300 THE HUMAN BODY 



blood which leaves the heart by the aorta, except that flowing 

 through the coronary arteries, is finally collected into the superior 

 and inferior vence cavce (cs and d, Figs. 103 and 104), and poured 

 into the right auricle. The jugular veins which run down the neck, 

 carrying back the blood which went out along the carotid arteries, 

 unite below with the arm-vein (subclavian) to form on each side an 

 innominate vein (Asi and Ade, Fig. 103) and the innominates unite 

 to form the superior cava. The coronary-artery blood after flow- 

 ing through the capillaries of the heart itself also returns to this 

 auricle by the coronary veins and sinus. 



The Pulmonary Circulation. Through this the blood gets back 

 to the left side of the heart and so into the aorta again. The pul- 

 monary artery, dividing into branches for each lung, ends in the 

 capillaries of those organs. From these it is collected by the pul- 

 monary veins, which carry it back to the left auricle, whence it 

 passes to the left ventricle to recommence its flow through the 

 Body generally. 



The Course of the Blood. From what has been said it is clear 

 that the movement of the blood is a circulation. Starting from any 

 one chamber of the heart it will in time return to it; but to do this 

 it must pass through at least two sets of capillaries; one of these 

 is connected with the aorta and the other with the pulmonary 

 artery, and in its 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 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. 



