136 MICROBES AND HEALTH. 



sons in the system are increased. The waste interferes 

 with the circulation in the small peripheral vessels, 

 elimination by the skin is clogged, the kidneys do what 

 they can, but are hopelessly behind all the time. There 

 is one other avenue by which the system may purify 

 the blood; the lungs. 



The lungs are supplied with two sets of vessels, 

 which, from their minute size are called capillaries. 

 The arteries divide and redivide until they penetrate 

 all parts of the lung substance. One set of capillaries 

 supplies nourishment, the other set envelopes the air 

 cells for the purpose of absorbing oxygen from the air 

 we breathe. This set of capillaries lay just beneath 

 the delicate mucous membrane which lines the cells. 



Animal membrane has the power of admitting gases 

 (oxygen) and yet remain impervious to fluid (blood). 

 It is estimated that here are six hundred million air 

 cells in the lungs, and that their combined surface is 

 more than seven times greater than the whole outer 

 surface of the body. This surface is literally covered 

 with small vessels (capillaries) through which the blood 

 is continually pouring. Nearly all the blood in the 

 ~body passes through the heart and lungs once every 

 minute. One-twelfth of the weight of the body is the 

 weight of the blood. This is equivalent to nearly two 

 thousand pounds of blood which must pass through 

 the lungs every twenty-four hours. 



The system of vessels which supply the lungs with 

 nourishment, and the system through which the blood 

 is oxydized are entirely separate. The system which 

 nourishes the lungs is given off from the left heart, 



