These different kinds of food, water, salts, sugar, starch, fat, and albumen, must be combined in our diet; for a simple substance which fulfils only one of the purposes required in our food will not support life. A man cannot live on water or salt, yet he would soon die without them. A diet composed exclusively of fats, starch, or sugar is equally incapable of supporting life. The albuminous foods, though they are considered the most nutritious, must be combined with the others to produce the desired result.

Milk is the only substance prepared by nature expressly as an article of food. Seeds grow, and produce plants and fruits after their kind; but milk is prepared as the natural food of the young of all mammalia. A baby fed on milk develops in every part; therefore milk must supply every requisite for the growth of the young body, and the proportions in woman's milk may be taken as the standard for human food. Milk analyzed is found to contain water, salt, fat, sugar, and caseine, - the five elements of food. It has enough of flesh-producing elements to restore the daily waste, and enough heat-giving elements to feed the oxygen in breathing. But when the teeth are formed, the child's system needs a greater proportion of some elements than are supplied by the milk, and other foods are gradually added to the diet.

Eggs also contain all the necessary elements; a diet of seven eggs per day furnishes all the nutrition a person needs. But the elements in eggs are too highly condensed, and are not properly proportioned for a continuous diet.

Many articles of food do not contain all these elements, and it is very important to proportion our food rightly, so that one kind will supply what another lacks. A certain bulk is also necessary in our food to stimulate a thorough action of the digestive fluids. If the quantity be not sufficient to distend the stomach, so that the churning motion of the muscular coats can affect every part of the food, the digestion is imperfect. Dyspepsia is often caused by lack of sufficient quantity of food, and by weakness of the muscles of the stomach. If this requisite quantity be wholly nutritious or highly condensed food, capable of entire absorption, the effect is too stimulating, and serious disorders of the alimentary canal are the result. To remedy this, it is necessary to have a certain amount of innutritious food, which furnishes the bulk required, and gives all parts of the digestive apparatus their proper amount of work to perform.

Many persons argue in favor of a vegetable diet, as we can obtain from vegetables all the necessary elements. There have been many cases known where people lived to an extreme age who used exclusively a vegetable diet, and others who lived equally as long upon animal food. But the general rule is, that we find the highest degree of bodily and mental vigor only among those who make use of a mixed diet. One of the strongest physiological arguments in favor of this rule is found in the structure and conformation of the teeth and alimentary canal. Part of the teeth are of the carnivorous, or flesh-eating kind, and part of the herbivorous, or vegetable-eating kind. The alimentary canal is equally well adapted to the digestion of animal or vegetable food, or an admixture of both. The proper proportion, by weight, is one third of animal, and two thirds of vegetable food.

"Nature has given us an unerring guide to a proper choice of diet. An unperverted appetite is the voice of the physical system making known its needs, and it may always be trusted to indicate the food necessary to the preservation of health. But as the voice of nature is often unheeded, it is necessary to exercise intelligence in selecting our food and adapting it to the circumstances of life. By a proper choice we can often counteract the effects of a violation of nature's laws."