106 THE BTJMAN BODY. 



are two chief sets of these one to receive oxidizable things, 

 and the other to receive oxygen. The first set is represent- 

 ed by the alimentary canal, consisting of mouth, gullet,* 

 stomach, and intestines. It takes in food and drink. The 

 second set consists of the lungs, with the air passages lead- 

 ing to them ; their business, as receptive organs, is to 

 absorb oxygen. 



The organs whose duty it is to get rid of waste materials 

 formed in the body are called excretory organs. The three 

 most important excretory organs are the lungs, the kidneys, 

 and the skin; the lungs pass carbon dioxide gas out to the 

 air, and also water ; the kidneys get rid of urea and water ; 

 and the skin, of water and a little urea. 



The Intermediate Steps between Reception and Ex- 

 cretion. Between the taking of oxidizable substances into 

 our mouths and oxygen into our lungs, on the one hand, 

 and the return of oxidized matters from our bodies to the 

 surrounding world on the other, a great many intermediate 

 steps take place. The alimentary canal (see Fig. 1) is a 

 tube which runs through the body but nowhere opens into 

 it ; so long as food lies in this tube it therefore does not 

 really form a part of the body, and is of no use to it : it 

 resembles coals in the tender of a locomotive, waiting to be 

 transferred to the furnace. In our bodies the furnace is 

 everywhere; wherever there is a living tissue things are 



What are the functions of the two chief sets? Name those con- 

 cerned in receiving oxidizable things. Those whose business it is to 

 absorb oxygen. 



What is meant by the excretory organs? Name the most im- 

 portant. What does each get rid of? 



Why is food in the stomach not really a part of the body? To 

 what may we liken it? Where is the furnace of the body? Why 

 must food be carried all over the body? 



* The technical name for the gullet is oesophagus. 



