CHORDATA — VERTEBRATA — BIRDS 369 



Class F, Aves (Birds) 



Body a compact mass with five spindle-like extremities. 

 Thorax shifted very far back, hence the long flexible neck, 

 and hence, too, the small head. Bones hollow and light. 



The external protective skeleton consists of feathers, covering 

 most of the body, a horny beak, claws on feet and sometimes 

 on hands, and reptilian-like scales upon the lower portions of 

 the legs and feet. The feathers are developed from reptilian- 

 like scales fringing at their edges. The fore limbs are modified 

 to form wings, upon the tips of which are three reduced fingers, 

 representing probably the first, second and third of the typical 

 hand. There are four toes upon the hind limbs, the fifth toe 

 of the typical foot being absent. In modern birds, one of the 

 two rows of the ankle bones (tarsus) is fused with those of the 

 foot, the other with the bones of the lower leg (tibia and fibula) ; 

 the joint is thus between the two rows of tarsal bones and not as 

 in mammals between the bones of the lower leg and tarsus 

 (Fig. 161). In all flying birds the breastbone has a well-devel- 

 oped keel for the attachment of the flying muscles. 



Teeth are absent in all adult birds from the Tertiary to the 

 present ; Mesozoic birds had functional teeth. The esopha- 

 gus is dilated to form a crop for the storage of the food which 

 is usually eaten so rapidly. From here the food passes to the 

 stomach, a division of which, the gizzard, has in grain-eating 

 birds {e.g. pigeon) its wall so thickened with muscles and its 

 inner lining so hardened and horny that, with the aid of the small 

 stones the bird swallows, it forms an excellent grinding apparatus. 

 In flesh-eating birds {e.g. gulls, owls) it is thin-walled with a 

 non-horny lining. If the gull is fed upon grain, its gizzard 

 wall gradually become thick, with a horny lining ; the opposite 

 effect will occur if the diet of the pigeon is changed to meat. 



The excellent respiratory apparatus produces a higher body 

 temperature (100° F.) than in any other animal; the air is 

 pumped into the lungs and forced out by the elevation and de- 



2B 



