MOVEMENrS.] PHYSIOLOGY. ' 37 



and joints, but the muscle must be supplied with 

 blood. 



19. We can now go a step further and ask the ques- 

 tion, What is there in the blood that thus gives 

 to the muscle the power of contracting, that 

 in other words keeps the muscle alive ? The 

 answer is very easily found. What is the name 

 commonly given to this power of a muscle to con- . 

 tract? We generally call it strength. Lay your 

 arm straight out on the table, put a heavy weight 

 in your hand, and try to bend your arm. If you 

 could do it, one would say you were strong ; if you 

 could not, one would say you were weak — all the 

 stronger or weaker, the heavier or lighter the weight. 

 In the one case your biceps had great power of con- 

 tracting ; in the other, little power. Try and find out 

 the heaviest weight you can raise in this way by bend- 

 ing your arm, some morning, not too long after break- 

 fast, when you are fresh and in good condition. Go 

 without any dinner, and in the afternoon or evening, 

 when you are tired and hungry, try to raise the same 

 weight in the same way. You will not be able to do 

 it. Your biceps will have lost some of its power of 

 contracting, will be weaker than it was in the morning. 

 What makes it weak ? The want of food. But how 

 can the food affect the muscle ? You do not place the 

 food in the muscle ; you put it into your mouth, and 

 from thence it goes into your stomach and into the 

 rest of your alimentary canal, and there seems to 

 disappear. How does the food get at the muscle? 

 By means of the blood. The food becomes blood. 

 The things which you eat as food become 

 changed into other things which form part 



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