There are two portals, namely, the lungs and the alimentary canal, by which new materials normally enter the animal body.

Within the lungs the blood comes into close relation with the air, and takes from it oxygen. The oxygen is then carried to the various tissues, where it aids in the combustion accompanying the life and functions of these tissues. Oxygen is the most abundant element in the body, taking part in almost every chemical change, and its continuous supply is more immediately necessary for life than that of any other substance, yet it is not counted as food, because tissue oxidation is distinguished from tissue nutrition.

The details of the union of oxygen with the blood will be found in the Chapter (xix) on Respiration.

It is then only to the liquid and solid portions of the material income of an animal - that, in short, which it must busy itself to obtain - that the term "food" is applied. These are introduced into the alimentary canal, where the nutrient materials are separated and prepared for absorption, while the portions which are not useful for nutrition are carried away as excrement. We are, therefore, quite prepared to hear that the really nutritious food stuffs are composed of materials which are chemically like the tissues, although, as we shall see, we have no grounds for believing that the different chemical groups of nutritive stuffs are exclusively destined to replace corresponding substances in the body. On the contrary, we have good reason to think that within the body the conversion of one group into another is common.

In Chapter III (Chemical Basis Of The Body) the tissues of the animal body were shown to consist of chemical compounds, which have been classified into certain groups. It has also been stated that the tissues are constantly undergoing chemical changes inseparable from their life, and that for these changes a supply of nutritive material is necessary.

The nutriment required for an animal is made up of substances which may be divided into the same chemical groups as the tissues of the body, viz., proteids, fats, carbohydrates, salts and water. So that each of the various substances which we make use of as food, contains in varying proportions several of the different kinds of nutrient material, either naturally or artificially mixed so as to form a complex mass, the important item water being the only one which is commonly used by itself. These substances may be considered to be the chemical bases of the food, as they are also the chemical bases of the animal body.

The following classification shows the relationships between the chief constituents of food, from a chemical point of view, and their distribution in the various substances we eat: -

I. Organic

1. Nitrogenous -

(A) Albuminous - abundant in eggs, milk, meat, peas, wheaten flour, etc.

(B) Albuminoid - in soups, jellies, etc.

2. Non-nitrogenous -

(A) Carbohydrates (sugar, starch) - abundant in all kinds of vegetable food, and in milk, and present in small quantity in meat, fish, etc.

(B) Fats - in milk, butter, cheese, fat tissues of meat, some vegetables, oils, etc.

II. Inorganic

1. Salts - mixed with all kinds of food.

2. Water - mixed with the foregoing or alone.

The nutritive value of any kind of food depends upon a variety of circumstances, which may be thus summed up: -

I. Chemical composition, of which the main points are i. The proportion of soluble and digestible matters (true food stuffs) to those which are insoluble and indigestible, such as cellulose, keratin, elastic tissue, etc.

2. The number of different kinds of nutrient stuffs present in it.

II. Mechanical Construction

The degree of subdivision in which the substance is introduced into the stomach materially influences its nutritive value, since the smaller the particles the greater the amount of surface exposed to the action of the digestive juices.

The relation of the nutrient to the non-nutrient parts is also of importance, as is seen where the nutritious starch of various vegetables is enclosed in insoluble cases of cellulose, which, if not burst by boiling, prevent the digestive fluids from reaching the starch.

III. Digestibility

This depends partly upon how the substances affect the motions of the intestines, and partly upon their construction. Thus, some substances, such as cheese, though chemically showing evidence of great nutritive properties, by their impermeability resist the digestive juices, and are not very valuable as food.

IV. Idiosyncrasy

In different animals and in different individuals, and even in the same individuals under different circumstances, food may have a different nutritive value.