EXERCISE 285 



obtained, must be in good condition, and that the air which 

 we take into them must be rich in oxygen. Bad ventilation 

 diminishes the oxygen content of the blood, and diminishes 

 the working power of the muscles by just so much. 



Every one knows that it is necessary at times to let 

 the engine fires go out in order that the accumulated 

 wastes may be removed, and in order that the heated 

 parts may cool. So, with the muscles, the constant use 

 of the cells soon causes an accumulation of waste too 

 great for the circulation to remove, the body becomes 

 overheated, and soon the muscle loses its power to respond 

 to a stimulus. Such a condition is called muscle fatigue. 

 We feel tired, and this tired feeling is an indication that it 

 is time to rest and allow the cells to recover from their 

 activity. If we heed this call and permit the used part 

 to rest for a while, the blood is able to remove the accumu- 

 lated wastes diu'ing this period, and stock the cells with 

 new fuel, and, in general, restore the muscle cells to their 

 working condition. On this account, children and manual 

 laborers who use their muscles much more constantly 

 than brain workers, require much longer rest periods. 

 Often a change of work will accomplish a restoration of 

 the muscles, provided the new work calls into play a 

 different set of muscles and allows the others to rest. A 

 certain amount of sleep, however, is necessary for every 

 healthy man, woman, or child. 



Exercise. — Muscles possess another property besides 

 contractility, namely, elasticity. For example, if we sus- 

 pend weights at one end of a freshly dissected frog's 

 muscle, the muscle readily stretches to accommodate the 

 increase in weight, and as readily returns to its original 

 length when the weight is removed, provided we do not 



