THE HEART AND CIRCULATION. 



105 



into the auricles, while the semilunar valves open to let 

 the blood out. The cardiac cycle is, therefore, as follows: 

 Circulation. The blood, after it has given off its oxy- 

 gen and collected carbon dioxide, returns to the heart 

 through two main channels, the superior and inferior vence 

 cavce, the former bringing the blood from the upper part of 



Pulmonary 

 artery 



Superior 

 vena cava 



Right auricle 

 Inferior 

 vena cava 

 Right 

 ventricle 



Pulmonary 

 capillaries 



-* Left ventricle 



Portal 

 circulation 



Systemic 

 capillaries 



FIG. 42. Diagram of the circulation. (After Kirke.) 



the body, including the head, neck, and arms, and the lat- 

 ter from the lower part below the diaphragm. The two 

 vessels empty along with the coronary sinus, which is 

 guarded by the coronary valve, into the right auricle. At 

 the same time that they empty into this auricle the four 

 pulmonary veins, the only veins that carry arterial or 

 oxygenated blood, are emptying the fresh blood from the 

 lungs into the left auricle. When both auricles are full, 





