342 THE ANIMALS AND MAN 



muscles of the lungs and heart, and so "tones" up the whole 

 system. Exercise, to be of most value, should be regular. 

 It should not be taken immediately after a meal, because 

 then blood would be withdrawn from the digestive system 

 at a time when it is particularly needed there. 



Fatigue and rest. A feeling of weariness or muscular 

 fatigue follows prolonged exercise at irregular intervals, or 

 exercise of muscles occasionally used. If the heart has not 

 been trained by constant exercise to keep up a vigorous 

 movement of the blood, or if the heart is overworked, the 

 waste matter (carbon dioxide and urea) produced by muscular 

 activity accumulates in the tissues and the feeling of fatigue 

 follows. Thus the waste products, as such, fulfill an im- 

 portant function as indicators that rest is needed. A rest 

 of a few minutes, even, gives the blood time to carry off 

 the waste; and the feeling of fatigue ceases. 



