THE BIRDS 



217 



filled with mixed blood. By an ingenious arrangement of 

 arteries and valves the blood is not evenly mixed, but the 

 part which is pumped to the head is richest in oxygen, that 

 to the body organs next richest, and that to the lungs and 

 skin is poorest in oxygen and richest in carbon dioxide. 

 As in the fish, the red corpuscles are nucleated. In its 

 early life the frog breathes by gills. 



The reptiles — These animals have, in 

 general, the same sort of circulatory 

 system as the amphibians. There is, 

 however, one modification in the struc- 

 ture of the heart that shows an ad- 

 vance. In most reptiles the ventricle 

 is partly divided by a partition, and 

 this partition tends to keep separate 

 the aerated and non-aerated blood so 

 that the aorta ^^•hich arises from the 

 left side of the ventricle receives blood 

 freer of carbon dioxide than the pulmo- 

 nary arteries which arise from the right 

 side. This partition is interesting as a 

 step toward complete separation of the 

 two kinds of blood. The red corpus- 

 cles are nucleated. 



The birds. — In these vertebrates the 

 ventricle is completely divided by the 

 partition, and the bird heart, like that 

 of man, is four chambered. This' fact 

 alone would enable us to classify birds 

 as higher forms than reptiles. The 

 rate of flow in birds is much more rapid than in man, and 

 the temperature of the blood is much higher. This is due 



Fig. 80.— Plan of circu- 

 lation in amphibians 

 and reptiles ; a, right 

 auricle receiving ve- 

 nous blood from the 

 system ; 6, left auri- 

 cle receiving arte- 

 rial blood from the 

 lungs ; c, common 

 ventricle; c/i/jC, sys- 

 temic arteries, capil- 

 laries, and veins ; g, 

 pulmonary artery ; 

 /.-, ft, pulmonary cap- 

 illaries and vein. 



