Page:Popular Science Monthly Volume 3.djvu/460

446 of the aid derived from our appetites, there is the great advantage of having foods which contain a proportion of nearly all these elements; and combinations of foods have been effected by experience which protect even the most ignorant from evil consequences.

Thus flesh, or the muscular tissue of animals, contains precisely the elements which are required in our flesh-formers, and, only limited by quantity, our heat-generators also; and life may be maintained for very lengthened periods upon that food and water when eaten in large quantities. Seeing, moreover, that the source of flesh in animals which are used as food is vegetables, it follows that vegetables should have the same elements as flesh, and it is a fact of great interest that in vegetables we have foods closely analogous to the flesh of animals. Thus, in addition to water and salts, common to both, there is vegetable jelly, vegetable albumen, vegetable fibrine, and vegetable caseine, all having a composition almost identical with animal albumen, gelatine, chondrine, and caseine.

Hence our appetites and the bountiful provision made for us extend our choice to both the vegetable and animal kingdoms, and it is possible to find vegetable foods on which man could live as long as upon animal food alone. Bread is in vegetable foods that which flesh is in animal foods, and each within itself contains nearly all the elements required for nutrition.

When, however, we bring knowledge of a special kind to the aid of our appetites, we are able to discover both the deficiencies in any given food and the kind of food which would meet them. Thus a knowledge of the requirements of the system and of the available uses of food leads to the proper combinations of food, or to the construction of dietaries.

We have thus placed face to face the requirements of the body and the qualities of the foods to be used to supply them, but it is of very common observation that the effect of the supply is but temporary, and needs renewal at definite periods. Hence we show that the needs of the body are tolerably uniform, while the effect of the supply is temporary, or that both the need and the supply are intermittent. This may be readily represented by showing the line of change in the degree of vital action on the body during the twenty-four hours, as produced by my own investigations, and delineated in the graphic diagrams of the present work.

It is there illustrated that, during the repose of the night, the amount of vital action, as shown by the respiration and pulsation, is low and tolerably uniform, while under the influence of food it is high, and varies during the day extremely, but the general course is such that a large increase takes place after a meal, and a considerable decrease before the following meal. This increase, followed by decrease, being due to the action of food, proves that the influence is temporary, and that after a sufficient interval another supply of food is required.