94 



VERTEBRATED ANIMALS. 



explained 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 fowl, 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 ad- 

 mitted becomes considerably rarified by the tempera- 

 ture of the Bird, by which provision the whole body is 

 rendered considerably lighter than it otherwise would 

 have been. 



In all this we cannot but observe a wonderful adap- 

 tation in the construction of the animal to its habits, and 

 the element in which it lives. 



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 dependent 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 respiratory 

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

 to the head. Thus the bulk and consequently the centre 

 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 animals. 



In the Birds, on the contrary, the viscera are placed 

 as far back as possible, and a long flexible neck is con- 

 tained 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 1 What reiiiarkabk differ- 

 ence is there, between the construction of birds and that of fishes 1 



