[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.

Craniad of the optic foramen the surface is smooth and marked near its ventral border by a longitudinal ridge which forms part of the dorsal boundary of a fossa, the external pterygoid fossa ([Fig. 40], p).

The caudal end presents ventrally a quadrangular rough surface for articulation with the body of the sphenoid. The cranial end presents the two sphenoidal sinuses separated by a median partition.

The median partition articulates by its free border with the lamina perpendicularis of the ethmoid. At its ventral end is the abruptly truncate end of the median ridge of the ventral surface, which is continuous with the ventral cartilaginous portion of the lamina perpendicularis. The lateral walls of the sphenoidal sinuses are continued craniad of the dorsal and ventral walls and of the median partition, and articulate ventrally with the nasal portion of the palatine bones, and dorsally with the orbital plate of the frontal. Between them is received the caudal ends of the labyrinths of the ethmoid in the middle, while between their dorsal edges is received the caudal end of the cribriform plate, and between their ventral edges the expanded end of the vomer.

The wings (b) arise each from nearly the whole of the dorsolateral angle of the body. They form prominent nearly horizontal triangular projections over the optic foramina.