ORGAN AND FUNCTION. 



73 



simple inspection of the sternum in different species ; for 

 this bone measures, in some degree, the length of the pectoral 

 muscles which are lodged in its lateral cavities. Thus, birds 

 with long wings, have a wide and short sternum ; the others 

 have one which is long and slender. 



Fig. 14. —Skeleton of a penguin : sternum very long-, wing very short. 



The comparison of homologous muscles in mammals of 

 different kinds is not less instructive under the aspect in 

 which we are now considering them. But one is often em- 

 barrassed in this comparison by the difficulty of recognizing 

 the homology. The discrepancies are sometimes so striking 



