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 carbon di- 

 oxid, water, and other 

 wastes. One can often 

 rest to good adva,ntage 

 by changing 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. Roimd 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 



A = Correct posture. B = Incorrect posture. 



Fig. 48. — Standing positions. 



