204 



Anatomy of Skeleton 



the parietal to a less degree. Observe that this suture makes almost a right angle 

 with the sagittal. 



(3) Posterior or lambdoid ; makes an obtuse angle with the upper border, 

 is much serrated, and articulates in its whole length with occipital. 



(4) Lower border ; hinder part, starting at the angle, overlaps and articulates 

 with the petro-mastoid. In front of this the curved margin is bevelled-off on 

 the surface and shows fluted markings for the squama. A little distance from 

 the antero-inferior angle this surface changes abruptly into a smoother surface, 

 slightly concave, overlapped by the sphenoid. 



Observe that the lower and front part of the bone is drawn out in a narrow angle : 

 this proclaims the side to which the bone belongs when only seen at a distance. 



The parietal covers the parietal lobe, but in addition has lying deep to it portions 

 of all the other lobes that are seen on the surface of the brain. It must be understood 



lint. 



FIG. 168. Approximate positions of main sulci and sinuses on the surface. On the parietal, 

 F. is frontal lobe, P. parietal lobe, O. occipital lobe, and T. the hinder part of temporal lobe. 

 A', is the position of the " sylvian point." 



that the bone is as much concerned in forming the back as the side of the skull, so that 

 the fissure of Rolando is only about ij inches behind the line of the coronal suture. 

 The position of the fissure and lobes is approximately shown in Fig. 168. 



Development. 



From two centres appearing in membrane in the eighth week and rapidly fusing : 

 sometimes apparently only a single centre, though this may be only an instance of 

 very early union. The centres are generally upper and lower, and occur in the region 

 of the parietal eminence ; from thence ossification radiates out toward all the margins. 

 The four corners of the bone are not ossified at birth, and the spaces left in this way 

 constitute, with the neighbouring bones, the fontanelles (see p. 266 for account of 

 fontanelles). 



OCCIPITAL. 



A tabular bone situated in the lower and back part of the skull and resting on 

 the atlas, so that its cerebral surface is markedly concave and its superficial surface 

 convex. It is divisible, as we have seen, into parts grouped round the foramen magnum. 



