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).
[Illustration: FIG. 21.--PRESPHENOID BONE, VENTRAL VIEW.
_a_, body; _b_, wings; _c_, optic foramina.]
The body (_a_) lies in the base of the skull in the median line, craniad of the basisphenoid. It has the form of a rectangular prism about twice as long as broad. It is hollow, and the cavity is divided by a median longitudinal partition into two cavities (=sphenoidal sinuses=, Fig. 43, _l_). The sphenoidal sinuses are continued craniad into the cavities of the ethmoid. The body has six surfaces:
The dorsal or internal surface (Fig. 42, _n_) looks into the cranial cavity and is continuous with the dorsal surface of the wings. The caudal end of the body is depressed, and when united to the basisphenoid aids in forming the cranial wall of the sella turcica. At each caudolateral angle is a short spine, the =anterior clinoid process=. At about one-third the length of the bone from the caudal end is a transverse groove (=chiasmatic groove=, Fig. 42, _m_) for the optic chiasma. Its ends lead into two round foramina (the =optic foramina=, Fig. 42, _l_; Fig. 21, _c_) which pass craniolaterad between the body and the wings of the presphenoid and transmit the optic nerve and the ophthalmic artery.
The ventral surface (Fig. 21) is hour-glass-shaped and marked by a smooth median ridge, continuous with the ridge on the basisphenoid and overlaid at its cranial end by the vomer. The caudal end presents a rough triangular area on each side, for articulation with the pterygoid process of the sphenoid bone, while the cranial end has similar areas overlaid by the nasal portion of the palatine bones.
The lateral surface looks towards the orbitotemporal fossa. It is notched near the caudal end by the ventral border of the optic foramen (_c_). Caudad of this foramen the surface presents an oblique groove which forms in the natural condition the medial boundary of the orbital fissure.