204 ANIMAL ACTIVITIES. 
Besides this machinery of bone and muscle, the 
whole body of the bird is to some extent a balloon. 
Fic, 160,—A, arm of man; B, fore leg of dog; C, wing of bird; 4, 
humerus; 7, radius; #, ulna; c¢, carpus’ m, metacarpus; #, 
phalanges. 
a b 
Fic. 161.-—A Bird’s Wing. «a, when flying; 4, at rest. 
The feathers increase greatly the amount of air dis- 
placed, and hence aid the buoyancy without adding 
