366 



BONES OP THE TRUNK OF BIRDS. 



generally larger than the others, and elevated into a projecting 

 crest. 



328. The ribs of Birds also present some peculiarities of 

 structure, which tend to give solidity to the thorax. The car- 

 tilage, which in the Mammalia fixes them to the sternum, is 

 here replaced by a bone ; and each of them is provided in the 

 middle with a flattened process, which is directed obliquely 

 backwards above the next rib ; so that all these bones have 

 points of support on one another. But the most remarkable 

 part of the osseous structure is the sternum ; which, as it gives 

 insertion to the muscles of flight, presents in Birds a very high 

 development, and constitutes a sort of convex buckler, usually 

 square, which covers the thorax and a large part of the abdomen. 



PIG. 170. SKELETON OF OSTRICH. 



In the Cassowary and Ostrich, which cannot rise in the air, and 

 which have only rudimentary wings, the sternum does not present 



