HUMAN STRUCTURE AND LIFE- ACTIVITIES 489 



The valves (Fig. 161) in all but the largest veins also aid 

 in the circulation of blood, especially when contracting mus- 

 cles press upon them between two valves. The region so 

 pressed acts like a syringe-bulb which has outlet and inlet 

 valves, each compression driving blood towards the heart. 



H 



FIG. 161. A, pocket-like valves of veins. Arrow pointing from c to h showa 

 valves opening towards the heart, while the lower figure shows how the 

 valve would float up and close if blood started to flow backward towards 

 the capillaries through which it came. 



412. Heart a Duplex Pump. Like some pumping-ma- 

 chines with two pumps united side by side, the heart of man 

 and other mammals is double. To illustrate it correctly 

 with the rubber bulb mentioned in above paragraph, it would 

 be necessary to place two bulbs side by side. One bulb 

 would represent the right and the other the left ventricle of 

 the heart. The right ventricle pumps blood into arteries 

 which extend to the lungs, and the left pumps blood to all 

 the other organs. Moreover, the blood pumped from the 

 right side of the heart to the lungs comes back in veins which 

 connect with the left side, and the blood from the left ventricle 

 comes back to the right side of the heart. In other words, the 

 two sides are constantly supplying each other. Both right 

 and left ventricles contract at the same time, the right one 

 forcing blood into the lungs, and thence into the left collect- 

 ing chamber (left auricle), while the left ventricle forces blood 

 into all other organs, from which the blood returns to the 

 right collecting chamber (right auricle). 



It is evident from the above that the two sides of the heart 



