THE ANIMAL CLIMAX-THE CHORDATES 



313 



focorotid arch 

 to systemic arch 

 carotid arch 



systemic arcb 



pulmocutoneous 

 ^ arch 



to pulmocutoneous arch 



lonqitudinol valve 

 semilunon valve 



entrance of pulmonary veins 

 entrance of sinus veno3u5 



ouhculo-ventricular valve 



Fig. 13-35. Ventral view of the frog's heart, sectioned to show chambers and valves. 



coming to the heart has lost its oxygen and 

 must be sped on its way to the oxygen- 

 replenishing station, the gills. The function 

 of the heart of the fish is simply to keep 

 this mass of blood in motion. In the air- 

 breathing frog, however, a new complica- 

 tion arises: there are two circuits of the 

 blood. One circuit carries blood rich in 

 oxygen to the tissues, and then brings the 

 "used" blood, poor in oxygen, back to the 

 heart, while tlie other carries this depleted 

 blood to the lungs and brings it back after 

 oxygenation to the heart. These two circuits 

 must be kept separate in order to do an 

 efficient job. Since the amphibians were the 

 animals to take the first step onto land, 

 they were the first ones to begin the solu- 

 tion of this complex problem, and are in- 

 teresting to study because they give some 

 information as to how this development 

 and evolution all came about. The higher 

 reptiles (crocodiles), birds, and mammals 

 have solved the problem very nicely by 



producing two complete hearts, but the 

 amphibians seem to have been unable to 

 make the complete transition, and have 

 gotten along these millions of years with a 

 rather crude system. 



The blood coming from all parts of the 

 bodv first enters the sac-like sinus venosus 

 and then the right auricle; simultaneously 

 blood rich in oxygen enters the left auricle 

 from the lungs. It would seem that upon 

 syncronous contraction of the two auricles 

 the blood would be badly mixed and that 

 all blood leaving the heart could be mixed 

 blood. This is not the case, however, be- 

 cause of the anatomical arrangement of the 

 ventricle. By studying Fig. 13-35, it is seen 

 that the opening of the right auricle leads 

 into the ventricle to the left of the opening 

 from the left auricle and then delivers its 

 blood nearer the exit, the conus arteriosis. 

 When the blood from the rio;ht auricle flows 

 into the ventricle it is just a little ahead of 

 the blood from the lungs. When the ven- 



