CHAPTER XII 

 THE RESPIRATORY ORGANS 



The daily distribution of food materials in a city seems 

 a wonderful accomplishment; delivery of milk and eggs, for 

 example, from thousands of farms to, perhaps, a million 

 inhabitants, scattered in thousands of houses, apartments 

 and flats is made at least every twenty-four hours. In the 

 body, however, there are more than a million times as many 

 cells as there are people in any city on earth; yet each cell re- 

 ceives its food much more often than once a day. 



Besides food, another substance is delivered to the body 

 cells by the blood; this is oxygen. 



THE FUNCTION OF RESPIRATION 



No living thing, animal or plant (except a few bacteria) can 

 subsist without it. Unicellular and many other lower organ- 

 isms absorb it directly through the body surface; others (in- 

 sects) are provided with branching tubes which lead air from 

 the exterior to the internal cells; while others have special 

 breathing organs, such as lungs or gills, from which oxygen is 

 absorbed by the blood. 



To understand why we need oxygen we have only to return 

 to the comparison of the body to a furnace. If all the dampers 

 in a stove are closed, the fire burns slowly, or else goes out. If 

 all the cracks and joints in the stove could be closed air-tight, 

 the fire would never burn. If the drafts are opened and air let 

 in, the fire burns rapidly. It is the union of the wood or coal 

 with oxygen which results in its "burning," and giving off heat. 



Of course there is no flame in the body; the food materials 

 inside it never glow like coals, but they do combine with 

 oxygen and give off heat, very slowly it may be, but none the 

 less certainly. The process is essentially the same as that 



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