124 BRANCH VERTEBKATA. 



FIG. 216. 



Alliga'tor mis sis sip pi en' sis. Alligator. ( S L.) 



States, and have been captured on the Mississippi as far 

 north as the mouth of the Ohio. 



CLASS AVES (a'vez). 



Though the blood of reptiles as well as of fishes is 

 warmer than that of the surrounding medium, the active 

 BIRDS have blood considerably warmer ; and lest the heat 

 should too rapidly leave the body, the scales take on a 

 new structure, that of the feather. The blood is forced 

 over the body by means of a four-chambered heart, the 

 aorta (a or'ta) passing to the right, rather than to the left 

 as in man. The blood is purified in the lungs, branches 

 of which, as air-sacs, are distributed to different parts of 

 the body, some even penetrating the bones.* 



The skeleton offers many important structural pecul- 

 iarities. The head bears toothless jaws, and is articulated 

 to the neck by but a single condyle (kon'dil). But the 

 most interesting facts are those connected with the aerial 

 (a e'rl al) life of the animal. The bones are light f and hol- 

 low, though of great strength. Those of the fore limb, 

 bearing the strong feathers of flight, are highly specialized, 



* It is said that a bird will breathe through, the end of a broken bone when 

 the windpipe is tied. 



t This is owing to their being composed largely of phosphate of lime, and the 

 marrow in many of them being replaced by air. Singularly, at one stage, they are 

 solid, like those of all vertebrates, but the bony tissue is afterward absorbed. 

 "The thinnest-walled and widest air-bone of the bird of flight," says Owen, "was 

 first solid, next a marrow-bone, and finally became the case of an air-cell." 



