It is impossible to assign any especial office to the different food principles; that is, it cannot be said that the carbohydrates perform a certain kind of work in the body and nothing else, or that the proteids or fats do. The human body is a highly complex and intricate organism, and its maintenance is carried on by complex and mysterious processes that cannot be followed, except imperfectly; consequently, we must regard the uses of foods in the body as more or less involved in obscurity. It is, however, generally understood that the proteids, fats, and carbohydrates each do an individual work of their own better than either of the others can do it. They are all necessary in due amount to the nutrition of the body, and doubtless work together as well as in their separate functions. They are, however, sometimes interchangeable, as, for instance, in the absence of the carbohydrates, proteids will do their work. The carbohydrates are eminently heat and energy formers, and they also act as albumen sparers.

The body always has a store of material laid by for future use. If it were not for this a person deprived of food would die immediately, as is the case when he is deprived of oxygen. (Air being ever about us, and obtainable without effort or price, there is no need for the body to lay by an amount of oxygen; consequently only a very little is stored, and that in the blood.)

The great reserve forces of the body are in the form of fatty tissues, and glycogen, or the stored-away carbohydrates of the liver; the latter is given out to the body as it is needed during the intervals of eating to supply material for the heat and energy of daily consumption, and in case of starvation. That they are true reserves is shown by the fact that they disappear during deprivation of food. The glycogen, or liver-supply, disappears first; then the fat (Martin). The heat of the body can be maintained on these substances, and a certain amount of work done, although no food except water be taken.

The principal function of the liver is to form glycogen to be stored away. It constantly manufactures it, and as constantly loses it to the circulation. Glycogen is chemically allied to starch, having the same formula (C6H10O5), but differing in other ways. Its quantity is greatest about two hours after a full meal; then it gradually falls, but increases again when food is again taken. Its amount also varies with the kind of food eaten: fats and proteids by themselves give little, but starch and sugars give much, for it is found in greatest quantity when these form a part of the diet.

Inorganic Matter and Vegetable Acids. Water and other inorganic matter, as the salts of different kinds, and vegetable acids, as vinegar and lemon-juice, can scarcely be said to be digested. Water is absorbed, and salts are generally in solution in liquids and are absorbed with them.

Water is found in all parts of the body, even in the very solid portions, as the bones and the enamel of the teeth; it also constitutes a large proportion of its semisolids and fluids, some of which are nearly all water, as the perspiration and the tears.

Water usually is found combined with some of the salts, which seem to act as regulators of the amount which shall be incorporated into a tissue. Water is a necessary constituent of all tissues, giving them a proper consistency and elasticity. The power of resistance of the bones could not be maintained without it. It is also valuable as a food solvent, assisting in the liquefying of different substances, which are taken up by the various absorbent tubes, conveyed into the blood, and so circulated through the body. Most of the water of the body is taken into it from without, but it is also formed in the body by the union of hydrogen and oxygen.1

Sodium chlorid, or common salt, is found in the blood and other fluids, and in the solids of the body, except the enamel of the teeth; it occurs in greatest proportion in the fluids. The part that this salt plays in nutrition is not altogether understood. "Common salt is intermediate in certain general processes, and does not participate by its elements in the formation of organs" (Liebig). Salt is intimately associated with water, which plays an intermediate part also in nutrition, being a bearer or carrier of nutritious matters through the body.

Salt seems to regulate the absorption and use of nutrients. It is found in the greatest quantity in the blood and chyle. It doubtless facilitates digestion by rendering foods more savory, and thus causing the digestive juices to flow more freely. Sodium chlorid is contained in most if not all kinds of food, but not in sufficient quantity to supply the wants of the body; it therefore becomes a necessary part of a diet.

Potassium chlorid has similar uses to sodium chlorid, although not so generally distributed through the body. It is found in muscle, liver, milk, chyle, blood, mucus, saliva, bile, gastric juice, and one or two other fluids.

Calcium phosphate is found in all the fluids and solids of the body, held in solution in them by the presence of C02; both it and calcium carbonate enter largely into the structure of the bones.

Sodium carbonate, magnesium phosphate, and other salts play important parts in nutrition.

1 Martin.

The various salts influence chemical change as well as act in rendering food soluble. For example, serum albumen, the chief proteid of the blood, is insoluble in pure water, but dissolves easily in water which has a little neutral salts in it.1 Salts also help to give firmness to the teeth and bones.

To recapitulate, food is eaten, digested, assimilated, and consumed or transformed in the body by a series of highly intricate and complex processes. It is for the most part used for the different powers and activities of the system; there is, however, always a small portion which is rejected as waste. The first change is in the mouth, where the food is broken up and moistened and the digestion of starch begins; these changes continue in the stomach until the whole is reduced to a more or less liquid mass. As the contents of the stomach pass little by little into the duodenum, the mass becomes more fluid by the admixture of bile, pancreatic juice, and intestinal juice, and, as it passes along, absorption takes place; the mass grows darker in color and less fluid, until all good material is taken up and only waste left, which is rejected from the body.