Every part of our organism is the subject of continual change. The flesh of the arm to-day is not precisely the same flesh as yesterday; some of its cells have been used up, dissolved, and carried away by the blood, and have been cast out of the system through the kidneys or bowels, whilst their places have been supplied by new molecules formed by digestive changes from the food taken into the stomach. If the stomach or the blood do not do their work properly, or if, doing the best they can, they are not supplied with suitable materials in the food furnished them, the effete molecules of the arm do not get fully replaced, and so the muscles become flabby and dwindle away; or, on the other hand, if, by unusually nutritious food and the stimulus of exercise, a greater number of new muscle cells are elaborated in the blood than are carried away by it in a worn-out condition, growth and development of the arm is the result. And so on with all the different parts of the body.
Another very important office of food, especially of the fatty (highly carbonised) articles of our diet, is to supply the bodily heat by being slowly burnt up within our systems, exactly as the coal (mineral carbon) burnt up in fireplaces warms our dwellings, except that the process of combustion is so managed in our bodies that it goes on slowly, and only a very little at a time, giving out no light and but a moderate amount of heat.
Materials of Food.—The materials which make up our food, besides water and saline ingredients, are: first, the nitrogenous (such as meat, eggs, cheese, the gluten of wheat flour, animal jellies, etc.); second, the fatty (as the fat of animals, butter, olive-oil, etc.); and third, the saccharine, comprising starch, sugar and molasses in all their varieties (bread, potatoes, rice, etc.).
The office of the first of these groups is to supply the waste of muscular substance or tissue caused by pulsation of the heart, breathing, eating, etc., and by physical exercise, such as manual labour, walking, or riding. Fatty articles of diet are chiefly employed to sustain the heat of the body by their gradual combustion, and the saccharine elements contribute to the same end.
Quantity of Food.—A healthy, full-grown man, doing a moderate amount of work, requires daily about 4½ ozs. of dry nitrogenous, 3 ozs. of fatty, and 15 ozs. of sugary and starchy food, besides 1 oz. of saline matter.
Under ordinary circumstances, the penalty for taking less than this amount of food is loss of flesh and strength, more or less rapid in proportion to the degree in which economy of nutriment, forced or otherwise, is actively carried on.
The penalty incurred by eating more than these quantities is derangement of the stomach, the liver and intestines, by thus over-loading them; and a consequent production of dyspepsia, biliousness, diarrhoea, or constipation, with their innumerable attendant evils, which, perhaps,