330 



ANIMALS 



formed out of skin. In the ancient bird, the wing was 

 much like a simple fore limb, instead of the greatly altered 

 structure that we see in the wings of modern birds. 



While modern birds still have some of the charac- 

 teristics of reptiles, in other respects they have gone far 

 beyond them. Instead of being cold-blooded, birds have 

 a higher blood temperature than any other animal; viz., 

 from 100 F. to 110F. Man's blood is at 98.6 F. 

 Horny scales, which cover reptiles, appear only on the 

 legs and feet of birds, while the main portion of the body 

 is covered with feathers. In flying birds the body is 

 wonderfully fitted for flight; in wading birds the neck 

 and feet are long; while in the case of swimming birds the 

 feet are webbed. The foot of a bird usually has four 

 toes. In climbing birds two of the toes are directed 

 forward and two backward. In perching birds three 



toes are directed forward, 

 and one backward. 



The gullet of birds is enlarged 

 into a crop for the storing of food 

 (Fig. 270). The gizzard is the 

 muscular part of the stomach. In 

 grain-eating birds the inner lining 

 of the gizzard is hard and rough. 

 This rough lining, together with 

 the gravel and sand which these 

 birds eat, takes the place of teeth 

 in grinding and crushing the food. 

 The brain and the nervous system of birds are highly developed. 

 We ought to expect this from the wonderful ways in which birds adapt 

 themselves to their surroundings, and from the great care that they 

 give to their young. 



Pelvis 



Stoma 



Sternun 



~ -^ 



Intestine 



FIG. 270. 

 Digestive System of a Bird. 



