THE SKELETAL SYSTEM 



1 57 



in which the ribs were so extensive. The hypothesis that the sternum 

 is a rib-sternum has at least so much foundation. 



The facts of mammalian ontogenesis, however, do not appear to 

 support this view. As stated above, the mammalian sternum arises 

 independently of the ribs by the union of a pair of longitudinal cartilages 

 which arise near the mid-ventral line. The connexion of these cartilages 

 with the ribs is secondary. 



If an opinion were to be based upon the relation of the sternum in 

 Necturus and of the ontogenesis of the mammalian sternum alone, we 



NTERCUkVICLE 



XIPHOID PROCESS 



Fig. 147. — Types of vertebrate sterna. A, Squalus; B, Salamandra; C, Necturus; 

 D, Rana; E, Felis; F, Crocodilus; G, Homo. The sternum is shown in black. While 

 there is no doubt of the homology of the various amniote sterna, their homology with 

 those of anamnia is in dispute. 



should have to conclude that the sternum arose from paired segmented 

 cartilages formed near the mid-ventral line independently both of the 

 girdle and of the ribs. Under the circumstances, and until more decisive 

 evidence is discovered, suspension of judgment is necessary. 



In the reptiles the sternum is converted into a metameric structure 

 composed of a series of sternebrae and connected with the ribs as in 

 mammals. 



The mammahan sternum differs little from that of reptiles. It is 

 divided into the same three elements as those of reptiles and man, pro- 

 meso-, and meta-sternum. 



