XV THE CIRCULATORY SYSTEM 277 



in the female several branches from the oviducts. The 

 system of veins which lead blood to the kidney is known as 

 \hQ renal portal sy stein. There is dX'^o 2, hepatic portal sys- 

 tem which carries venous blood to the liver. The latter con- 

 sists of (i) \.\\Q anterior abdominal vt'in, which receives blood 

 from the femoral veins, bladder, and ventral body wall, and 

 (2) the portal YQ'm, which carries blood from the stomach, 

 intestine, spleen, and pancreas, the terminal portion passing 

 through the latter organ to empty into the left lobe of the 

 liver. The abdominal vein, just before it enters the liver, 

 receives a small branch, the vena bulhi cordis, from the bul- 

 bus cordis ; the other parts of the heart are devoid of 

 special blood vessels. 



The Action of the Heart. — In the beating of the heart, 

 which may readily be observed in a frog that has recently 

 been killed, the contraction first occurs in the sinus venosus ; 

 and this is followed by successive contractions of the 

 auricles, ventricle, and bulbus. As we have seen, the ar- 

 rangement of the valves of the heart is such as to keep 

 the blood flowing through these parts in the order named. 

 Although the frog does not possess a complete double circu- 

 lation, such as occurs in birds and mammals, in which the 

 systemic and the pulmonary circulations are entirely sepa- 

 rated, the impure and the oxygenated blood are, neverthe- 

 less, not allowed to completely mix, but are kept more or 

 less apart and sent out to different parts of the body. It 

 was formerly held that the blood from the two auricles was 

 completely mingled in the ventricle, but Mayer showed in 

 1835 that if the tip of the ventricle be cut off, two blood 

 streams, a dark and a red, issue from the cut end. Later 

 (1851) the noted physiologist Rriicke studied the structure 

 and action of the frog's heart in detail and explained the 

 mechanism by which the two kinds of blood were kept 



