132 ANIMAL PHYSIOLOGY. 



till they reach the capillaries, and that the veins emerging 

 from these bring the blood directly back to the heart. 

 Looking at vertebrate animals generally, one finds many 

 instances of arteries breaking up into small branches which 

 reunite before reaching the capillaries, and such an arrange- 

 ment is called a rete mirabile. There is only one instance of 

 such a thing occurring in the human subject, namely, in the 

 Malpighian corpuscles of the kidney. But there is a notable 

 instance, occurring in man and all vertebrate animals, of a 

 venous trunk branching again into twigs, which open into a 

 second set of capillaries; and that is the portal vein. The 

 portal vein receives all the blood returning from the stomach, 

 intestines, and spleen, and divides into a right and a left 

 branch, which enter^the liver, and break up into branches 

 which pour their contents into the capillaries of that organ, 

 and then discharge tneir blood into the hepatic veins, which 

 open into the vena cava inferior. Thus all the blood which 

 goes to the stomach and intestines has to pass through two 

 sets of capillaries before returning to the heart, and this is 

 the blood on which the liver exercises its purifying power. 



