ADDITIONAL TOPICS IN HUMAN BIOLOGY 153 



nervous system. Before the muscles of the arm or leg contract, a 

 " message " must come to them from the brain or spinal cord ; hence 

 nerve tissue is likewise developed by exercise. 



226. Rest. If physical exertion is carried beyond a certain 

 point, exhaustion results, and the muscles cannot be made to con- 

 tract until after a period 

 of rest. Since all muscu- 

 lar contraction involves 

 oxidation of tissue, pe- 

 riods of rest must be al- 

 lowed for the muscles to 

 get rid of their wastes and 

 to build up new tissue in 

 place of the old. The 

 feeling of weariness after 

 long-continued exercise is 

 probably due to the pres- 

 ence in the body of great 

 quantites of 

 oxid, water, 

 wastes. One 

 rest to good 

 by changing 



A = Correct posture. B = Incorrect posture. 



FIG. 48. Standing positions. 



carbon di- 



and other 



can often 



advantage 

 _ _ from one 

 form of activity to an- 

 other, but from eight to 

 nine hours of sound sleep 

 each night are indispen- 

 sable for the health of a growing youth. The necessity for sleep 

 will be further discussed in the study of the nervous system. 



227. Relation of muscles to proper posture. An erect posture 

 and graceful carriage not only add to pleasing appearance, but are 

 important in maintaining the health. Round shoulders and stoop- 

 ing position decrease the capacity of the chest and interfere seriously 

 with the action of its organs. It is important that boys and girls 

 acquire a good posture early in life, and that they realize that this 



