304 PHYSIOLOGY AND HEALTH. 



TJie development of strength is progressive through the 

 several periods of early life, and each period has its appro- 

 priate means and opportunities. The plays of childhood 

 prepare the muscles for the light employments of youth ; 

 these increase the muscular energies, and prepare them for 

 the full power of labor in manhood. Each is necessary, and 

 none can be omitted, nor can they be interchanged. Some 

 children are exclusively devoted to study, and have no incli- 

 nation to active play. They prefer their books, while other 

 children are playing abroad. These develop their nervous 

 systems, but their muscular powers are dormant. They 

 become good scholars, but are weak in body, and ultimately 

 their mental energies sometimes falter. 



CHAPTER XVI. 



Labor requires healthy Organs of Digestion and Nutrition. Good 

 Food, sound Lungs, and fresh Air. Healthy Skin, sound Con- 

 dition, and Cooperation of the Brain and Nervous System. Exer- 

 cise most beneficial when the Brain is lively and Spirits cheerful. 



702. MUSCULAR action presupposes waste, ( 252, p. 115,) 

 which is supplied immediately from the blood, and remotely 

 from the food. The active have, comparatively, new flesh, 

 and the inactive old flesh. ( 257, p. 117.) In order to sup- 

 ply this waste, and make new muscular atoms, the organs 

 that effect the changes in the food, and the channels through 

 which it passes in its progress from its condition as food in 

 the mouth to its new condition as flesh in the tissues, should 

 be in good health. A working person must, therefore, have a 

 sound stomach and good digestion. The organs of circulation 

 and nutrition must also be in good order ; for otherwise the 

 food cannot be converted into blood, nor the blood into flesh, 

 to meet the changes that the action of the muscles demands. 



703. Next in importance to a sound condition of the 

 organs of digestion and nutrition is the supply of good food. 

 Food alone is the source of all our bodily strength ; and it 



