nary food-materials may be succinctly classified as follows:

1. Edible Substance: e. g., the flesh of meats and fish, the shell-contents of oysters.

2. Refuse: e. g., bones of meat and fish, shells of oysters.

The edible substance consists of

1. Water. 2. Nutritive Substances or Nutrients. The refuse may, for our present purpose, be left out of account, and our attention confined to the edible substance. And, as the water which forms a part of the edible substance, though indispensable, is nevertheless inexpensive and distinct from the nutritive ingredients, we may consider simply the nutrients.

Speaking as chemists and physiologists, we may say that our food supplies, besides mineral substances and water, albuminoids, carbohydrates, and fats; and that these are transformed into the tissues and fluids of the body, muscle and fat, blood and bone, and are consumed to produce heat and force. Viewed from a chemico-physiological stand-point, then, the nutritive ingredients of food can be classified as follows. Of the actually nutritive substances or nutrients of foods the most important groups (exclusive of water) are

1. Protein (proteids, albuminoids, etc.): e. g., albumen ("white") of egg, fibrin of blood, "lean" of meat, gluten of wheat.

2. Fats: e. g., fat of meat, butter, olive-oil. 3. Carbohydrates: e. g., starch, sugar, gly

cogen.

4. Mineral Matter or Ash: e. g., calcium and potassium phosphates and chlorides.

The terms protein, proteids, and albuminoids, are applied somewhat indiscriminately, in ordinary usage, to several or all of certain classes of compounds characterized by containing nitrogen. The most important are the proteids or albuminoids, of which albumen, the white of egg, and myosin, the basis of muscle, are types. Allied to these, but occurring in smaller proportions in animal tissues and foods, are the nitrogenous compounds that make the basis of connective and other tissues. Gelatin is derived from some of these tissues, and may be taken as a type of the compounds of this class. As these constituents are of similar constitution and have similar or nearly similar uses in nutrition, it is customary to group them together as protein. The muscular tissues of animals, and hence the lean portions of meat, fish, etc., contain small quantities of so-called nitrogenous extractives-creatin, carnin, etc. (contained in extract of meat, etc.) which contribute materially to the flavor and somewhat to the nutritive effect of the foods containing them. They are not usually deemed of sufficient importance, however, to be grouped as a distinct class in tabular statements of the composition of foods. Details regarding the nature and functions of the several classes of nutrients may be found in the article in the "Annual Cyclopædia" for 1881, referred to above. Concerning their

composition, it will suffice, then, to state that' the compounds classed together as protein contain carbon, oxygen, hydrogen, and nitrogen, while the carbohydrates and fats contain no nitrogen, but consist chiefly of carbon, oxygen, and hydrogen. The fats are much richer in carbon than the carbohydrates. Animal foods, as meats, fish, etc., contain but little of carbohydrates, their chief nutrients being protein and fats. Milk, however, and some shell-fish, as oysters, scallops, etc., contain more or less of carbohydrates. Vegetable foods, as wheat, potatoes, etc., contain less protein and consist largely of starch, sugar, cellulose, and other carbohydrates.

Functions of Nutrients.-The different nutrients have different offices in nourishing the body, in building up its tissues, repairing its wastes, and serving as fuel to produce animal heat and muscular and intellectual energy. The chief part borne by each in nutrition is shown below:

The protein of food

The fats of food

forms the (nitrogenous) basis of blood, muscle, connective tissue, etc.

is transformed into fats and carbohydrates, and stored as such in the body.

is consumed for fuel.

are stored as fat.

are consumed for fuel. The carbohy- are transformed into fat.

drates of food are consumed for fuel.

