UREATHING.] 



PHYSIOLOGY, 



93 



It presses on the outside of the abdomen, and so 

 presses on the under surface of the diaphragm, and 

 drives it up into the chest as far as it will go. But it 

 will not go very far, because its edges are fastened 

 to the firm walls of the chest. The air also presses 

 on the outside of the chest, but cannot squeeze that 

 much, because its walls are stout. 



If the walls of the chest were soft and flabby, the 

 atmosphere would squeeze them right up, and so 

 through them press on the outside of the lungs ; since 

 they are firm it cannot. 1 . ^e chest walls keep the pres- 

 sure of the atmosphere off the outside of the lungs. 



The lungs then are pressed by the atmosphere on 

 their insides and not on their outsides ; and it is this 

 inside pressure which keeps them on the stretch or 

 expanded. When you blow into a bladder, you put 

 it on the stretch and expand it because the pressure of 

 your breath inside the bladder is greater than the pres- 

 sure of the atmosphere outside the bhdder. If, instead 

 of making the pressure inside greater than that outside, 

 you were to make the pressure outside less than that 

 inside, as by putting the bladder under an air-pump, 

 you would get just the same effect ; you would expand 

 the bladder. That is just what the chest walls do ; 

 they keep the pressure outside the lungs less than 

 that inside the lungs, and that is why the lungs, as 

 long as the chest walls are sound, are always expanded 

 and on the stretch. 



When you make a hole into the chest, and let the 

 air in between the outside of the lungs and the chest 

 wall, the pressure of the atmosphere gets at the out- 

 side of the lungs ; there is then the same atmospheric 

 pressure outside as inside the lungs ; there is nothing 



