BREATHING 365 



any germs which may enter the wound. When the 

 white corpuscles are killed by the germs or in any other 

 way, they form pus. 



The pale transparent liquid which separates from the 

 blood on standing is called the serum. It is this which 

 fills a blister. It contains liquid food such as is taken 

 up from the intestine during digestion. The serum 

 goes all over the body, supplying both food and water 

 where they are needed. In this way it becomes poor 

 in food and water, and then the sensations of hunger 

 and thirst tell us that we need to eat and drink to renew 

 the supply. 



How is the blood changed in the lungs? If we put a 

 little fresh blood in a bottle with plenty of air, cork it, 

 and shake vigorously, it will turn a bright color, because 

 it takes up oxygen from the air. It is the same way 

 in the lungs; the blood entering the lungs is dark in 

 color because it is poor in oxygen. In the air sacs it 

 takes up oxygen and becomes bright in color ; thence it 

 goes to the heart and is pumped all over the body and 

 gives up its oxygen wherever it is needed, and thus it 

 makes possible the burning which goes on everywhere 

 in the body. It was thought at one time that all the 

 burning was done in the lungs, but this cannot be so, 

 for the lungs are no warmer than any other part of the 

 body. 



The blood not only carries fuel (food) to be burned, 

 but also the air (oxygen) to make it burn, and the burn- 