In classifications formerly maintained and frequently met with still, the protein compounds were regarded as the "flesh-formers " and the sources of muscular energy, while the carbohydrates and fat were looked upon a "fat-formers" and "heat-producers." A vast deal of painstaking research, however, has shown that these distinctions were not correctly drawn. The albuminoids are flesh-formers, it is true; indeed, flesh, i. e., muscular and other nitrogenous tissue, according to the nearly unanimous testimony of the most trustworthy experiments, is made from the nitrogenous constituents of the food exclusively. But the balance of testimony is decidedly against the production of muscular energy by nitrogenous compounds exclusively or mainly. Each of the three groups of nutrients probably shares, directly or indirectly, in this function. So, too, it appears that the combustion which produces animal heat is not confined to the carbohydrates and fats, but the protein compounds, or the products of their decomposition, are also used for this purpose. Again, the production of fat in the body was formerly ascribed to the fats and carbohydrates alone. The view was held at the same time, and is still maintained by some physiologists, that the carbohydrates can not be transformed into fats, and that a very large part of the fat of the body is formed from the disintegration of the albuminoids. The weight of evidence to-day is decidedly in favor of the assumption that all three of the great classes of nutrients in our foods-the

albuminoids, the carbohydrates, and the fats -are transformed into fat, and that the fat thus formed is consumed, either before or after being stored as body-fat.

It appears, then, that protein is the most important constituent of our food, because, while it performs the functions of each of the other two chief nutrients in being transformed into fat and in being consumed for fuel, it has a most weighty office of its own in forming the basis of the blood and in building up the muscular and other nitrogenous tissues, an office which no other nutrient can perform at all. And, as we shall see further, in examining the pecuniary cost, protein is the dearest as well as most important of the ingredients of foods.

The same experimental research which has revealed to us the ways in which our food supplies our bodily wants, has shown us how to estimate the relative nutritive values of different foods from their chemical composition. The estimates are only approximate, because the nutritive effects are influenced by various conditions, some of which research has not yet definitely explained, while others vary with the nature of the food or of the user, so that the value of a given food in a given case may vary from the standard set by the analysis. These sources of uncertainty are nevertheless so narrowed down by late investigation, and the errors confined within such limits, that by intelligent use of the facts at our disposal we may judge very closely from the chemical com

position of a food what is its value as compared with others of the same class, at any rate, for our nourishment.

Chemical Analysis of Foods. The following tables give the composition of a number of the more important kinds of animal and vegetable foods. The details will perhaps be best explained by an example. A sample of beef, sirloin, of medium fatness, was found to consist of about one fourth bone and three fourths flesh, edible substance. The flesh was analyzed and found to contain, nearly: water, 60 per cent.; protein, 19 per cent.; fats, 20 per cent.; mineral matters, 1 per cent. Calculated upon the whole sample of meat, of which one fourth, or 25 per cent., was bone and other refuse, and 75 per cent. flesh, the analysis would stand as in the schedule below, in which the composition of the flesh by itself and that of the meat, bone, and all, are both given:

Table I, on page 345, gives the composition of a number of animal foods, mostly from late American analyses. It is only a short time since analyses of American meats, fish, etc., have been undertaken in any considerable number, and those as yet accomplished are far from sufficient for a complete survey of the subject. Indeed, the work already done can be regarded only as a beginning. Still, the figures will give a tolerably fair idea of the composition of the articles named.

The analyses of this table, with the exception of a few from European sources and indicated by italics, are selected from the results of the investigation referred to above, as conducted under the auspices of the Smithsonian Institution and the United States Fish Commission. The specimens of meats were purchased from a dealer in Middletown, Conn., and said by him to be "fair average samples of the better kinds

Per cent. Per cent. Per cent. Per cent. Per cent, 45 80 14.8 15 0.7

of meats." A side of beef, freshly brought in winter from Chicago, and said to be a good specimen of first-class "Chicago beef," was cut into about twenty-five pieces in the ordinary way. From each a sample fairly representing the whole cut was taken and analyzed. Thus the composition of each piece and of the whole side was learned. The composition of one of the leanest portions, the round, a moderately fat piece, sirloin, a very fat portion, flank, and of the whole side, together with a tongue, liver, and heart from another animal, are given in the table. The samples of a side of mutton and of parts of the same side were obtained and analyzed in like manner, as were those of the other meats and fowl. The speci

*The tables contain also columns for carbohydrates, etc., which occur in milk and in some shell-fish, but are not found in ordinary meats in sufficient amount to warrant their inser. tion in such tables as these.