Page:Popular Science Monthly Volume 3.djvu/456

442 which, when introduced into the body, supplies material which renews some structure or maintains some vital process; and it is distinguished from a medicine in that the latter modifies some vital action, but does not supply the material which sustains such action.

This is certainly correct so far as relates to the substances which supply nearly all our nourishment, and which the Germans class under the term Nahrungsmittel, but there are certain so-called foods known as Genussmittel, which seem to form a connecting link, in that they increase vital actions in a degree far beyond the amount of nutritive material which they supply. They thus resemble certain medicines in their action, but, as they supply a proportion of nutritive material, they should be ranked as foods.

It is essential to the idea of a food that it should support or increase vital actions; while medicines usually lessen, but may increase, some of them.

It is not necessary that a food should yield every kind of material which the body requires, for then one might suffice for the wants of man, but that it fulfils one or more of such requirements, so that by a combination of foods the whole wants of the body may be supplied. Neither is it essential that every food should be decomposed or broken up, and its elements caused to enter into new combinations when forming or maintaining the structures of the body, since there are some which in their nature are identical with parts of the body, and, being introduced, may be incorporated with little or no change.

But there are foods which are more valuable to the body than others, in that they supply a greater number of the substances which it requires, and such are known as compound foods, while others, which supply but one element, or which are incorporated without change, may be termed simple foods. Other foods are more valuable because they are more readily changed into the substance of the body, or act more readily and quickly in sustaining vital actions, and these may be called easily-digested or easily-assimilated foods. Others are preferred because they supply a greater quantity of useful nutriment at a less proportionate cost, and are known as economical foods; and foods varying in flavor are classed as more or less agreeable foods.

Some foods are classed according to the source whence they are derived, as animal and vegetable foods; and others according to the density of their substance, as fluid and solid foods.

There are foods which nourish one part of the body only, and others which sustain one chief vital action, and are called flesh-forming or heat-forming foods, while others combine both qualities.

Besides these larger divisions, there are qualities in foods which permit of further classification, such as those which render them particularly fit for different ages, climates, and seasons, and others which possess a special character, as sweetness, acidity, or bitterness.

There are also effects produced by foods apart from or in addition