THE SKULL THE CEANIUM 261 



than the round foramen, and is directed more obliquely toward the side 

 of the skull ; a small foramen is often found piercing its inner wall. 



Each elevated and concave lateral part of the upper surface is 

 sickle-shaped, and is formed by the alisphenoid (Fig. 188). It presents 

 inner, posterior, anterior, outer, and superior borders. The sharp 

 inner border is elevated and overhangs the flat surface lying medial 

 to it. It is directed from behind forward and somewhat inward, and 

 is emarginate at its middle, over the entrance to the oval foramen. 

 It forms in front the outer part of the upper margin of the sphe- 

 noidal fissure, and terminates by making an obtuse angle with the 

 anterior border. The anterior border is arcuate and directed upward, 

 outward, and backward, so that it appears as an upper border to the 

 prominent front part of the surface. Its inner or median half is 

 straighter and thinner than the rest, and is serrated to articulate with 

 the posterior border of the orbitosphenoid. The outer half, or less, is 

 broader and jagged ; it curves backward and joins the posterior border 

 of the vertical plate of the frontal. The superior border varies in 

 shape and direction : sometimes it is straight, sometimes pointed ; it 

 may run almost horizontally or may assume a vertical position. It is 

 always thin, and overlaps the produced lower anterior angle of the 

 parietal. The outer border is deeply emarginate, more especially in 

 front, the concavity facing outward and backward. It is sharp and 

 slightly serrated, but bevelled at the expense of the lower surface to 

 articulate with the front of the squamous of the temporal. The pos- 

 terior border is bevelled in both directions ; the upper rough surface is 

 overlapped by the tentorial plate of the parietal bone (Fig. 170). The 

 lower part is a narrow, triangular, transverse fossa which is applied to 

 the rounded anterior border of the petrous portion of the temporal, 

 lateral to the hiatus Fallopii. This posterior end of the alisphenoid is 

 the alisphenoidal root of the tentorium. 



The superior surface of the alisphenoid is deeply concave from 

 before backward, rising higher in front than behind. It is also some- 

 what concave transverselv. It enters into the formation of the middle 



V 



cranial fossa. Near the middle of its anterior border is seen a foramen 

 from which a groove crosses the surface transversely. 



The inferior surface of the posterior sphenoid comprises three 

 parts, a central portion of the body, an inferior surface of each wing, 

 and an inferior surface of each pterygoid process (Fig. 190). 



