THE SKULL 271 



the orbits, uniting suturally with the ali- 

 sphenoids and with the squamosals. 

 iii. The J rentals 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. 



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, 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. 



2. The sense-capsules. Each of the organs of special sense 

 has, as in the dog-fish, a protecting investment of 

 cartilage or strong connective tissue. In the case of 

 the olfactory and auditory organs these capsules are 

 closely connected with the cranium, and are more or 

 less extensively ossified. Membrane-bones are also 



