterygoid fossa ([Fig. 40], s). The laterocaudal margin of the pterygoid process projects caudad, as a curved triangular spine, the hamulus or hamular process ([Fig. 40], t; [Fig. 43], i).
The Presphenoid Bone. Os presphenoidale
([Fig. 21]).—In a young cat this bone is in three pieces, a basal portion (presphenoid) and two wings (orbitosphenoid bones). These bones remain distinct throughout life in many lower vertebrates, but in the adult cat they fuse to form a single bone. We may nevertheless conveniently describe this bone as made up of a body (a) (the basisphenoid), and two wings (b), the orbitosphenoids (the alæ parvæ of the human sphenoid).
