About nine billion eggs are produced annually in the United States (Clark). Eggs contain all the ingredients necessary to support life and develop the organism. Like milk, they constitute a complete food, for out of an egg the entire structure of the bird - its bones, nerves, muscles, viscera, and on some birds even feathers - are developed previous to hatching. The inner portion of the shell is dissolved by phosphoric acid to furnish phosphates for the bones.

The average weight of a hen's egg, according to Bauer, is 50 grammes, or about 2 ounces, divided as follows: Shell, 7; white, 27; yolk, 16 grammes. The eggs of pullets are smaller than those of old hens.

Composition of a Hens Egg (Lawes and Gilbert)

Fresh weight

1.8 ounce.

Dry weight

0.45 "

Fat

0.198 "

Mineral matter

0.025 "

Nitrogen

0.036 "

Carbon

0.275 "

Or in percentage:

Water

70.00 per cent.

Dry matter

30.00 "

100.00 per cent.

Nitrogen

2.00 per cent.

Carbon

17.52 "

In artificial digestion experiments it is customary to estimate about four hundred grains of albumin to the egg.

Hens' eggs principally are used for food, but the eggs of ducks, geese, and guinea-fowl are also eaten, and occasionally those of such wild birds as the plover and sea birds. Along the Texan coast the eggs of gulls, terns, and herons were formerly gathered for food, those of the gull and murre on the Farallone Islands off the coast of California, and those of the laughing gull off the eastern coast of Virginia. Terrapin eggs are eaten with the flesh of the animal, and various forms of fish eggs are esteemed for food, as those of the sturgeon (caviare) and shad (roe).

The shell of a hen's egg weighs about one hundred grains. Its colour bears no relation to the nutrient value of the egg, but white shells are heavier than brown. An egg consists approximately of -

Shell - 11 parts

Carbonate of lime.

Yolk - 32 parts: Proteid (vitellin)

15.7 per cent.

Fat

33.3 "

Ash

1.1

Water

49.5 "

Total calories

1,705

White - 57 parts: Proteid

12.3 per cent.

Fat

.2 "

Ash

.6 "

Water

86.2 "

Total calories

250

The yolk is very complex. Besides the proteid vitellin, it contains three fats, colouring matter, nuclein, lecithin, and salts of iron, calcium, magnesium, and potassium.

Egg albumin is not pure, but consists of four albuminoid and mucoid bodies, the chief of which is ovalbumin, together with a trace of carbohydrate. The phosphorus equivalent is 0.03 per cent phosphoric acid (C. F. Langworthy), whereas that of the yolk exceeds 1 per cent.

If the absorption of eggs from the intestine is delayed, decomposition ensues with production of sulphuretted hydrogen and ammonia, which cause considerable gastro-enteric disorder. The yolk of egg is particularly responsible for this. It is therefore a matter of great importance to serve only eggs which are absolutely fresh to invalids. No egg having an odour of stale, old straw should ever be offered. There are two simple methods by which the nurse may test the freshness of an egg:

1. Hold the egg between the hands so that the light of a candle shines through it. If fresh, it is more transparent in the centre; if stale, at the ends.

2. Make a solution of two ounces of common salt in a pint of water. An egg one day old will sink in this solution, but will not quite reach the bottom; an egg three days old will barely float beneath the surface, and an egg a fortnight old will float above the surface, only partially dipping beneath it (Siebel). This difference is due in part to loss of water, which after ten days equals 1.60 per cent; after twenty days, 3.16 per cent; and after thirty days, 5 per cent. It is also due in part to development of gases of putrefaction.

With some persons eggs have a slight aphrodisiac effect, and they also promote costiveness. They should not be eaten in the following diseases: Flatulent dyspepsia, gastric dilatation, or any form of severe gastric derangement, severe acute Bright's disease. The flavour of eggs is modified by the food of the hen, those from hens fed on nitrogenous food instead of carbohydrates have poor odour and flavour and small yolks. An excess of onions in the hen's diet imparts the strong odour and taste of the vegetable to the eggs, about fifteen days after such food has been eaten. Eggs packed in stale bran or straw, or kept near decaying apples, acquire a bad flavour. Micro-organisms may penetrate the eggshells and give rise to fermentation and decomposition.