THE FROG 191 



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through the kidneys, where it is again broken up into capil- 

 laries. This is the course of the blood which returns from 

 the leg through the renal-portal vein (Fig. 92 rp), but the rest 

 of the blood from the legs is diverted to an abdominal vein, a, 

 which enters the heart without passing through the liver. 

 Both the liver and the kidneys have their own supply of blood 

 from the aorta, as well as that received from the veins. 



The vessels thus far described are called the systemic cir- 

 culation, in distinction from the pulmonary circulation, which 

 is the circulation in the lungs. The lungs are elastic bags 

 (Fig. 92), capable of much expansion when inflated with air, 

 but collapsing if the air is removed. They are connected with 

 the mouth by the larynx, which opens at the base of the tongue 

 through the glottis. Through the glottis and the larynx air 

 is taken into the lungs to purify the blood. The arteries which 

 supply the lungs, the pulmonary arteries, pu, arise from the 

 main arteries near the heart. From each of these an artery 

 is given off to the skin under the arm, the cutaneous, cu. Since 

 in the lungs the blood is purified by the oxygen of the air, 

 and through the skin it is purified by the oxygen in the water, 

 the frog can live either in the water or in the air, i. e., it is 

 amphibious. The blood that is purified in the lungs enters the 

 heart again by a pulmonary vein, puv, which flows into the left 

 auricle. The pure blood in the left auricle is thus kept separate 

 from the impure blood in the right auricle, but as soon as the 

 auricles contract the blood of both auricles is forced into the 

 single ventricle, and intermingles. Although the blood in the 

 ventricle is really mixed, still the blood upon the right side of it, 

 since it received blood directly from the right auricle, will con- 

 tain more impure blood than that on the left side, which is 

 connected directly with the left auricle. The pure and impure 

 blood are kept partly separate by muscular partitions ex- 

 tending irregularly through the ventricle. 



The blood is composed of a colorless liquid, called the plasma, 

 in which float two types of corpuscles. The larger, the red 



