THE SKULL. 29 



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 FIG. 21. -PRE- 

 (/>), the orbitosphenoids (the alae parvae of the V^RAL VEW 

 human sphenoid). a, body; 4, wings; 



The body (a) lies in the base of the skull in f ' P tic foramina, 

 the median line, craniad of the basisphenoid. It has the form 

 of a rectangular prisrn about twice as long as broad. It is 

 hollow, and the cavity is divided by a median longitudinal par- 

 tition into two cavities (sphenoidal sinuses, Fig. 43, /). 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 (chias- 

 matic groove, Fig. 42, m) for the optic chiasma. Its ends 

 lead into two round foramina (the optic foramina, Fig. 42, /; 

 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 



