OUR LUNGS 139 



firm. All the air you breathe goes from your nose or 

 mouth down to the lungs by this road ; but when a bit 

 of food or water tries to make the same journey, we 

 cough and almost strangle in trying to get it out. At 

 the same time we know that the trouble is with the little 

 swinging door of cartilage between the windpipe and 

 the food pipe. It did not shut down soon enough and 

 the food went the wrong way. 



You can see that the windpipe has two branches: 

 one goes to one lung and one to the other. After that, 

 each of these branches divides and keeps on subdivid- 

 ing into smaller and smaller branches and twigs, until 

 the real skeleton of the lung is a beautiful tree like 

 the one in the picture. 



The air sacs are too small to show in the picture, but 

 there is one of them on the end of each twig. They are 

 so tiny that nobody sees them without a microscope, and 

 there are so many of them that all the blood of the body 

 comes to them and goes away again. It brings carbon 

 dioxid gas when it comes, and it takes oxygen when it 

 goes away. 



That is why the lungs are so important: the blood 

 must have oxygen for the body to use. The reason for 

 breathing is now plainer than ever: whenever we take a 

 good breath of fresh air we send a quantity of this oxy- 

 gen into our lungs; and of course, the more we draw 

 in the deeper down it goes and the more the blood gets. 



