94 SCIENCE PRIMERS, [§ vi. 



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to keep them on the stretch, and so they shrink up to 

 their natural size, just as does the bladder when you 

 leave off blowing into it, or when you take it out of 

 the air-pump. 



When before you made the hole in the diaphragm 

 you pulled the diaphragm down, you still further 

 lessened the pressure on the outside of the 

 lungs ; hence the pressure inside the lungs caused 

 them to swell up and follow the diaphragm. But 

 this put the lungs still more on the stretch, so that 

 when you let go the diaphragm and ceased to pull on 

 it, the lungs went back again to their former size, 

 emptying themselves of part of their air and pulling 

 the diaphragm up with them. When there is a hole in 

 the chest wall, pulling the diaphragm down does not 

 make any difference to the pressure outside the lungs. 

 They are then always pressed upon by the same at- 

 mospheric pressure inside and outside, and so remain 

 perfectly quiet. 



When in an air-tight chest the diaphragm 

 is pulled down, the pressure of the atmo- 

 sphere drives air into th'e lungs through the 

 windpipe and swells them up. When the 

 diaphragm is let go, the stretched lungs 

 return to their former size, emptying them- 

 selves of the extra quantity of air which they 

 had received. 



Suppose now the diaphragm were pulled down and 

 let go again regularly every few seconds : what would 

 happen ? Why, every time the diaphragm went down 

 a certain quantity of air would enter into the lungs, 

 and every tinie it was let go that quantity of air would 

 come out of the lungs again. 



