270 SKELETON OF THE BABBIT 



At the posterior border of each ali-sphenoid 

 is a deep notch for the passage of the third 

 division of the fifth nerve. 



iii. The parietals are a pair of flattened mem- 

 brane-bones, articulating with each other in 

 the mid-dorsal line at the sagittal suture, and 

 forming a great part of the roof of the cranial 

 cavity. ' • 



Prom 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. 



c. 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 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. In front of the optic 

 foramen the pre -sphenoid is continued for- 

 wards as a pair of vertical lamina, diverging 

 from each other at an acute angle, and forming 

 parts of jibe inner walls of the orbits. 



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

 which are fused with the pre-spheiioid. They 

 surround the optic foramen, behind which they 

 form part of the wall of the cranium and of 



