xi SKULL 491 



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 (p. 495) 

 and which, on the external face of the skull, 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. sph), and two 

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

 pressed and is connected with the basisphenoid by carti- 

 lage, 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, II), 

 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 com- 

 pletely 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-orbital 

 process. 



The brain-case is closed in anteriorly by a bone 

 riddled with numerous small holes for the passage of 

 the olfactory nerves (I) : this is the cribriform plate of 

 the ethmoid (eth). 



H H 2 



