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TEXT-BOOK OF PHYSIOLOGY. 



is phosphate of lime, and this is the chief source of this agent in the formation 

 of bones. Sodium and potassium chlorids are also present. 



Eggs are also to be regarded as complete natural foods, inasmuch 

 as they contain all the necessary food principles. The analysis given in 

 the foregoing table represents the composition of the entire egg. The 

 white of the egg contains 12 per cent, of protein and 2 per cent, of fat. 

 The yolk, however, contains 1 5 per cent, of protein and 30 per cent, of fat. 



Composition of Cereal Foods. The average composition of the 

 principal cereals is shown in the following table: 



That the cereals are most important and useful articles of diet is evident 

 from their composition, consisting, as they do, of proteins and carbohydrates 

 in large proportion. Owing to the cellulose or woody fiber which envelops 

 and penetrates the grain, they are somewhat difficult of digestion. A section 

 of a grain of wheat shows the external cellulose envelope, the husk, beneath 

 which is a layer of large cells containing the chief protein the gluten. 

 The interior of the grain consists of small cavities, the walls of which are 

 formed of cellulose and which contain the granules of starch, fat, small 

 quantities of protein, and inorganic salts. All other cereals have a similar 

 structure. 



In the preparation of white flour from wheat it is customary to remove 

 the husk, a process which involves the removal also of a portion, if not all, 

 of the gluten cells, so that such flour contains less nitrogenized material 

 than the original grain. It is possible, however, in the milling of wheat, 

 to remove only the husk and retain the gluten in the flour, as in the prepa- 

 ration of whole wheat flour. 



Bread is an artificially prepared food made either of wheat or rye. 

 Owing to the fact that the proteins of the other cereals do not possess the 

 same adhesive properties when kneaded with water, they cannot be used 

 for bread-making purposes. In the making of bread, the flour is kneaded 

 with water until a glutinous mass dough is formed. During this process, 

 salt, sugar, and yeast are added. It is then kept in a temperature of about 

 100 F. In the presence of heat and moisture the natural ferment of the 

 flour diastase converts a portion of the starch into sugar, which in turn 



