OSTEOLOGY AND ARTHROLOGY 



17 



but do not fly, the sternum has the form of a plate of bone 

 sHghtly convex, but without a keeL 



The shape of the sternum, correlated to the faculty of 

 flight (or of swimming ; apropos of which we may cite the 

 penguin, of which the rudimentary wings resemble fins, 

 and perform their functions only), or the absence of this 

 faculty, has furnished the division of birds into two groups. 

 In one are included, under the name Carinates {carina, keel), 

 those in which the sternum is provided with a keel ; in the 



7'\ 



Fig. 7. — Sternum of a Bird (the Cock) : Left Side, External Surface. 



I, Keel ; 2, internal slot ; 3, external slot ; 4, internal process; 5, external 

 process ; 6, inferior ribs ; 7. costal process ; 8, surface for articulation 

 with the coracoid bone. 



other division are those in which the sternum is not furnished 

 with one. These latter, on account of their unique mode of 

 progression, are more nearly allied to the mammals. 



The keel is developed in flying mammals (bats). 



Ribs and Costal Cartilages. — There are on each side 

 of the thorax as many ribs as there are dorsal vertebra; . 

 In animals, as in man, the ribs which articulate with the 

 sternum by their cartilages are called true, or sternal ribs ; 

 those whose cartilages do not articulate with the sternum 



