TUP: CIRCULATION. 65 



2, The heart-muscle is remarkable for its endurance. 

 No other muscie could do its work. Through a whole 

 lifetime, sometimes a hundred years, it never pauses for 

 one minute. 



3, It might seem, therefore, to be an exception to the 

 general law, that rest is necessary for all organs; but, after 

 each contraction, it has a very short time of relaxation and 

 rest. This time is not more than two-fifths of a second; 

 but, as it comes every second, its whole amount in twenty- 

 four hours would be eight or nine hours. Moreover, in 

 sleep the heart beats less rapidly, and is not obliged to 

 make the special efforts which are so often required of it 

 during the day by rapid movements or excitement. 



4, The heart is, doubtless, tired after labor, and con- 

 tributes to the general sense of fatigue; but in health we 

 have no special feeling of the heart. Great and long-con- 

 tinued care such as business men are often subjected to, 

 or protracted muscular exertion like that of soldiers on a 

 march, sometimes so exhausts the heart, that it acts irreg- 

 ularly, and feels distress. This condition, known to 

 physicians as " irritable heart," maybe brought on by 

 prolonged dissipation. 



5, The number of heart-beats in a minute varies at dif- 

 ferent periods of life. In an infant it is one hundred and 

 twenty or more; in a child under fourteen, eighty or 

 more; in a grown person, about seventy -two. But it may 

 be ten beats more or less; and, in rare cases, there is a still 

 wider variation in health. 



6, Many things cause a temporary variation. It is less 

 in sleep: it is greater in active exercise. One reason why 

 we soon become exhausted by running, is that the heart 

 is stimulated to such japid action. It is greater after eat- 



