THE STRUCTURAL AND FUNCTIONAL SYSTEMS 59 



of the ventricle, where a certain amount of mixing has occurred, in 

 general enters the systemics, or the second pair of arterial vessels, 

 and passes to all parts of the body save the head. The oxygenated 

 blood from lungs and left auricle, which was emptied into the side of 

 the ventricle on the observer's right is the last to enter the truncus, 

 and passes into the carotids, or anterior pair of arterial trunks, to be 

 distributed to the brain and spinal cord. Hence, by the arrange- 

 ment of the parts and the timing of their contractions, the 

 oxygenated and the unoxj'genated portions of the blood, after 

 being together for an instant in the ventricle and truncus, are 

 actually distributed to suitable parts of the body with a minimum 

 of mixture. The mechanism is less effective than that of the 

 four-chambered device in birds and mammals. Nevertheless, the 

 heart of the frog is the more remarkable, because a separation of 

 oxygenated and unoxygenated blood is fairly well accomplished, 

 under conditions that make this result seem impossible when the 

 structural relationships are first examined. 



Another factor of the frog's circulation is the existence of what 

 are called the portal systetns. Blood passing from the capillaries 

 of the digestive tract goes by the hepatic portal vein (Fig. 34), 

 to the Hver, where it is again distributed through capillaries, 

 which combine to form the hepatic veins leading from this 

 organ. Similarly, in the refial portal sysfeyn of a vertebrate like 

 the frog, blood that has passed through capillaries in the hind 

 hmbs passes to the kidneys by the renal portal veins (Fig. 34). 

 These break up into the capillaries of the kidneys, which in turn 

 form the renal veins leading from the kidney to the post-caval vein. 



We may consider briefly the circulatory systems of other classes 

 of vertebrates for comparison with that of the frog and other Am- 

 phibia. In the fishes (Fig. 35), where there are no lungs, the 

 gills with their capillaries are merely inserted at one place in the 

 general circuit, so that, diagrammatically, the blood may be 

 shown passing from the heai-t to the gills where oxygenation occurs, 

 and thence to all parts of the body, returning through the veins to 

 repeat the process. The heart of the fish is two-chambered, con- 

 sisting of a single auricle, which is thin-walled since it serves only 

 to " charge " the ventricle, and a ventricle, which is heavy-walled 

 since its contraction must force the blood through the entire 

 system. The only further complexity is the presence of an ad- 

 ditional set of capillaries where the blood flowing from the digestive 



