4 6 



HUMAN BIOLOGY 



food, gives a good appetite, and sets the digestive organs to 

 work ; it uses up the oxygen and sets the lungs to work ; 

 but most of all, every contraction of a muscle helps the blood 

 to flow. As a muscle contracts, it presses upon the veins 

 and lymphatics, and, by this pressure, forces the blood 



and lymph along (Fig. 

 48). In any ordinary 

 activity, dozens of mus 

 cles are being used. 

 That the effect upon the 

 circulation is very pow- 

 erful, is shown by the 

 rosy skin, deep breath- 

 ing, and rapid heart beat. 

 The many benefits of an 

 active circulation of the 

 blood and lymph will 

 be discussed in the next 

 chapter. See page 67. 

 A grave danger from athletics is that of developing the 

 muscles, including the heart, to an enormous extent by 

 training; then when training ceases the muscles undergo 

 fatty degeneration from disuse. Heart disease and other 

 diseases may follow. Many athletes die young, killed by 

 trying to turn their bodies into mere machines for run- 

 ning, boxing, or rowing, instead of living complete lives. 

 The athletic ideal is not the highest ideal of health ; gen- 

 eral activity, resembling the occupations of hunting and 

 farming by which the early race supported itself, is 

 best for health. Many kinds of factory work use only 

 one set of muscles. The savage did not lead a monoto- 

 nous life, and monotony is bad for both muscles and 

 nerves. 



FIG. 48. CAPILLARIES among fibers of 

 voluntary (cross striped) muscle. (Peabody.) 



