THE ANATOMY OF THE HEART AND BLOOD-VESSELS 335 



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 back at the point from which it started, but is sepa- 

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

 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, pan- 

 creas, and intestines (M, Fig. 99). After traversing the capillaries 

 of those organs it is collected 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 (P, Fig. 99). From 

 these it is collected by the hepatic veins, which pour it into the 

 inferior vena cava, which carries it to the right auricle, so that 

 it has still to pass through the pulmonary capillaries to get back 

 to the left side of the heart. The flow from the stomach and in- 

 testines 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 im- 

 pervious partition, although placed in proximity in the Body, 

 we may conveniently represent the course of the blood as in the 

 accompanying diagram (Fig. 107), in which the right and left 

 halves of the heart are represented at different points in the 

 vascular system. Such an arrangement makes it clear that the 

 heart is really two pumps working side by side, each engaged in 

 forcing the blood to the other. Starting from the left auricle, 

 la, and following the flow, we trace it through the left ventricle 

 and along the branches of the aorta into the systemic capil- 

 laries, sc; from thence it passes back through the systemic 

 veins, vc. Reaching the right auricle, ra, it is sent into the right 



