THE CRANIUM. 



163 



(2) For the parietal, at the position of the parietal eminence ; 



(3) For the squamosal — at the base of the zygoma (Fig. 130) ; 



(4) For the membranous part of supra-occipital (part above 

 superior curved line). 



bregma 



ant. font. 



Sag. font, 

 parietal. 



lambda 



aster/on. 



oceip. (memb.) 

 occip. (oartilag.) 



■frontal 



pterion 



great wing of Sphen. 



Squamosal 

 tympanic ring 



petro-mast 



Fig. 130. — The Centres of Ossification for the Dermal Bones of the Skull. The Bones 

 which are formed in Cartilage are stippled. 



The two occipital centres fuse early into one at the position of 

 the external occipital protuberance. The two frontal ossifications 

 fuse about the end of the first year ; the metopic suture which 

 separates them disappearing then. This suture occasionally per- 

 sists. The parietal bones fuse together, at the sagittal suture, 

 late in life, commonly between the 35 th and 45 th year. The 

 squamosal partly covers the petro-mastoid cartilaginous element 

 and fuses with it in the first year, the temporal bone being thus 

 formed. 



The Manner in which these Bones are Developed. — In 

 Fig. 131 a vertical section of the skull of a foetus 4 \ months 

 old is represented. The coverings of the brain are seen to be then 

 (1) scalp, (2) a stout white fibrous capsule, (3) a fine membrane 

 lining it — the inner layer of the dura mater — (4) the arachnoid 

 covering the brain (not shown in figure). Spicules of bone which 

 form the parietal are seen developing within the fibrous capsule 

 and radiating out from the centre of ossification. Lower down 



