THE SKULL. 299 



hind 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. In front of the optic fora- 

 men the orbito-sphenoids are 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. 



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 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. 



d, 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- 

 wards in front of the pre-sphenoid as the 

 septum narium separating the right and left 

 olfactory cavities from each other. 



Its upper and posterior border is ossified 

 and expanded laterally into the cribriform 

 plate, which is placed obliquely across the 

 anterior end of the cranial cavity, in the plane 

 of the base of the skull, and is perforated by 

 a number of holes through which the olfactory 

 nerves pass from the brain to the nose. 



