THE SKULL. 27 



Its lateral surfaces are mostly covered by the wings. They 

 appear at the sides of the elevated cranial end of the dorsal 

 surface as triangular areas. 



The caudal end is concave, rough, and has the form of the 

 cranial end of the basilar part of the occipital. 



The cranial end is nearly square and rough for articulation 

 with the body of the presphenoid. 



The Wing (alisphenoid ; ala magna of the human sphenoid) 

 (Fig. 20, b). — This is a thin quadrilateral plate of bone attached 

 by its medial border to nearly the whole of the lateral surface 

 of the body. Its middle portion lies nearly in the same plane 

 as the body, but its ends are curved dorsad so that its internal 

 surface is concave and its external surface is convex. The 

 curvature is most pronounced near the long lateral border, so 

 that this border forms nearly a semicircle. 



The internal surface supports the occipital lobe of the cere- 

 brum. It is marked by a rounded groove (//) which is parallel 

 with the lateral surface of the body. The dorsal margin of the 

 groove projects mediad in the form of a sharp ridge which is 

 broadest caudad, where it often reaches nearly to the posterior 

 clinoid process. The groove passes craniad into three fora- 

 mina. The first (cranial) of these, the orbital fissure (/), is 

 large and lies between the wing, the body, and the pterygoid 

 process. It is incomplete, but is completed by the presphenoid. 

 The second foramen is small and rounded; it is the foramen 

 rotundum {j). The third, foramen ovale {k), is larger and 

 oval and penetrates the wing through about the middle of its 

 longitudinal axis. Another minute foramen penetrates the 

 sphenoid between the wing and the body of the bone, just 

 laterad of the tuberculum sellse. This foramen is continuous 

 craniad with a groove on the dorsal surface of the pterygoid 

 process; the groove and foramen constitute the pterygoid 

 canal. It transmits a nerve. 



The external surface shows the orbital fissure, the foramen 

 rotundum and the foramen ovale, bounded ventrally by a sharp 

 ridge, which is continued onto the pterygoid process. Between 

 this ridge and the body the surface is longitudinally grooved 

 for the tuba auditiva or Eustachian tube. 



