VERTEBRAL COLUMN, RIBS, AND STERNUM 309 



the tail bore ribs. In higher forms they do not extend behind 

 the sacrum. 



The sternum first appears in Amphibia. It arises from 

 paired rudiments of cartilage which may become replaced by 

 cartilage-bone. In the Amphibia which are alive to-day, the 

 sternum has no connexion with the ribs. The sternum in the 

 Amniotes is however connected with the ribs, and this was 

 probably the condition in the Stegocephalia also. The sternum 

 is also usually in contact with the coracoids and clavicles. 

 In the mammals, the sternum is often broken into a number of 

 pieces or sternebrae. In the birds (with the exception of the 

 Struthiones : ostrich and its allies) the sternum bears a median 

 projection forming the " keel " or carina to which the flight 

 muscles are attached. Analogous but not homologous keels 

 are developed on the sterna of Pterosaurs and bats. 



Overlying the sternum on the ventral side there is in many 

 forms a dermal bone, the interclavicle. It is present in the 

 Stegocephalia but has been lost in the living amphibia. Among 

 the reptiles, it is present in all except the snakes. In birds 

 it is apparently absent, unless it is represented by the keel of 

 the sternum. Only the Monotremes preserve the interclavicle 

 among the mammals. 



