230 THE LUNGS'. 



are about to swallow, the motion of the tongue 

 pulls down this covering and closes up the open- 

 ing with the utmost nicety. 



How wonderful and beautiful this is, as we are 

 not sensible of it; and if before swallowing we had 

 always to think and exercise our will in shutting 

 this aperture, how different would be the process 

 of eating, and how frequently must accidents hap- 

 pen from our forgetfulness ! 



Why is it that breathing is so absolutely essen- 

 tial to our life ? For this reason, that the blood, 

 in its passage through the body, is changed, be- 

 comes dark coloured, and is no longer fit to sup- 

 port life. In this state it reaches the heart, and 

 before it is sent back into the arteries, it is passed 

 through the lungs. Here it is purified by an ex- 

 posure to the air, through the thin linings of the 

 air-cells, with which the lungs are quite filled, and 

 all of which communicate with each other, and 

 with the windpipe. After this exposure, the blood 

 becomes of a bright red colour, is taken back to 

 the heart, and thrown into the body for its support. 



The blood being thus changed, let us see what 

 change has been undergone by the air in our 

 lungs. If you put a piece of quill into the nozle 

 of a pair of bellows, and blow into a cup of lime 

 water, you will find no change in its appearance. 

 You blow into it just the same kind of air which 

 you draw into your lungs. But now put the quill 

 into your mouth, and blow into the lime water, 

 and you will see that it becomes turbid and white, 



