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EVOLUTION OF ANIMALS 



Carotid arteries 



Part V 



Pulmonary 

 vein 



Jugular vein 

 Cutaneous artery 



Left 



Pulmonary 

 artery 



^Subclavian rein, 



Intestine 



Renal 

 portal 

 vein 



Iliac artery 



Fig. 34.18. Circulation of blood in the bullfrog. Veins in black. (Courtesy, 

 Wolcott: Animal Biology, ed. 3. New York, McGraw-Hill Book Co., 1946.) 



Circulation of Blood. Blood containing various substances from the body is 

 poured into the sinus venosus which opens into the right auricle. At the same 

 time, well-oxygenated blood flows through the pulmonary veins into the left 

 auricle. Both auricles then contract and force their contents onward into the 

 ventricle. Blood is kept from going back into the sinus by blood behind it, 

 which pours in from the veins, and from going into the pulmonary veins by 

 the pressure of the distended wall of the auricles against their openings. Well- 

 oxygenated blood fills the left side and sparsely oxygenated blood the right side 

 of the ventricle with blended blood between. The ventricle then contracts. 

 With the valves into the auricles closed behind it, the blood takes the only free 

 road, into the truncus arteriosus. As it does so it passes the semi-lunar valves, 

 three soft cups, and the current approaches from beneath and completely 

 flattens them. Muscles in the truncus contract upon the blood and it fills the 

 cups behind it bringing their soft edges together. This creates a backstop. 

 Muscular contraction continues in a wave over the arteries of the body. 



Blood with a low-oxygen content enters the truncus from the right side of 



