THE SKULL 279 



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 laminae, diverging 

 from each other at an acute angle, and forming 

 parts of the inner walls of the orbits. 



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

 which are fused with the pre-sphenoid. They 

 surround the optic foramen, behiad which 

 they form part of the wall of the cranium 

 and of the orbits, uniting suturally with 

 the ali-sphenoids and with the frontals and 

 squamosals. 



iii. The frontals are a pair of large membrane-bones 

 completing the frontal segment. They unite 

 suturally with each other in the mid-dorsal 

 line, with the parietals behind, and with the 

 orbito-sphenoids and pre-sphenoids below. 



The frontals form the roof and sides of the 

 anterior part of the cranial cavity, and meet 

 each other on its floor in front : they extend 

 forwards into the region of the nose. The 

 outer surface of each frontal forms the upper 

 part of the wall of the orbit, and bears a 

 prominent crescentic supra-orbital process, in 

 front of which it sends forwards a slender 

 process between the maxilla and premaxilla. 



The ethmoidal region lies entirely in front of the 

 cranial cavity, and is in close relation with the 

 olfactory organs. 



i. The mes-ethmoid is a median vertical plate of 

 cartilage, which extends forwards and down- 



