318 THE HUMAN BODY. 



plex substances in it, the food must contain them either in 

 the form of such complex substances, or in forms which the 

 Body can build up into them, Free nitrogen and hydrogen 

 are no use as foods, since they are neither oxidizable under 

 the conditions prevailing in the Body (and consequently can- 

 not yield it energy), nor are they capable of construction by 

 it into its tissues. (2) Food after it has been swallowed is 

 still in a strict sense outside the Body; the alimentary canal 

 is merely a tube running through it, and so long as food lies 

 there it does not form any part of the Body proper. Hence 

 foods must be capable of absorption from the alimentary 

 canal; either directly, or after they have been changed by the 

 processes of digestion. Carbon, for example, is useless as 

 food, not merely because the Body could not build it up into 

 its own tissues, but because it cannot be absorbed from the 

 alimentary canal. (3) Neither the substance itself nor any 

 of the products of its transformation in the Body must be 

 injurious to the structure or activity of any organ. If so it 

 is a poison, not a food. 



Alimentary Principles. The articles which in common 

 language we call foods are, in most 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 examination of all the common 

 articles of diet shows that the actual number of important 

 foodstuffs is but small: they are repeated in various propor- 

 tions in the different things we eat, mixed with small quan- 

 tities of different flavoring substances, and so give us a pleas- 

 ing 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 espe- 

 cially sought after and so raise their market value. The 

 alimentary principles may be conveniently classified into 



