THE SKULL. 307 



or sphenoidal fissure, through which the third, 

 fourth, and sixth nerves, and the first and 

 second divisions of the fifth nerve pass out 

 from the cranial cavity into the orbit. 



At the posterior border of each ali-sphenoid 

 is a deep notch, the foramen ovale, for the 

 passage of the third division of the fifth nerve. 



iii. The parietals are a pair of flattened mem- 

 brane-bones, meeting each other in the mid- 

 dorsal fine at the sagittal suture, and forming 

 a great part of the roof of the cranial cavity. 



From the posterior and outer angle of each 

 parietal a thin plate runs down on the inner 

 surface of the squamosal, which latter bone 

 separates the parietal from the ali-sphenoid. 



iv. The interparietal is a small median bone in the 

 roof of the skull, between the parietals and 

 the supra-occipital. 



The frontal segment surrounds the anterior end of 

 the brain : it is placed in front of the parietal 

 segment, with which it articulates closely. 



i. The pre-sphenoid is a median laterally compressed 

 bone in front of the basi-sphenoid, and con- 

 nected with it by cartilage. It forms the 

 lower margin of the optic foramen, a large 

 aperture through which the optic nerves enter 

 the orbits from the cranial cavity, and the two 

 orbits communicate in the dried skull with 

 each other. The upper and posterior border 

 of the pre-sphenoid is produced backwards 

 into the small anterior clinoid processes, 

 which bound the sella turcica in front. 



ii. The orbito-sphenoids are a pair of lamellar bones, 

 which are fused with the pre-sphenoid, the 

 boundary between the two being indistinguish- 

 able. They surround the optic foramen, be- 



