204 
ANIMAL ACTIVITIES. 
Besides this machinery of bone and muscle, the 
whole body of the bird is to some extent a balloon. 
Fig. 160.— A, arm of man; B , fore leg of dog; C, wing of bird; h , 
humerus; r, radius; u, ulna; c, carpus- m } metacarpus; 
phalanges. 
Fig. 161.—A Bird’s Wing. <7, when flying; 6, at rest. 
The feathers increase greatly the amount of air dis¬ 
placed, and hence aid the buoyancy without adding 
