The digestion, absorption, and distribution of food are not its use, - they are the preliminaries necessary to use. Not until the nutrients have been converted to available forms and have passed into the blood do they in the slightest degree furnish energy or building material to the animal organism. We have followed to a certain extent the chemical changes which the digested food suffers, but no detailed statements have been made as to the part taken by each class of nutrients in constructing the human body and in maintaining its complex activities.

152. Food Used In Two General Ways

The animal organism uses food in two general ways: viz., for constructive purposes, which involve the building or repair of tissue and the formation of milk; and as fuel for supplying different forms of energy mainly through oxidation of the food nutrients. The tissues which are to be formed are of several kinds; principally the mineral portion of the bone; the nitrogenous tissue of the muscles, tendons, skin, hair, and various organs and membranes; and the deposits of fat which are quite generally distributed throughout the body substance.

163. Forms Of Energy

Energy in the forms in which it is used by the animal organism may appear as muscular activity, such as walking, working, breathing, the beating of the heart, the movements of the stomach and intestines, as heat, and as chemical energy necessary for carrying on digestion and other metabolic changes. The human body is certainly the seat of greatly varied and complex constructive and destructive activities, which are sustained by the matter and potential energy of the food. How this is done we do not fully understand, but we know many facts which are of great scientific and practical importance and which must be consciously or unconsciously recognized if we would not come into conflict with immutable laws.

154. Functions Of Water

Water fills an important place in the nutrition of all forms of life. In both plants and animals it acts as a solvent of the building materials which it carries from one part of the organism to another. It also serves as a carrier of wastes, particularly those excreted through the kidneys, and the free use of water is recommended as promoting thorough cleansing of the tissues. It is proper to speak of water as building material for the animal body, for it is an abundant constituent of animal tissue and takes part in chemical changes such as hydrolysis. It fills an essential office in regulating the heat processes of the body through varying rates of evaporation (see p. 168).

155. Functions Of The Mineral Compounds

We have learned that mineral compounds are abundant in the human body. The tissues, the blood, digestive fluids, and especially the body framework, contain a variety of these bodies, which are as essential as any other substances to the building and maintenance of the animal organism. Bone formation without phosphoric acid and lime is not possible, and to deprive the gastric juice of the chlorine which it contains would be to destroy its usefulness. Sodium salts are an essential part of the bile, iron must be supplied to the haematin of the blood, and potassium phosphate is always present in muscular tissue. Children would fail to develop if given no mineral food, and adults, if entirely deprived of even one substance, sodium chloride, would become weak, inactive, and finally die. Not only must the growing child have the ash compounds for constructive purposes, but the mature man must be supplied with them in order to sustain the nutritive functions. It is especially true of mammalian females, which store combinations of phosphoric acid, lime, and potash so abundantly in the milk that they must have an adequate supply of these substances. Nothing is clearer than that these materials must of necessity be furnished in the food. They cannot originate in the animal organism, neither can carbon compounds take their place.

Mineral compounds evidently have important offices outside of supplying constructive material. Their physiological influence or reactions must also be taken into account. While the whole subject is not well understood, abundant evidence exists that the chemical environment of animal tissues has an important influence in several directions. It has been shown that the eggs of certain marine fishes multiply (segment) normally only in water containing a mixture of certain salts of a certain concentration. In the same way muscular contractions are greatly influenced by the salts that are in contact with the tissues. Osmotic pressure or the rate of transfer of liquids in the animal body is also influenced by the inorganic salts present. Indefinite as our knowledge is, it is quite evident that the soluble salts of calcium, magnesium, potassium, and sodium play an important rôle in the vital activities of the animal organism, other than for constructive purposes.