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 
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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 
