120 HUMAN BIOLOGY 



this organ may beat as often as twice a second. Exercise, 

 when properly regulated, is undoubtedly beneficial to every 

 organ of the body, for the heart should be kept in such a 

 vigorous condition that it is ready to meet not only the ordi- 

 nary requirements of everyday life, but even the strain that 

 may come in such emergencies as necessary escape from 

 danger or recovery from disease. 



It is easily possible, however, to overstrain the heart, 

 muscle by exacting from this organ too violent or too pro- 

 longed activity {e.g. in sprinting or in long distance runs and 

 bicycle rides). These often result in permanent thickening 

 of the walls of the valves of the heart. Before a youth takes 

 part in athletic contests, he should consult a competent 

 physician as to the wisdom of his taking violent exercise. 



170. Effect of exercise on the blood vessels. — When 

 one is using the muscles actively, greater oxidation of the 

 tissues goes on, and a larger amount of blood is needed to 

 supply the oxygen and to remove the added wastes formed 

 by this increased oxidation. The muscular walls of the 

 arteries relax in the organs that are specially active, thus 

 supplying these organs with more blood. It is manifestly 

 impossible to have an increased supply of blood in the organs 

 of digestion, in the muscles, and in the brain all at the same 

 time. This is the reason why it is unhygienic for an ad\ilt 

 to exercise violently or to carry on any considerable degree 

 of mental activity immediately after a hearty meal. Per- 

 sistence in violating this rule usually results in attacks of 

 indigestion. 



171. Stopping of blood flow in wounds. — One can tell 

 when an artery has been cut by the fact that blood comes 

 out in spurts. Since the blood is on its way from the heart, 

 the flow can be stopped or lessened in this kind of accident 



