Eggs and milk occupy a unique place in the catalog of foods. The purpose
for which they were produced in nature throws much light upon their
value as food.
[Sidenote: Every form of life exists for itself alone]
As will be learned from the lesson, "Evolution of Man," no living
creature exists for the sole benefit of other creatures, but because
once created, the inherent struggle of all living matter to survive
and to reproduce itself has evolved wonderful and various adaptations.
Every organic substance is primarily produced in nature for a specific
purpose in the life of its species. The lumber in our houses owes its
existence to the plant's struggle for sunlight, which made it necessary
for the tree to possess a strong storm-withstanding stem to hold aloft
its leaves above the shade of other foliage.
The leaves and the stems of grass are primarily an essential part of the
life of the plant, and not food for animals. The greater part of the
human food of plant origin represents in nature the nutrient material
supplied by the parent plant for the early life of the seedling. All
grains, nuts, fruits and roots, and tubers are merely modified forms of
food material adapted to the rapid nourishment of the young plant.
The starch and the oil of seeds, the sugar of fruit, and the lesser
quantities of nitrogen contained in all seeds, are in a more available
form for cell-nourishment than would be the original mature portions of
plant life.
Milk and eggs in the animal world occupy a position identical to that of
seeds and fruit in the plant world; that is, they are created for the
first nourishment of the offspring.
In the process of evolution, a fundamental distinction between birds and
mammals is in the manner in which the young are nourished. The egg of
the bird supplies sufficient nourishment to develop the young bird to a
point where it can exist upon the ordinary food of the adult bird.
The hen's egg must contain all food material necessary to form all
portions of the body of the chick, and to supply it for a time with heat
and energy.
[Sidenote: Composition of eggs]
An average egg weighs two ounces; of this weight about 10 per cent is
shell, 30 per cent yolk, and the remainder white. The white of the
egg is composed of albumin and water. The yolk consists of globulin,
egg-fat, and lecithin; this latter substance contains a considerable
proportion of phosphorus, and is one of the essential contingents of
brain and nerves. The egg-shell contains 13 per cent protein, 10 per
cent fat, and one per cent ash.
[Sidenote: Milk and eggs not a balanced adult diet]
The younger the animal, the more rapid is the growth of the animal
body compared with the amount of energy expended. For this reason the
percentage of nitrogen in milk and in eggs is much too great to form a
balanced adult diet, and should be supplemented by articles containing
larger proportions of heat-producing materials, preferably carbohydrates.
[Sidenote: Eggs for emaciation and convalescents]
The proteid material of eggs is in a form especially adapted to the
construction of new cells. For this reason it is one of the best known
foods for use in cases of emaciation, where new tissue is to be added
rapidly to the body. An egg contains about fourteen decigrams of
nitrogen. Ten eggs, therefore, would supply an ample amount of nitrogen
for the daily needs of the average body, were no nitrogen taken from
other sources. In feeding patients who are convalescing from fevers or
other wasting dis-eases, it is sometimes necessary to prescribe a diet
of from ten to twelve eggs daily for a limited time.
The consumption of five eggs a day, when we rely wholly upon this
article for animal proteids, is quite sufficient for one performing
ordinary labor, when supplemented by one succulent and one tuber
vegetable.