90 VERTEBKATED ANIMALS. 



in another place. The air not only circulates through 

 the lungs by this means, but also penetrates the cavities 

 of the bones through vessels constructed for this purpose. 

 In birds not formed for extensive flight, this provision, 

 however, is much less extensive than in others. Thus 

 in the domestic ^>wl> the humerus, or first bone of the 

 wing, is the only one filled with air. But in the eagle, 

 and other tribes, which spend much of their lives in the 

 air. nearly all the bones are hollow, and are filled with 

 the element in which they live. The air thus admitted 

 becomes considerably rarefied by the temperature of the 

 bird, by which provision the whole body is rendered 

 considerably lighter than it otherwise would have been. 



257. In all this we cannot but observe a wonderful 

 adaptation in the construction of the animal, to its habits, 

 and the element in which it lives. 



258. Structure of the Back-Bones of Birds and Fishes. 

 In the structure of the two classes of vertebrata, the 

 birds and fishes, we may trace remarkable differences 

 which are obviously dependant on the adaptation of 

 each to the elements in which they are respectively des- 

 tined to live. In the fish, the chest and all the viscera 

 are placed as far forward as possible ; the respiratoiy 

 organs, which are the gills, and the heart being also close 

 to the head. Thus the bulk, and consequently the cen- 

 tre of gravity, being placed near the head, the tail is left 

 light and flexible for the purpose of motion. In the fish, 

 the neck, or rather that portion usually occupied by the 

 neck in the other classes, Disappears, its place being filled 

 with those parts usually found in the chests of other ani- 

 mals. 



259. In the birds, on the contrary, the viscera are pla- 

 ced as far back as possible, and a long flexible neck is 

 contained between the trunk and head, so as to place them 

 at a considerable distance asunder. In fish, progressive 

 motion is effected by the tail, the impulse being given by 

 its horizontal flexures from one side to. the other. In 



What difference is there in the different kinds of birds with respect to 

 the quantity of air contained in their bones? What remarkable differ- 

 ence i-s *here between the construction of birds and that of fishes? 



