THE SKULL 277 



the foramen magnum, and a small part of 

 each occipital condyle. 



ii. The ex-occipitals bound the foramen magnum 

 laterally, and form the greater part of the 

 condyles. Each is produced at its outer side 

 into a long, downwardly directed par-occipital 

 process, which is closely applied to the pos- 

 terior surface of the tympanic bulla. 



iii. The supra-occipital is a large median bone, 

 which completes the occipital segment above. 

 The whole bone is markedly cancellous, and its 

 outer surface is raised into a prominent shield- 

 shaped elevation, giving attachment to the 

 muscles raising the head. 



. 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- 

 ossified 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 

 ah-sphenoid there projects downwards a ver- 

 tical plate of bone, the external pterygoid 



