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 4, 
conveniently describe this bone as made up of “= 2 
a body (a) (the basisphenoid), and two wings Fic. 21, —PRE- 
(0), the orbitosphenoids (the ala parve of the {fuinolp seca 
human sphenoid). a, body; 5, wings; 
The body (a) lies in the base of the skull in % optic foramina. 
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 par- 
tition into two cavities (sphenoidal sinuses, Fig. 43, 7). The 
sphenoidal sinuses are continued craniad into'the cavities of 
the ethmoid. The body has six surfaces: 
The dorsal or internal surface (Fig. 42, 2) 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, ¢) whith 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 
