A STUDY OF THE FROG 9 



which carry blood to the greater part of the body and to the 

 viscera. The pulmonary arteries, or pulmo- cutaneous arteries 

 as they are sometimes called, divide just before they reach 

 the lung, one branch passing out to the skin. 



The arteries entering the lungs at once divide into smaller 

 and smaller vessels and finally into the small capillaries where, 

 as already said, the blood is purified by giving off its carbon 

 dioxide and taking up oxygen. The capillaries collect again 

 into larger and larger veins and the blood is returned from the 

 lung to the left auricle of the heart through the pulmonary 

 vein. All of the arteries that pass out to the various parts of 

 the body also divide into smaller and smaller vessels and into 

 capillaries which in turn unite to form veins. Some of the 

 veins carry blood through the kidneys, where urea and other 

 waste matter is taken out; others carry blood to the liver; but 

 all of the veins from the different parts of the body finally 

 unite into three large veins which open into the sinus venosus, 

 a thin-walled sac on the dorsal side of the heart. From the 

 sinus venosus the blood enters the right auricle. The right 

 auricle thus becomes filled with the impure blood, that is, 

 with blood that has given up its oxygen to the tissues in all 

 parts of the body and is carrying carbon dioxide as waste. 

 The left auricle, as we have seen, is filled with blood that has 

 just returned from the lungs and the skin, hence it is pure, that 

 is, it contains much oxygen and no carbon dioxide. The two 

 auricles contract simultaneously and send the blood into the 

 ventricle, that from the right auricle into the right side of the 

 ventricle, that from the left auricle into the left side. When 

 the ventricle contracts the impure blood in the right side is 

 forced out first and passes into the pulmonary arteries, and the 

 blood in the left side, which has already received its oxygen, 

 is sent out through the carotid and systemic arteries, carrying 

 its oxygen and nourishment to all parts of the body. A 

 longitudinal section through the heart and the beginnings of 

 the arteries will show the valves that keep the blood from 

 flowing back when the heart contracts. 



The Excretory System. The reddish glandular kidneys lie 

 close to the dorsal body-wall. They are composed of connec- 



