120 



OSTEOLOGY. 



whilst inferiorly it is thinned at the expense of its outer table. The mastoid 

 angle is a truncated angle lying between the inferior and posterior borders. 

 It is deeply serrated, and articulates with the mastoid part of the temporal 

 bone. Not infrequently there is a channel in this suture which transmits an 

 emissary vein. 



Connexions. The parietal bone articulates with, its fellow, with the frontal, occipital, 

 mastoid and squama temporalis of the temporal, and with the sphenoid. Occasionally the 



Depressions for arachnoideal granulations (O.T. Pacchionian bodies) 



Frontal angle 



Sphenoidal angle 



Grooves for middle meningeal 

 artery and accompanying 

 venous sinuses 



Mastoid angle 

 Groove for transverse sinus (O.T. lateral sinus) 



FIG. 132. THE RIGHT PARIETAL BONE (Cerebral Surface). 



sphenoidal angle may not reach the great wing of the sphenoid, being separated from it by the 

 articulation of the squama temporalis of the temporal with the frontal (Appendix B). 



Ossification.- Ossification takes place in membrane by two centres which appear, 

 one superior to the other, about the end of the second month (Toldt) ; these gradually 

 unite during the fourth month and correspond in position to the future tuber parietale ; 

 from this, ossification spreads in a radial manner towards the edges of the bone, 

 where, however, the membranous condition still for some time persists, constituting the 

 fontanelles. These correspond in position to the angles of the bone. Ossification is also 

 somewhat delayed in the region of the parietal foramina, constituting what is known as the 

 sagittal fontanelle, a membranous interval which is not infrequently apparent even at birth. 



Os Occipitale. 



The occipital bone, placed at the posterior and lower part of the cranium, 

 consists of four parts, arranged around a large oval hole, called the foramen 

 occipitale magnum or foramen magnum. At birth these parts are all separate. 

 The expanded curved plate posterior. to the foramen is the squama occipitalis 

 or tabular part. The thick rod-like portion anterior to the foramen is the basilar 

 part. On either side the foramen is bounded by the lateral or exoccipital parts. 



