STRUCTURE OP BIRDS. 97 



strongest feathers; though made of very light 

 materials, they are moved by powerful muscles. 

 These propel them forwards with great rapidity, 

 and enable them to sustain long flights during their 

 migrations. Their bones are also exceedingly 

 light and thin ; and to make them still more 

 buoyant, air-cells, connected with their lungs, are 

 extended almost over the whole body. In the 

 mammalia, the lungs are confined to the chest, 

 but in this class they pass along. the sides of the 

 body into the bones, and even -into the pinions or 

 wing-feathers. All these are filled when the bird 

 breathes, and it is thus made almost as light as the 

 air, and floats in it very easily and lightly. Next to 

 the wings, the largest feathers are in the tail, and 

 this is used as a kind of rudder, to guide and regu- 

 late their motions when flying. 



Birds have no teeth for masticating their food, 

 and therefore either swallow it whole, or crush 

 and tear it with their bills. Some of them, which 

 feed principally on grain, always swallow it with- 

 out breaking; and in these the food does not pass 

 at once into the proper stomach, or gizzard, as it is 

 termed, but is received into a pouch called the 

 crop, where it is softened and rendered fit for 

 digestion. These birds are in the habit of swal- 

 lowing pebbles and gravel, which appear to be 

 useful in the crop, and to assist in bruising the 

 hard skin of the grain. 



The greatest part of birds live in trees, some on 

 the water, and but very few on the ground only. 

 I 



