ANATOMY OF BIEDS. 181 



part of the thorax only, where they are confined on each side to a 

 cavity, bounded above by the ribs, and below by an imperfect 

 diaphragm, they are perforated by tubes, which communicate 

 with membranous cells, distributed over the thoracic and abdo- 

 minal cavities, between the muscles, and beneath the skin, often 

 in all parts of the body. What distinguishes the bird, in fact, is 

 not the wing ; for certain of the Mammalia, as the Bat, and even 

 some fishes, as the Gusard and Exocretus, can traverse the air by 

 expanding their wings. In birds the diaphragm which arrests 



Fig. 45. Skeleton of the Swan. 



the air in the Mammalia is scarcely perceptible, so the external 

 air penetrates into every part of the body by the respiratory tubes, 

 which ramify the whole cellular tissue, the interior of the bones, and 

 even the feathers, and between the muscles. Their bodies, dilated 

 by the air inhaled, lose a proportionate amount of weight ; balloon- 

 like, they float in the air, and, from their peculiar forms, they 

 can swim, so to speak, in any direction in the gaseous element. 

 Wings alone, then, would fail to support the bird in space. 



