18 THE VERTEBRATE SKELETON. 



of the primitive cranium, and greatly modifies it. Thus the 

 auditory or periotic capsules press on the parachordals till 

 they come to be more or less imbedded in them. Perhaps 

 owing to the pressure of the nasal capsules the trabeculae 

 fuse in front, and then grow out into an anterior pair of 

 processes, the cornua trabeculae, and a posterior pair, the 

 antorbital processes, which together almost completely sur- 

 round the nasal capsules. The sclerotic capsules of the eyes 

 greatly modify the cranium, although they never become 

 completely united with it. 



The cartilaginous cranium formed of the basal plate, to- 

 gether with the sense capsules, does not long remain merely 

 as a floor. Its sides grow vertically upwards, forming the 

 exoccipital region of the cranium behind, and the ali- 

 sphenoidal and orbitosphenoidal regions further forwards. 

 In many forms, such as Elasmobranchs, all these upgrowths 

 meet round the brain, roofing it in and forming an almost 

 complete cartilaginous cranium. But in most vertebrata, 

 while in the occipital region, the cartilaginous cranium is 

 completed dorsally, in the alisphenoidal and orbitosphenoidal 

 regions the cartilage merely forms the lateral walls of the 

 cranium, the greater part of the brain having dorsal to it a 

 wide space, closed by merely membranous tissue in connection 

 with which the large frontal and parietal bones are subse- 

 quently formed. 



The SKULL includes 



a. the cranium, 



b. the jaws and visceral skeleton. 



The cranium can be further subdivided into 



(1) an axial portion, the cranium proper or brain 

 case; 



(2) the sense capsules. The capsules of the auditory 

 and olfactory sense organs are always present, and as has been 



