EXPANSION OF THE L UNGjS. 239 



Why the Lungs remain expanded. We may best under- 

 stand this by considering a thin india-rubber bag, such as a 

 toy balloon. If the neck of the bag is open it collapses. 

 Why ? Because the atmosphere, which pushes on all 

 things near the earth's surface with a pressure of about 

 15 Ibs. on each square inch (1033 grams on each square cen- 

 timetre), presses equally on the outside and the inside of 

 the bag. These pressures balance one another, and the bag 

 collapses on account of its elastic contractility. 



We can expand such a bag in two ways. We may blow 

 air into it forcibly, and so make the pressure inside suffi- 

 ciently greater than the opposing aerial pressure outside to 

 overcome the elasticity of the bag. But we can also dis' 

 tend the bag, not by increasing the aerial pressure inside it, 

 but by diminishing that outside it. 



Suppose (Fig. 67) we tie our rubber bag (d) on the lower 

 end of the tube , which, like the tube c, passes 

 air-tight through a cork, and then fit the cork 

 tightly into the glass bottle A. The air will 

 then press with its full weight on the inside of 

 the bag through 1) and on the outside of it 

 through c, and the bag will remain collapsed. 

 If now we suck air out through c we diminish 

 its pressure on the outside of the bag without FlG - *?: Di ^- 



* gram mustrat- 



altering the atmospheric pressure on its inside: refa^shlpTof 

 tf will therefore begin to expand, because the Sorax gs in the 

 pressure inside it is no longer counterbalanced by the pres- 

 sure outside. The more air we suck out of c (that is, the more 

 we diminish the atmospheric pressure on the exterior of d) 



Why does a thin rubber bag collapse when its neck is open? 



How can we expand such an elastic bag? Describe a model illus- 

 trating the means by which the lungs are kept expanded in the chest- 

 and explain its action? 



