
THE BONES OF THE SKULL. 143 
and to a certain extent with the orbitosphenoid, the superior 
orbital fissure. The foramen lacerum is formed by the pos- 
terior margin of its root in association with the petrotympanic. 
The external surface of the alisphenoid is convex, both toward 
the orbit and toward the ventral surface of the skull. In the 
posterior portion of the orbit this surface bears a jagged elevation, 
the crista alae magnae. The internal surface forms a portion 
of the floor and anteroventral wall of the middle cranial fossa. 
At its base a broad groove, the sulcus sphenoidalis, indicates 
the position of the root of the fifth nerve and the related semilunar 
(Gasserian) ganglion. 
The pterygoid process comprises the two plates described 
above as the medial and lateral laminae. The former is vertical, 
and its medial surface is directed toward the nasopharynx. The 
latter is almost horizontal. The medial lamina ends ventrally in a 
hooked projection, the hamular process (hamulus pterygoideus). 
In the young animal this portion is formed of an elevation of 
cartilage tipped by a separate membrane element, the pterygoid 
bone. The pterygoid fossa is formed in part by the medial and 
lateral laminae and in part by the divided posterior end of the 
palatine bone. The posterior basal portion of the lateral lamina is 
extensively excavated, like the adjacent portions of the alisphenoid. 
It bears a shallow groove, representing a pterygoid canal (canalis 
pterygoideus), and is perforated by the three apertures described 
above as the anterior, middle, and posterior sphenoidal 
foramina. 
3. THE ANTERIOR SPHENOID. 
The anterior sphenoid (0s sphenoidale anterius) consists of 
two portions, namely, a median portion, the body, or presphenoid, 
and a pair of lateral expansions, the lesser wings (alae parvae), or 
orbitosphenoids. 
The presphenoid is a constricted bony splint which continues 
the basis cranii forward from the basisphenoid. It is joined 
anteriorly with the perpendicular plate of the ethmoid and with the 
cartilaginous nasal septum. In the divided skull, or better in one 
from which the roof has been removed, the actual dorsal surface 
