A STUDY OF THE CIRCULATION OF BLOOD 157 



most oxygen is switched off into the arteries that go to 

 the head; the blood supplied with a moderate amount of 

 oxygen is forced to the stomach, intestines, and other organs 

 in the lower part of the body; while the blood with the 

 least oxygen is turned into the arteries that carry it to the 

 lungs. Blood from the lungs then returns, as stated above, 

 through pulmonary veins to the left auricle ; the rest of the 

 blood conies back to the right auricle. 



Circulation in the Reptiles. In the group of reptiles (snakes, 

 alligators, turtles) we first find the beginnings of a four- 

 chambered heart. But while the two auricles are entirely 

 separated from each other, the partition between the ven- 

 tricles is not quite complete. In spite of this fact, the pul- 

 monary and systemic circulations are carried on without 

 any considerable mixing of blood in the two sides of the 

 ventricle. As in man, the right side of the heart has to do 

 with sending the blood to the lungs for oxygen; the left 

 side supplies the rest of the body with the oxygenated blood. 



Circulation in the Birds and Mammals. All birds and all 

 mammals (including man) have the two sides of the heart 

 completely separated from each other. There are, there- 

 fore, two distinct auricles, each communicating with one 

 ventricle only. Hence, we may speak of a distinct right 

 and left heart in all animals above the group of reptiles. 



Comparison of the Organs of Circulation Studied. Reviewing 

 the facts presented above, we see that, while in each of the 

 animals studied there is a complete circulation of the blood, 

 the means by which this circulation is accomplished varies 

 greatly. In the earthworm the force that propels the blood 

 is furnished, not by a heart, but by the contraction of the 

 muscular walls of certain blood vessels. 



All vertebrates (fishes, amphibia, reptiles, birds, and mam- 

 mals) have hearts, and this is also true of many kinds of in- 

 vertebrates. But one has only to compare the different 

 hearts we have described to see how much this organ may 

 be modified in structure. Fishes have a single auricle and 



