The food of man is derived from the three great kingdoms of nature : mineral, vegetable, animal. It may be conveniently classified into three principal groups : 1. Mineral constituents—incombustible or unoxidizable : water, phosphate of lime, chloride of sodium, etc. ; 2. Oxidizable—heat-producing and force-forming—carbon compounds : fat, sugar, starch, gum, etc. Nitrogenous — flesh-forming : albumen, fibrin, casein, etc. ; 3. Food adjuncts—alcohol, acids (citric, tartaric, etc.), alkaloids (caffeine, theine, etc.).

The members of the first group will be discussed hereafter, under the head of "agents promoting constructive metamorphosis" ; the second group, the most important, will be considered in this relation, with the foods ; and the third will have separate treatment under appropriate heads.

The classification of foods, originally formulated by Liebig, if not too strictly adhered to, is of much utility, as indicating the general purposes of these substances in the economy—viz.: carbonaceous or force-producers ; nitrogenous or flesh-formers. Under the first division are comprehended fat, starch, sugar, etc. ; under the second, substances containing nitrogen, as albumen, casein, etc. There is not, however, a rigid line of separation between these two classes, for both are more or less concerned in the functions attributed to each, but their most important position and office are as assigned by the classification.

The ultimate uses of food are two: to construct tissues or repair them when destroyed by wear; to supply force—muscular, nervous, secretory, etc. The reception, digestion, and absorption of food is known as the primary assimilation; the utilization of the material for the growth and repair of the tissues, and by the organs, as force, constitutes the secondary assimilation.

The first step in the primary assimilation is the mechanical subdivision of the food by mastication. The admixture of the saliva with the food facilitates the process of mastication, and, as it contains a ferment, ptyalin, which has the property of converting starch into sugar, a portion of this constituent undergoes conversion; but, probably, the chief use of the saliva is to give a slight alkaline reaction to the mass of food. According to the laws of osmosis, the entrance of an alkaline fluid into the stomach hastens the formation of the acid gastric juice. In the stomach, under the influence of the ferment, pepsin, and the acid (hydrochloric) of the gastric juice, the nitrogenous materials—the albuminous constituents of the food, the proteids—are transformed into peptones. Although fat is necessary to the stomach digestion, it does not undergo conversion in the stomach, and escapes in a coarse emulsion, with the chyme, into the duodenum. Starch, sugar, and gum, also pass into the chyme unchanged, although separated from their proteid envelopes, by the action of the gastric juice, except such portions as may diffuse directly into the stomach-veins. The peptones, to a large extent, diffuse into the blood from the stomach, and doubtless, also, such crystalloidal substances as sugar, to some extent. The chyme contains parapeptones, starch, fat, sugar, and refuse matter remaining undissolved. In the duodenum the acid chyme mixes with the alkaline intestinal and pancreatic juices and the bile, which are poured out freely as the materials from the stomach distend the canal. Here the conversion of starch into sugar takes place actively, and the fats are emulsionized and to some extent, also, saponified. The pancreatic juice not only emulsionizes the fats, but separates them into their component fat acids and glycerin, and the acids meeting alkaline bases form soaps, which are readily diffusible. The action of the bile is also very important. It renders the soaps formed soluble, and promotes the emulsionizing of the fats. Its agency in the digestion of the fats is well shown in the results of the experiment for forming a biliary fistula. When the bile is conveyed externally, the amount of fat entering the lacteals is much below normal, and instead, the fat appears in the stools. In the small intestine the proteids which escape conversion in the stomach are transformed, under the agency of the bile, pancreatic fluid, and intestinal juice, into peptones and other substances, the starch is converted into suger, and the fats are emulsionized, and in part, also, saponified. These nutrition materials diffuse into the portal veins and into the lacteals, so that, by the time the intestinal contents reach the ileo-caecal valve, they are composed, for the most part, of excrementitious matters and the refuse of the foods taken.

When the nutrition of the body goes on in the normal manner, there exists a certain ratio between the income and the outgo. The income consists of the proteids, fats, carbohydrates, salts and water of the food, together with the oxygen absorbed from the atmosphere. The outgo is made up of the excreta of the respiratory act, consisting of carbonic acid and water with a little hydrogen; of the perspiration, composed of water and salts; of the urine, which contains the nitrogen excreted from the body and a large quantity of saline matter; and of the faeces, composed of excreta from the immense glandular apparatus of the ileum and colon, and from the liver. In a perfectly healthy condition of the body, after it has attained its full growth, there should be an exact ratio between the income and outcome; the income should suffice to furnish the force necessary for the performance of the various functions and to repair the waste of the outcome. In an ideal dietary, the amount of the food should be sufficient to maintain this ratio at the normal standard.

As respects classification of foods, for the purposes of this work, they may be considered under the natural divisions of Animal and Vegetable.