THE BONES OF THE SKULL 183 



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 exca- 

 vated, like the adjacent portions of the alisphenoid. It bears a 

 shallow groove, representing a pterygoid canal (canalis ptery- 

 goideus), and is perforated by the three apertures described above 

 as the anterior, middle, and posterior sphenoidal foramina. 



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 anteri- 

 orly with the perpendicular plate of the ethmoid and with the carti- 

 laginous 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 orbito- 

 sphenoids, the dorsal surface of the presphenoid being thus excluded. 



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

 portion 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 



