IO2 ELEMENTS OF PHYSIOLOGY 



swells out; but if you rub your hand on the outside of 

 the pocket from the bottom toward the top, it flattens 

 down. So with the action of the blood upon the valves 

 in the veins (Figs. 94, 95). They all open toward the 

 heart. 



167. Suppose a muscle hardens as it contracts and 

 presses upon a vein which goes through the muscle; the 

 blood is pressed out of the vein. The 

 > blood cannot go toward the capillaries, 



i- *^ -* H for the valves fill and close when it starts 



_ that way ; so it is all pressed out toward 

 the heart. When the muscle relaxes, 

 the blood that has been pressed for- 

 FIG. 93 -Diagram of the war d cannot come back because of 



Valves 01 Veins. 



H, heart side ; c, capillary the valves, but the valves nearer the 



filled. When the muscle contracts 

 again, the same effect on the blood movement is re- 

 peated. We see, therefore, that every contracting muscle 

 converts into a pump the vein running through it, and 

 when a person works or exercises, many little pumps 

 are working all over the body, aiding the heart in its 

 function. 



168. This aid makes the blood flow faster and re- 

 lieves the heart of part of its work, so that it beats faster, 

 just as a horse might trot faster if half the load were 

 removed. All of the body gets fresher blood than it 

 got when the muscles were still and the blood flowed 

 more slowly. This help comes during active work, just 

 when the body is demanding more blood and the heart 

 needs help. 



169. Suppose a person engages in vigorous muscular ex- 

 ertion just after eating. The stomach and the muscles 

 both demand an increased supply of blood. The muscles 

 will get it because the valves in the veins will increase the 



