166 



REPTILES AND BIRDS. 



in all parts of the body. What distinguishes the bird is not the 

 wing ; for certain of the Mammalia, and even some fishes, possess 

 them. The diaphragm which arrests the air in Mammalia is scarcely 

 perceptible in Birds, so the external air penetrates into every part of 

 their body by the respiratory tubes, which ramify the whole cellular 



Fig. 42. Skeleton of the Swan. 



tissue, even the interior of the bones, the feathers, and between the 

 muscles. Their forms, thus inflated, lose a proportionate amount of 

 weight, enabling them to float in the gaseous element (see Fig. 42). 



Wings alone would fail to support the Bird in space. The 

 position renders a double system of breathing necessary. Vital 

 heat in animals is always in proportion to their respiration, for the 

 oxygen of the air, which penetrates every cell and cavity of their 

 bones, feathers, and body, warming and giving increased activity to 



