STRUCTURE OF BIRDS. 89 



gether with the positions and strength of the quills, and 

 feathers, all have a direct and beautiful relation to the 

 properties of the elements in which they are intended to 

 move. 



253. In no part of creation, therefore, do we see more 

 direct and positive marks of design in the Great Author of 

 Nature, in the adaptation of means to specific ends, than in 

 the construction of birds. 



254. 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 atjf, Fig. 70. This 

 bone not only covers the whole chest to a considerable 

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

 sertion of the legs. Its lower part forms a deep perpen- 

 dicular 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 development 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. 



255. But in addition to the general appearance of light- 

 ness, 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. 



256. 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 contractile 

 motion, like those of other animals ; hence respiration 

 in this order is carried on by alternately enlarging and 

 contracting the cavity of the chest, as will be explained 



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

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

 respiration carried on in birds ? 



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