xi SKULL 491 



ing of two laminae which converge and unite with one 

 another anteriorly. The parietals are a pair of thin, slightly 

 arched bones, forming a considerable part of the roof of the 

 brain-case, and united with one another by suture along the 

 middle line; the outer edge of each gives off a thin, 

 ventral process which is covered by the squamosal (sq), 

 a bone which will be referred to presently and which 

 separates the parietal from the alisphenoid. Interposed 

 between the parietals and the supraoccipital is a small 

 median interparietal* (int. pa). 



The frontal segment also consists of five bones a 

 presphenoid, (p. sph} two orbitosphenoids (o. spJi) and two 

 frontals* (fr). The small presphenoid is laterally com- 

 pressed and is connected with the basisphenoid by cartilage, 

 so that in the dry skull there is a considerable interval 

 between the two bones ; it forms the inferior and anterior 

 boundary of the optic foramen (opt. fo, //), which puts the 

 two orbits in communication with one another and both in 

 communication with the cranial cavity. The orbitosphenoids 

 are two wing-like laminae directed outwards and slightly 

 backwards, and completely fused with the presphenoid ; 

 they surround the rest of the optic foramen. The frontals 

 form the roof and side-walls of the anterior part of the 

 brain-case, and are united by suture with one another in the 

 middle line and with the parietals behind ; below they 

 meet with one another anteriorly on the floor of the brain- 

 case and unite with the presphenoid by suture ; the outer 

 part of each forms a prominent crescentic ridge, the 

 supra-orbita I process. 



The brain-case is closed in anteriorly by a bone riddled 

 with numerous small holes for the passage of the olfac- 

 tory nerves : this is the cribriform plate of the ethmoid 

 (ftK). 



