324 ELEMENTARY ZOOLOGY. 



stoniach, or gizzard, of the grain-eating birds (Fig. 131). 

 The action of such an organ is needed because of the lack 

 of teeth in the mouth. The intestines are long and 

 coiled. 



328. Respiration and Circulation. The heart is com- 

 pletely four-chambered and there is no mingling of pure 

 and impure blood. In addition to the lungs, there are 

 air sacs in various parts of the body which are connected 

 with the lungs. Some of these may even extend into 

 the larger bones. This gives a larger surface for the 

 exchange of oxygen and at the same time renders the 

 body lighter than if it were solid. The breathing of 

 birds and the heart beat are more rapid than in the 

 other groups. These facts contribute to the hot-blooded 

 condition of the birds and to their ceaseless activity. 



329. The Nervous Activities and Structures. The brain 

 is larger in proportion to the size of the animal than in 

 the reptiles. This increase is found especially in the 

 cerebrum and cerebellum (see 298). The larger brain is 

 associated with the great activity of the animal, especi- 

 ally the demands of balancing and flying, as well as the 

 highly developed instincts of mating, nesting, caring for 

 the young, and so forth. The eyes are large and efficient. 

 It appears that the sense of sight and hearing are better 

 developed in birds than the other senses. It is thought 

 that the sense of smell is not very acute. The bird is 

 what we would call a "nervous animal." It is alert, it 

 is sensitive, and it is quickly responsive. 



330. Reproduction and Development. All birds lay 

 large eggs in which there is stored enough food to bring 

 the embryo well along in its development before hatch- 



