THE SKULL THE CRANIUM 



183 



FIG. 119. 



In outline it is triangular ; one angle points directly forward, and the 

 opposite side is posterior and transverse. 



The interparietal presents an outer or upper surface and an inner 

 or lower surface, and these are 

 separated behind by the narrow 

 posterior surface, and in front 

 and at the sides by the lateral 

 borders, which together form a 

 strongly convex anterior surface. 



The outer 'surface (Fig. 120) 

 is triangular. The posterior mar- 

 gin is emarginate, and each lateral 

 margin is serrated and directed 

 from without inward and forward 

 and separated behind from the 

 posterior part of the lateral out- 

 line of the bone by part of the 

 bevelled lateral surface. The 

 outer surface is divided into 

 right and left halves by a me- 

 dian longitudinal ridge, which 

 forms part of the sagittal crest of 

 the sknll and is produced on this 

 bone in front into a spine, more 



or less distinct. On each side of the crest the surface is slightly 

 concave and slopes downward, outward, and forward, and is smooth 

 and covered by the temporal muscle. 



The inner surface is triangular, or may be transversely oblong if 

 the anterior angle be cut off transversely. Its posterior margin is 

 encroached upon by the bevelled posterior surface. Its lateral margins 

 are jagged, and either end in front in an angle or round into an 

 arcuate anterior margin. The surface is strongly concave from side 

 to side, smooth, and marked by small depressions. It is continuous 

 in front with the median part of the posterior surfaces of the ten- 

 torium of the parietals, and continuous behind with the upper part 

 of the internal surface of the vertical plate of the occipital, and 

 contributes to form the fossa for the cerebellum. 



The posterior border or surface is concave from side to side, 



POSITION OP INTEKPARIETAL BONE. 



