THE HEART AND THE BLOOD VESSELS 143 



The hearfc action, however, undergoes an interesting change 

 of rate with age, the average rate found in large numbers of 

 instances being as follows: In early babyhood, 140 beats per 

 minute. In childhood, 100 per minute. In youth, 90 per 

 minute. In adults, 75 per minute. In elderly persons, 70 

 per minute. In very aged persons, 75 to 80 per minute. 



The Work Done by the Heart. The muscular power of the 

 heart is very great. The work it does during one day is about 

 equal to the additional energy expended by a man in climbing 

 to the top of a mountain 3600 feet high. Assuming that the 

 man weighs about 150 pounds, this would be equal to an 

 amount of energy sufficient to lift 90 tons to a height of 

 three feet. The work of the left side is greater than that of 

 the right, since the former has to drive the blood all over the 

 body, while the latter has only to force it through the lungs 

 which are near by. For this reason the muscle walls of the 

 right are much thinner than those of the left ventricle. 



Defects in the Heart Mechanism. We can readily see how 

 a very slight injury to the heart might result seriously. Sup- 

 pose, for example, that the valves should fail to close the pas- 

 ! sages over which they are placed as guards. In some cases of 

 heart disease, a bit of clotted blood collects on the edges of 

 the valves, preventing their perfect closure, causing a leakage 

 land seriously interfering with the action of the heart. It 

 appears that the serious disease, influenza, may frequently 

 i affect the heart muscles and valves, causing faulty action and 

 'thus menace the general health. But one must not think 

 I that, because he has a pain around the heart, he is suffer- 

 ing from heart disease. Indeed, it often happens that those 

 who have some defect in the heart mechanism are quite 

 unconscious of the fact, since the effects are generally more 

 noticeable elsewhere. 



