THE SKULL. 



59 



While the first and the last of these are gradually surrounded 

 by cartilage, and, especially in higher types, more and more drawn 

 in to the skull proper, the lateral walls of the basal plate become 

 raised up, and begin to grow round the brain on both sides, 

 eventually extending even to the dorsal region. Thus a con- 

 tinuous cartilaginous capsule is formed, such as persists through- 

 out life in Elasmobranchs for example. But in by far the greater 

 number of Vertebrates, the cartilage does not play so great a part, 

 and is, as a rule, confined to the base and lower parts of the sides 

 of the skull and to the sense-capsules, except in the occipital 

 region, where it always extends over the brain. The rest of the 

 skull, more particularly the roof, becomes directly converted from 

 membrane into bone. Thus it may be stated generally, that the 

 higher the systematic position of the animal, the less 

 extensive are the cartilaginous constituents and t lie- 

 more important the bony. 



I. The Visceral Skeleton. 



The skeletal parts of the visceral arches, always formed in 

 hyaline cartilage, encircle the anterior section of the alimentary 

 canal, lying embedded in the inner part of the walls of the throat 



FIG. 45. DIAGRAMMATIC TRANSVERSE SECTION OF THE THIRD STAGE ix THE 

 DEVELOPMENT OF THE PRIMORDIAL SKULL. 



C, notochord ; Tr, trabeculre, \vhich enclose the brain (C) ventrally and laterally ; 

 0, auditory capsule ; EH, the cavity of the pharynx, enclosed by the visceral 

 skeleton ; 1 to 4, the individual elements composing each visceral arch, which 

 latter is united with its fellow by a basal piece (Cp). 



(Figs. 45 and 46. B,B]. Always present in a greater number (up to 

 as many as nine) in gill-breathing animals than in higher types, they 

 gradually become reduced, so that in the Amniota the remains of 

 the three or four anterior at most are seen : they further undergo 

 a change of function, for all but the first of these take on definite 

 relations to the auditory organ and larynx. 



The most anterior arch; serving as a support for the walls of 

 the mouth and receiving its nerve supply from the trigeminal, 



