362 THE HUMAN BODY AND ITS FOOD 



fatty acids, glycerine, maltose, etc., which are formed by 

 digestion, are not found in the blood. The blood does 

 contain fats, formed out of the glycerine and the fatty 

 acids ; proteids, formed out of the peptones and proteoses ; 

 and grape sugar, formed out of the maltose. From this it 

 may seem that absorption, so far as the fats and proteids 

 are concerned, undoes the work of the other stages of 

 digestion. But we must remember that the complex 

 molecules of our food are not the actual nutrients that the 

 cells can use, although they contain the " pieces " (small 

 molecules) out of which the body's fats and proteids can be 

 constructed. Digestion must be looked upon not simply 

 as a breaking down of large molecules, but also as a 

 recombining of the small molecules of digested food to 

 form the special fats and proteids of the body. 



The last stage in the digestive process is called assimila- 

 tion, which means "making similar to." Assimilation 

 takes place at the cells. Here the proteid brought by the 

 blood and lymph becomes a part of the living protoplasm. 

 Here also, in some mysterious way, the cell protoplasm 

 causes the sugar and fat, and, to some extent, the pro- 

 teids, to unite with oxygen (cf. 52). It is this chemical 

 reaction that sets free the energy which the cells must 

 have to do their work, and which keeps the body warm. 



Storage of Food. We eat only at intervals, while the body needs 

 food all the time. Hence there must be ways in which food can b 

 stored up during the time of abundance, in order that the body may 

 have a supply to draw upon when its cells wear out, and its energy is 

 low. We have already learned that carbohydrates are stored up by 

 the liver (cf. 366) as glycogen. The muscles also store glycogen. 

 When dextrose is taken from the blood, by the cells, glycogen is 

 changed to sugar to keep up the blood's supply. 



