CHAPTER X 

 THE FOOD-PREPARING SYSTEM 



Digestion. — In an earlier chapter (VII) we saw that 

 the complicated materials which are built up by plants 

 and stored within them have to undergo a breaking-down 

 into simpler compounds in order that they may be moved 

 about from place to place, or in order to make them use- 

 ful for the purposes of the plant. This breaking-down 

 of the complicated into the simpler substances we have 

 learned to call digestion. The food of animals consists 

 either of the protoplasmic body substance of animals or 

 plants or of the materials which they have manufactured 

 and stored away within themselves. Most of these ma- 

 terials are too complicated to be used by the cells of the 

 animal body in precisely the form in which they are 

 taken in, and so must undergo digestion before they 

 can be utilized. 



Classes of Foods. — As we have already learned, the 

 many kinds of substances called foods can be grouped 

 into a few classes. In the first place, we set off the foods 

 proper, which can yield energy or be built into proto- 

 plasm, from those which cannot; the former are often 

 called nutrients, while the foods which are of use for 

 other purposes are called food accessories. In the class 

 of food accessories fall inorganic materials like ordinary 

 table salt and the lime salts, which have to be taken by 

 young children in order that their bones may be properly 

 formed, and the series of organic substances whose im- 

 portance we have only lately commenced to realize, the 

 vitamins. These are complex organic substances which 

 are not nutritious in the sense that they furnish us with 



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