STRUCTURE OP BIRDS. 93 



gether with the positions and strength of the quills, and 

 feathers, all have a direct and beautiful relation to the 

 properties of the element, in which they are intended to 

 move. 



In no part of Creation, therefore, do we see more di- 

 rect and positive marks of design in the Great Author 

 of Nature, than in the adaptation of means to specific 

 ends, than in the construction of Birds. 



What is particularly striking in the skeletons of these 

 animals when compared with others, is the vast size of 

 the sternum, or breast bone as seen at/, Fig. 70. This 

 bone not only covers the whole chest to a considerable 

 depth on each side, but extends back nearly to the inser- 

 tion of the legs. Its lower part forms a deep perpendic- 

 ular crest, shaped, it is well known, like the keel of a 

 ship, the whole being remarkably thin and light, when 

 compared with the extent of its surface. The design of 

 this great developement is obviously to furnish an exten- 

 sive surface for the attachment of the pectoral muscles 

 to be employed in the motions of the wings. In many 

 Birds these muscles outweigh all the others of the body 

 put together, and it is owing to their great power that 

 the eagle and other birds have such amazing strength of 

 wing, as to carry animals heavier than themselves, and 

 that the Swan sometimes breaks a man's leg by a single 

 flap of his pinion. 



But in addition to the general appearance of lightness 

 which the bones of Birds present, the cylindrical ones 

 are hollow tubes filled with air. In this they differ from 

 all other living bones, those of other animals being filled 

 with marrow. 



The lungs of Birds are placed on the ribs, between 

 which their substance also projects. They are of 

 a compact texture, and so bound down to their places 

 among the ribs, as to have no expansive and contrac- 

 tile motion, like those of other animals ; hence respi- 

 ration in this order is carried on by alternately enlarg- 

 ing and contracting the cavity of the chest, as will be 



For what purpose is the breast bone peculiarly large in the birds? 

 With what substance are the bones of birds filled 1 In what manner is 

 ^aspiration carried on in birds 1 



