BREATHING.] 



PHYSIOLOGY. 



91 



up towards the chest, so that the under surface at 

 which you are looking is quite hollow. If you hold 

 the rabbit up by its hind legs with its head hanging 

 down, and pour some water into the abdomen, quite 

 a little pool will gather in the shallow cup of the 

 diaphragm. 



In the rabbit the diaphragm is very transparent ; 

 you can see right through it into the chest, and you 

 will have no difficulty in recognizing the pink lungs 

 shining through it. You will notice that they 

 cover almost all the diaphragm — in fact they 

 fill up the whole of the cavity of the chest 

 that is not occupied by the heart. 



If you seize the diaphragm carefully in the middle 

 with a pair of forceps, and pull it down towards 

 the abdomen, you will find that you cannot create 

 a space between the lUngs and the diaphragm, but 

 that the lungs follow the diaphragm, and are quite as 

 close to it when it is pulled down as when it is 

 drawn up. . * 



In other words, when the diaphragm is 

 arched up as you find it on opening the abdo- 

 men, the lungs quite fill the chest ; and when 

 the diaphragm is drawn down ^nd the cavity 

 of the chest made biggei^ the lungs swell 

 out so that they still fill up the chest. 



40. How do they swell out? By drawing air in 

 through the windpipe. If you listen, you will perhaps 

 hear the air rush in as you pull the diaphragm down — 

 and if you tie the windpipe, or quite close up the nose 

 and mouth, you will find it much harder to pull down 

 the diaphragm, because no fresh air can get into the 



lungs. -v.^ -;-..■::,-?.,. ;-.^ -■..■■ ■ _.,.•■•-.-,„; 



