92 SCIENCE PRIMERS, [§ vi. 



Now prick a hole through the diaphragm into the 

 cavity of the chest, without wounding the lungs. You 

 will hear a sudden rush of air, and the lungs will 

 shrink up almost out of sight. They are no longer 

 close against the diaphragm as they were before ; and 

 if you open the chest you will find that they have 

 sh/unk to the back of the thorax as you saw them in 

 the first rabbit. The rush of air is partly a rush of 

 air out of the lungs, and partly a rush of air into the 

 chest between the chest walls and the outside of the 

 lungs. 



But before you lay open the chest, pull the dia- 

 phragm up and down as you did before you made the 

 hole in the diaphragm. You will find that you have 

 no effect whatever on the lungs. They remain per- 

 fectly quiet, and do not swell up at all. By working 

 the diaphragm up and down, you only drive air through 

 the hole you have made, in and out of the cavity 

 of the chest, not in and out of the lungs as 

 you did before. 



We see then that the chest is an air-tight 

 chamber, and that the lungs, when the chest walls 

 are whole, are always on the stretch, are on the 

 stretch even when the diaphragm is arched up as high 

 as it can go. # ' 



Why is it that the lungs are thus always on the 

 stretch ? Because the chest is air-tight, so that no air 

 can get in between the outside of the lungs and the 

 inside of the chest wall. You know from your 

 Physics Primer (Art. 29, p. 34) that the atmosphere 

 is always pressing on everything. It is pressing 

 on all parts of the rabbit ; it presses on the inside 

 of the windpipe and on the inside of the lungs. 





