PURE AIR 109 



of the veins, with this difference : the sewer pipes 

 carry the waste and unclean water away, but the 

 veins conduct the impure blood back to our pumping 

 station, the heart, which sends it to the lungs to be 

 purified. The network of pipes that conduct a city's 

 water supply is, indeed, a remarkable contrivance. 

 Nevertheless, it does not begin to compare in work- 

 manship with the wonderful blood-conducting system 

 of our bodies ; for in the latter are many more tubes 

 of different sizes, much more delicately constructed 

 and skillfully put together. 



In pumping or forcing the blood, the two auricles 

 contract at the same time, both ventricles contracting 

 together immediately after the auricles. Skillfully 

 constructed valves between the auricles and ventricles 

 and between the ventricles and the tubes leading from 

 them keep the blood from going backward. The 

 passing of the blood through the arteries, capillaries, 

 and veins back to the heart again is called the circu- 

 lation (the act of traveling around). It is interesting 

 to know that before the fact of the circulation of the 

 blood was made well known to the medical world *by 

 Dr. William Harvey of England, in 1618, it was quite 

 generally thought that the arteries carried air. This 

 error arose because they were always found empty 

 immediately after a person's death. 



The amount of blood a healthy person has is esti- 

 mated at about one-thirteenth of his body's weight. 

 The heart of males usually beats about seventy times, 



