144 SYSTEMIC AND PULMONARY CIRCULATIONS. 



branches, b, to the lungs. There it flows through the 

 pulmonary capillaries, a, and is collected from them 

 into the pulmonary veins, r, which convey it to the left 

 auricle; thence it flows to the left ventricle, and com- 

 mences its round once more. 



The valves of the heart only let the blood flow in 

 the direction of the arrows in Fig. 39. If you start at 

 any point in that figure and follow along in the direc- 

 tion pointed by the arrows, you will find that the blood 

 cannot flow back at once, to the side of the heart it 

 started from. To make a complete circulation, it twice 

 leaves, and twice returns to, the heart. 



20. The Systemic and Pulmonary Circulations. To get 

 from the left side of the heart to the right, the blood 

 must flow through the arteries, capillaries, and veins of 

 the body in general. This flow, from left ventricle to 

 right auricle, is often named the systemic circulation. To 

 get from the right side of the heart to the left, blood 

 must flow through the arteries, capillaries, and veins of 

 the lungs. This flow, from right ventricle to left auri- 

 cle, is often named the pulmonary circulation. It is clear, 

 however, that neither is a circulation in the proper mean- 

 ing of the word, for after completing it, the blood is 

 not back again at the place it started from. In order 

 that it may be, it must go through both these circula- 

 tions. 



21. Illustration. We may compare the blood- supply 

 of the body to the water-supply of a city. The left side 

 of the heart answers to the reservoir, and the arteries to 



20. What is the systemic circulation? The pulmonary? Why is 

 neither a circulation, strictly speaking ? 



21. Compare the blood-supply of the body to the water-supply of a 

 city. In what respects is it essentially different ? What would have 



