VERTEBRATES. 



293 





region, the vertebra of which is connected with the bones 

 (girdle) supporting the hind limbs. In the higher verte- 

 brates the trunk vertebra? can be divided into thoracic 

 and lumbar regions, the former with, the latter without, 

 ribs. 



As we have just seen, there may be two kinds of ribs 

 those of fishes and those of the higher 

 vertebrates. In reptiles, birds, and mam- 

 mals the ribs of one side fuse at their ven- 

 tral ends with their fellows of the opposite 

 side. The fused regions separate from the 

 ribs and unite together, giving rise to the 

 breast-bone or sternum (fig. 103). In 

 some sterna the separate elements can be 

 traced; in others the fusion is complete. 

 The sternum in the Amphibia has no con- 

 nection with the ribs, and may therefore 

 be different from the breast-bone in the 

 Sauropsida and Mammalia. 



The skull consists of two portions: the 



FIG. 103. - - Ster- 



cramum and the face, or better, the ins- numofdog.show- 



777, nn f cc i n ^6 separate 



ceral skeleton. Ihe iormer anords protec- elements of which 



it is composed. 



tion to the brain and support to the organs 



of sense; the visceral portions cluster around the mouth, 



nose, and throat. 



In the sharks the cranium is a continuous box of carti- 

 lage, only perforated for the passage of nerves and blood- 

 vessels. In the other vertebrates some or all of this carti- 

 lage becomes replaced by bone, either by direct conversion 

 (ossification) or by substitution. The bony cranium (un- 

 like the cartilaginous cranium) is not a continuous wall, 

 but is composed of separate bones firmly united together, 

 the number varying between wide limits, being most 



