130 ELEMENTS OF COMPARATIVE ZOOLOGY. 



brates the trunk vertebrae 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 verte- 

 brates. In reptiles, birds, and mammals the 

 ribs of one side fuse at their ventral 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. In some sterna the separate ele- 

 ments can be traced; in others the fusion is 

 complete. The sternum in the Batrachia 

 has no connection with the ribs, and may 

 therefore be different from the breast-bone 

 in Sauropsida and Mammalia. 



The skull consists of two portions: the 

 cranium and the face. The former affords 

 P ro Action to the brain and support to the 

 mente p owh1ch or g ans f sense; the facial portions cluster 

 it is composed. ar0 und the mouth and nose. 

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

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



FIG. 57 Skull and branchial arches of a shark. 7i, hyoid, and Tim, 

 hyomandibular form the suspensor of the lower jaw, m (Meckel'a 

 cartilage); p</, upper jaw (palato-quadrate) ; s, spiracle; I V, gill-arches, 

 between which are shown the gill-clefts. 



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

 lage becomes replaced by bone, either by direct conversion 



