The Bones of the Skull. 87 



orbit and toward the ventral surface of the skull. In the posterior por- 

 tion of the orbit this surface bears a jagged elevation, the crista alae 

 magnae. The internal surface forms a portion of the floor and antero- 

 ventral 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 hori- 

 zontal. The medial lamina ends ventrally in a hooked projection, the 

 hamular process (hamulus pterygoideus). In the young animal this por- 

 tion is formed of an elevation of cartilage tipped by a separate mem- 

 brane element, the pterygoid bone. The pterygoid fossa is formed in 

 part by the medial and lateral laminae and in part by the divided pos- 

 terior end of the palatine bone. The posterior base 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 por- 

 tions, 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 of the bone is seen to be 

 exposed to the cranial cavity only in its posterior portion, where it is 

 occupied by the sulcus chiasmatis and the optic foramina. That part 

 of the floor immediately in front of the optic foramina is formed by the 

 coalesced roots of the orbitosphenoids, the dorsal surface of the pre- 

 sphenoid being thus excluded. 



The orbitosphenoid forms a long, low plate, lying in the ventral por- 

 tion of the orbit, and divided by a shallow notch at the level of the optic 

 foramen into a posterior portion, the orbitosphenoid proper, and an 

 anterior portion, the ethmoidal process (processus ethmoidalis). The 

 orbitosphenoid proper lies behind the optic foramen. It is in contact 

 dorsally with the orbital portion of the frontal, and ventrally with the 

 alisphenoid ; it assists the latter in the formation of the superior orbital 

 fissure. Its posterior tip is in contact with the squamosal. Its internal 

 surface forms a considerable portion of the anteroventral wall of the 

 middle cranial fossa. 



The ethmoidal process extends forward from the optic foramen. Its 

 dorsal margin is articulated with the orbital portion of the frontal, 

 and its ventral margin with the orbital portion of the palatine. Anteriorly 

 it projects toward the lacrimal bone,- thus occupying, in part, a space 



