PORTAL CIRCULATION. 215 



circuit the blood returns to the heart twice. Leaving the 

 left side it returns to the right, and leaving the right it 

 returns to the left: and there is no road for it from one side 

 of the heart to the other except through a capillary network. 

 Moreover it always leaves from a ventricle through an 

 artery, and returns to an auricle through a vein. 



There is then really only one circulation; but it is not 

 uncommon to speak of two, the flow from the left side of 

 the heart to the right, through the Body generally, being 

 called the systemic circulation, and from the right to the 

 left, through the lungs, the pulmonary circulation. But 

 since after completing either of these alone the blood is not 

 again at the point from which it started, but is separated 

 from it by the septum of the heart, neither is a " circulation 5 * 

 in the proper sense of the word. 



The Portal Circulation. A certain portion of the blood 

 which leaves the left ventricle of the heart through the 

 aorta has to pass through three sets of capillaries before it 

 can again return there. This is the portion which goes 

 through the stomach, spleen, pancreas, and intestines. 

 After traversing the capillaries of those organs it is col- 

 lected into the portal vein which enters the liver, and 

 breaking up in it into finer and finer branches like an 

 artery, ends in the capillaries of that organ, forming the 

 second set which this blood passes through on its course. 

 From these it is collected by the hepatic veins which pour 

 it into the inferior vena cava, which carrying it to the right 

 auricle, it has still to pass through the pulmonary capillaries 

 to get back to the left side of the heart. The portal vein 

 is the only one in the Human Body which thus like an 

 artery feeds a capillary network, and the flow from the 

 stomach and intestines through the liver to the vena cava 

 is often spoken of as the portal circulation. 



Diagram of the Circulation. Since the two halves of 

 the heart are actually completely separated from one 

 another by an impervious partition, although placed in 

 proximity in the Body, we may conveniently represent the 

 course of the blood as in the accompanying diagram (Fig. 

 84) in which the right and left halves of the heart are rep- 



