CHAPTER VII 

 DIGESTION IN THE MOUTH 



General Purpose of Digestion. The body is by no means a 

 permanent structure consisting always of the same molecules of 

 its constituents. It is a living, changing mass of material, fairly 

 uniform in its percentage composition, to be sure, but constantly 

 wearing out, and being rebuilt or repaired. The cells per- 

 form much work, they build up substances for secretion, they 

 produce heat by the oxidation of their own constituents or mate- 

 rials supplied to them, and they perform mechanical work. 

 To do all these things the cells require fuel, and materials out 

 of which to construct their products. Thus a living organism 

 must take food. For purposes of producing heat or mechanical 

 work the materials required are not restricted to particular 

 chemical substances. It is enough that the materials supplied 

 be such that they can enter the cells, and be "burned" by 

 them. For purposes of repair or building specific products the 

 situation is different, however. Unless the body can build up a 

 constituent such as a given amino acid required for the manu- 

 facture of a necessary substance, that particular constituent 

 must be supplied in the food. We are beginning to realize that 

 definite building stones are demanded by the cells for the con- 

 struction of their products. In some cases the body itself is able 

 to manufacture the necessary building stones from other ma- 

 terials. In other cases it is not able to do so, and serious conse- 

 quences result if the required building stone is not supplied in 

 sufficient amounts. This is another argument in favor of a 

 varied diet which will be likely to furnish all substances re- 

 quired. 



The materials of our food correspond only in a rough way to 

 the materials out of which our body is constructed. Many of 



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