The Boxes 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 (os 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 



