116 TEE HUMAN BODY. 



products of its chemical transformation in the "body shall he 

 injurious to the structure or action of any organ] other- 

 wise it would he a poison, not a food. 



Alimentary Principles. The substances which in ordi- 

 nary language we 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 these 

 are true foodstuffs, but mixed with them 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 proportions in the 

 different foods we eat, mixed with small quantities of dif- 

 ferent 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 artisan and in the " delicacies 

 of the season." The chief foodstuffs, which are found re- 

 peated in many different foods, are known as "alimentary 

 principles" and the nutritive value of any article of diet 

 depends on the proportion of these foodstuffs which is 

 present, far more than on the various agreeable flavoring 

 matters which cause certain things to be especially sought 

 after, and to have a high market value. Alimentary prin- 

 ciples may be conveniently classified into proteids, albu- 

 minoids, hydrocarbons, carbohydrates, and inorganic bodies. 



What is a poison? 



What are ordinary foods? Give an example. Why is cellulose 

 not a food? What does chemical examination of ordinary foods 

 show? How do we get variety in our foods? What are "alimentary 

 principles''? On what does the nutritive value of a food depend? 

 Into wLat groups are alimentary principles classified? 



