THE SKULL 269 



The parietal segment consists of both cartilage- 

 bones and membrane-bones. It is situated in 

 front of the occipital segment, ■with which it is in 

 contact above and below ; while at the sides the 

 two segments are separated by the interposition 

 of the auditory capsules and the squamosal 

 bones, 

 i. The basi- sphenoid is a median bone placed directly 

 in front of the basi- occipital, with which it is 

 connected by a thin plate of cartilage. Seen 

 from below it is triangular, with the apex 

 directed forwards. On its upper or dorsal 

 surface is a depression, the sella turcica : 

 this lodges the pituitary body, and is bounded 

 behind by the large overhanging posterior 

 clinoid process ; while its floor remains un- 

 ossifled as the pituitary foramen, 

 ii. The ali- sphenoids are a pair of wing-like bones 

 ankylosed with the sides of the basi-sphenoid, 

 and forming parts of the floor of the cranial 

 cavity. From the ventral surface of each 

 ali- sphenoid there projects downwards a ver- 

 tical plate of bone, the external pterygoid 

 process, which is placed nearly transversely 

 to the longitudinal axis of the skull, and is 

 connected along its inner edge with the pala- 

 tine bone : it is perforated at its base by three 

 foramina, of which the innermost and largest 

 transmits the internal maxiUary artery and 

 vein, and the two outer ones, branches of the 

 mandibular division of the fifth nerve. 



Between the ali-sphenoid and the anterior 

 end of the basi-sphenoid on each side is a 

 large sUt-like notch, the sphenoidal fissure or 

 foramen lacerum anterius, through which the 

 third, fourth, and sixth nerves, and the first 

 and second divisions of the fifth pass out from 

 the cranial cavity into the orbit. 



