Chap. 34 AMPHIBIANS 701 



the ventricle. It takes the path of least resistance and enters the pulmocutaneous 

 arches to the lungs and skin (Figs. 34.18, 34.19). The next to enter is the 

 partially mixed blood from the central part of the ventricle and this goes into 

 the systemic arches, the pair that offers next least resistance. The carotid 

 arches that supply the head region receive the remainder, the blood from the 

 left side of the ventricle, that carries the best supply of oxygen. The twisted 

 ribbon of tissue (longitudinal or spiral valve) in the truncus has been held 



renal 

 portal vein 



cutaneouj 

 artery 



fiu/monart/ 

 artery 



Fig. 34.19. Circulation of the blood in a vertebrate with two auricles (atria) and 

 one ventricle as in the frog. (Courtesy, Curtis and Guthrie: General Zoology, ed. 4. 

 New York, John Wiley and Sons, 1947.) 



important in keeping the blood rich in oxygen from that less well supplied 

 with it. This has not been supported by some recent experiments. 



The circulating blood of the frog makes two partially separated circuits, 

 each one passing through the heart. In one of these (pulmonary), the blood 

 flows from the heart to the lungs and back to the heart. In the other (systemic 

 circulation), the blood flows to all parts of the body, except the lungs, and 

 returns to the heart. Since it is constantly shifting, all of the blood is able to go 

 through each route very often. 



Lymph and Lymphatics. Lymph is a watery fluid similar to the blood 

 plasma. It contains colorless cells, the lymphocytes, but no red blood cells. 

 Several fluids contribute to its content — the plasma of the blood, tissue fluid, 

 and, in the frog, extra large quantities of water. It is contained in tubes, in 

 spaces between the tissues (lymph sinuses), and in lymph hearts. In frogs, the 

 lymphatic system is especially important and conspicuous. The smallest lymph 

 vessels have blind ends. They form networks of capillaries which join larger 

 and larger vessels and finally one or more main trunks that open into the veins. 

 Some lymph vessels are broken by lymph sacs in which the lymph is in direct 

 contact with the tissues. Such sacs are located directly beneath the skin, almost 

 surround the body and sometimes become pillowed out by abnormal accumu- 



