2Q2 



ANIMAL BIOLOGY 



greatly elongated cells. In the so-called voluntary 

 muscle, or muscle which we can control by the will, the 

 fibers are marked with fine cross striations. In involun- 

 tary muscle the fibers are smaller and usually devoid of 

 cross striations. Such muscular tissue occurs in the 

 coatings of the stomach and intestines, in the walls of 

 blood vessels and ducts, and in various other parts of the 

 body. Involuntary muscle acts usually 

 without our being aware of its contrac- 

 tion, and we have very little direct 

 control over its activity. In fact it is 

 best that we are not able to control it, 

 because the wisest of us would not 

 know how to direct its functioning. 



Muscles increase in size and strength 

 through exercise, and there is nothing 

 so good as exercise in the open air, not 

 only for muscles, but for the body in 

 general. Muscular exercise involves 

 stimulating to greater activity practi- 

 cally all parts of our organism. It 

 brings increased appetite, better tone 

 of the blood vessels, greater lung power, 

 increased elimination of waste, better 

 feelings and clearer thinking. When 

 exercise is carried too far, however, we 

 have the feeling of fatigue which is 

 caused by substances in the blood that result from an ex- 

 cessive breaking down of living tissue, but when we have 

 rested and the fatigue-producing substances are disposed 

 of, Nature not only restores what has been lost, but she 

 commonly adds a little more. Hence the strong biceps 

 of the blacksmith, which is in striking contrast to the 

 weak and flabby muscles of a sedentary clerk who never 



FIG. 210. Muscles 

 and tendons of fore 

 arm and hand. 



