Foods are (1) substances which are taken into the alimentary canal, which can be absorbed from it, and after absorption serve to supply material for the growth of the body, or for the replacement of matter which has been removed from it; or (2) they are substances which can be oxidized in the body to yield energy for its use; or (3) substances, which by dissolving nutritive or waste matters facilitate the transfer of material from the receptive organs to the working, and from the working to the excretory. Foods to replace matters which have been oxidized must be themselves oxidizable; they are force generators, but may be and generally are also tissue formers: they are nearly always complex organic substances derived from other animals or from plants. Foods to replace matters not oxidized in the body, as water and salt, are force regulators, and are for the most part tolerably simple inorganic compounds. Among the force regulators we must, however, include certain foods, which, although oxidized in the body and serving as sources of energy, yet produce effects totally disproportionate to the amount of energy which they thus set free. Their influence as stimulants in exciting certain tissues to activity, or as agents checking the activity of parts, is more marked than their direct action as force generators. As examples, we may take condiments: mustard and pepper are not of much use as sources of energy, although they no doubt yield some when oxidized; we take them for their stimulating effect on the mouth and other parts of the alimentary canal, by which they promote a greater flow of the digestive secretions or an increased appetite for food. Thein, again, the active principle of tea and coffee, is taken for its stimulating effect on the nervous system rather than for the amount of energy which is yielded by its own oxidation.

Illustrate the use of common salt in helping to keep important substances in solution. Illustrate the employment of non oxidizable foods in constructing the body. What must foods supply to the body besides fuel? Why? Are oxidizable foods used in machinery construction? Give an example showing the gain of using non oxidizable matters when possible.

Give a definition of foods.

To the above definition of a food should be added the condition that, neither the substance itself nor any of the products of its chemical transformation in the tody shall be injurious to the structure or action of any organ; other-wise it would be a poison, not a food.

What foods must be oxidizable? What are they called? Do they also make tissue? Are they complex or simple? What is their source? What is meant by force regulators? Examples? Are they chemically complex or simple? What oxidizable foods are included among the force regulators? How is their influence chiefly exhibited? Give examples.

Alimentary Principles

The substances which in ordinary 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 i different foods 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 artisan and in the " delicacies i of the season." The chief foodstuffs, which are found repeated 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 principles may be conveniently classified as albumens, albuminoids, 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 what groups are alimentary principles classified?

Albuminous Alimentary Principles

Of the nitrogen-containing foodstuffs the most important are albumens: they are an essential part of all diets, and obtained both from animals and plants. The most common and abundant are myosin and syntonin, which exist in the lean of all meats; egg albumen; casein, found in milk and cheese; gluten and vegetable casein from various plants.

Albuminoid Alimentary Principles

These also contain nitrogen, but cannot entirely replace the proteids as foods; though a man can manage with less albumen when he has some albuminoids in addition. The most important is gelatine, which is yielded by the connective tissue and bones of animals when cooked. On the whole, 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.