204 THE WONDERFUL HOUSE THAT JACK HAS 



of life brings such material and carries away the waste 

 products. As long as a normal supply of pure blood 

 continues to come to any part of the body, that part 

 will be warm, healthy, and vigorous ; but it will weaken 

 or die as the usual amount lessens or fails altogether. 



In a previous chapter we read that the passing of 

 the blood from the heart through the arteries, capil- 

 laries, and veins back to the heart again is called the 

 circulation. We also learned that the pumping or 

 beating of the heart is the force that propels the blood, 

 and that its passage is helped by the elastic arteries 

 vibrating along their course. 



The arteries have small muscles that by relaxing 

 and contracting allow these blood canals to grow larger 

 or smaller, thus increasing or decreasing the amount 

 that flows through them at any particular time. Ex- 

 ercise of any part quickly increases the supply of 

 blood there. This follows because muscles, when 

 active, require a larger amount of blood than when at 

 rest, and the arteries expand to allow the increased 

 supply to come. Rubbing or massage also stimulates 

 the free flow of blood in the part of the body to which 

 it is applied, for friction tends to relax these little 

 controlling muscles. Hot-water bags or hot cloths 

 are good remedies in colds or other congestive troubles, 

 for applying heat to any part of the body also causes 

 the arterial muscles to expand and allow a larger 

 supply of blood to pass. 



On the contrary, cold, by contracting the arterial 



