90 THE HUMAN BODY. 



should not he exposed to any considerable continued 

 pressure, since this interferes with the flow of blood 

 and lymph through them which is essential for their nu- 

 trition. 



Exercise is necessary for the best development of the 

 muscles. A muscle long left unused diminishes in bulk 

 and degenerates in quality, as is well seen when a muscle is 

 paralyzed and remains permanently inactive because of 

 disease of its nerve ; although at first the muscle itself 

 may be perfectly healthy, it alters in a few weeks, and 

 when the nerve is repaired the muscle may in turn be in- 

 capable of activity. The same fact is illustrated by the 

 feeble and wasted state of the muscles of a limb which 

 has been kept motionless in splints for a long time : when 

 the splints are removed it is only after careful and per- 

 sistent exercise that the long idle muscles regain their 

 former size and power. The great muscles of the 

 "brawny arm" of the blacksmith illustrate the converse 

 fact the growth of muscles when exercised. 



Exercise, to be useful, must be judicious ; taken to 

 the point of extreme fatigue, day after day, it does harm. 

 When a muscle is worked its substance is used up ; at 

 the same time and afterwards more blood flows to it, 

 and if the exercise is not too violent and the intervals of 

 rest are long enough, the repair and growth will keep pace 

 with or exceed the wasting : but excessive work and too 

 short rest will lead to diminution and enfeeblenient of the 

 muscle ju-st as certainly as too little exercise. 



Few persons can profitably attempt to work hard daily 



Why should muscles not be exposed to continuous pressure? 



What happens when a muscle is not used? Illustrate by examples. 

 Why are the muscles of a blacksmith's arm large? 



When doet exercise do harm? Why? Can most persons work 

 hard with both brain and muscle at the same time? 



