6o 



PURE AIR 



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How we breathe. We all know that somewhere inside 

 of our bodies there are spongy structures we call lungs. 

 The diagrams (page 70 and below) show us that from the 

 mouth cavity down the front of the neck leads a tube known 

 as the air tube, or windpipe, and that this tube branches 

 into two pipes, or bronchial tubes, which in turn branch 



again and again and 

 end in tiny grapelike 

 clusters of little air 

 sacs, which are too 

 small to be seen with 

 the naked eye. In 

 these air sacs, which 

 have extremely thin 

 walls, the oxygen of 

 the air comes in con- 

 tact with the blood, 

 the blood taking 

 some oxygen and 

 giving up in return 

 some carbon diox- 

 ide, which has been 

 brought there from 

 the various parts of the body where work is being done. 

 Around these delicate lungs are placed the ribs, so that as 

 the diagram shows, the two lungs are held within a sort 

 of bag or cavity. This chest cavity is bounded at the 

 bottom by a muscular wall, the diaphragm, which extends 

 across from the front to the back of the body, and makes 

 an air-tight surface surrounding the lungs. If we now study 

 a simple piece of apparatus shown in the picture on the next 

 page, we shall be able to understand how we breathe. 



The human breathing organs. Notice one lung is cut 

 open to show the air sacs. 



