ALIMENTARY PRINCIPLES. 299 



commonly call foods are, in nearly all cases, mixtures of 

 several foodstuffs, with, substances which are not foods at 

 all. Bread, for example, contains water, salts, gluten (a 

 proteid), some fats, much starch, and a little sugar; all true 

 foodstuffs: but mixed with these is a quantity of cellulose ) 

 (the chief chemical constituent of the walls which surround 

 vegetable cells), and this is not a food since it is incapable 

 of absorption from the alimentary canal. Chemical exami- 

 nation of all the common articles of diet shows that the 

 actual number of important foodstuffs is but small: they 

 are repeated in various proportions in the different tilings 

 we eat, mixed with small quantities of different flavoring 

 substances, and so give us a pleasing variety in our meals; 

 but the essential substances are much the same in the fare 

 of the workman and in the "delicacies of the season." 

 These primary foodstuffs, which are found repeated in so 

 many different foods, are known as ' ' alimentary principles;" 

 and the physiological value of any article of diet depends 

 on them far more than on the traces of flavoring matters - 

 which cause certain things to be especially sought after and 

 so raise their market value. The alimentary principles 

 may be conveniently classified into proteids, albuminoids, 

 hydrocarbons, carbohydrates, and inorganic bodies. 



Proteid Alimentary Principles. Of the nitrogenous 

 foodstuffs the most important are proteids: they form an 

 essential part of all diets, and are obtained both from 

 animals and plants. The most common and abundant are 

 myosin and syntonin which exist in the lean of all meats; 

 gg albumen; casein, found in milk and cheese; gluten and 

 vegetable casein frpm various plants. 



Albuminoid Alimentary Principles. These also con- 

 tain nitrogen, but cannot replace the proteids entirely as 

 foods; though a man can get on with less proteids when he 

 has some albuminoids in addition. The most important is 

 gelatin, which is yielded by the white fibrous, tissue of 

 animals when cooked. On the whole the albuminoids are 

 not foods of high value, and the calf's-foot jelly and such 

 compounds, often given to invalids, have not nearly the 

 nutritive value they are commonly supposed to possess. 



