The subject of Hygiene naturally divides itself into two parts, the health of the individual, and that of the community, or Personal and Public Health.
The former treats of the influence of habits, cleanliness, exercise, clothing, and food on health; while the latter is concerned with the interests of the community at large, as affected by a pure supply of air and water, the removal of all excreta, the condition of the soil, and with the administrative measures required to secure the removal of evil conditions. It is obvious, however, that these two divisions are not mutually exclusive. What is important to the health of the community, is equally so to each individual member of it. The purity of air and water, for instance, is of immense importance both personally and collectively.
It will be convenient to study first the three main factors in relation to health—food, water, and air—subsequently considering other matters of importance to health (see pages 4-157).
[CHAPTER II.]
FOOD.
Physiological Considerations.—All substances are foods which, after undergoing preparatory changes in the digestive organs (rendering them capable of absorption into the circulation), serve to renew the organs of the body, and maintain their functions. Foods have been classified as tissue producers or energy producers, the first class renewing the composition of the organs of the body, and the second class supplying the combustible material, the oxidation (or more correctly the metabolism) of which is the source of the energy manifested in the body. The two main manifestations of energy in the body are heat and mechanical motion, which are to a large extent interchangeable.
All foods come under one of these heads; they are either tissue or energy producers. They may be both, and in many cases are so. Thus, all nitrogenous foods (as meat, legumens, etc.) not only help to form the nitrogenous tissues of the body, but their largest share becomes split up into fats and urea, and so forms a source of heat to the body. Similarly fats may possibly, after assimilation, enter into the composition of the various tissues containing fat (of which the brain is the most important), though they usually supply an immediate source of heat. Proteid foods are, however, the tissue producers par excellence, other foods serving as the immediate sources of energy when metabolised in the body.
Certain foods do not directly serve either as tissue or energy producers, but are useful in aiding the assimilation of food. Such are the various condiments which may be classed as adjuncts to food. Salt is so necessary to the assimilation of food and to the composition of the various tissues, that it may be ranked as an important food. Water, again, though already oxidised, and so not an immediate source of energy, is absolutely necessary to the assimilation of food, to the interchange between the various tissues and the blood, and to the elimination of effete products.
Classification of Foods.—Inasmuch as milk supplies all the food necessary for health and growth during the first year of life, it may reasonably be expected to afford some guidance as to the necessary constituents of a diet for the adult; although the conditions of life being altered in the latter, we can hardly expect the same proportions of the different materials to hold good. In the infant rapid growth and building up of new tissues and organs are going on, involving the necessity for a larger proportional amount of nitrogenous food than in the adult.
The following is the average composition of 100 parts of