ANIMAL FOOD 1 13 



thirtieth is left over in their digestion. They are thus a 

 valuable food, but yet do not rank so high as milk. 



167. Digestion of egg albumin. When boiled for a minute 

 or two, the albumin of eggs is partly coagulated to a soft, jelly like mass. 

 Boiling for three minutes coagulates all the albumin to an elastic, 

 slippery mass ; while after boiling for ten minutes the albumin becomes 

 brittle, and is easily crushed to fine particles. 



A lump of albumin of a raw egg is digested with less expense of 

 time and energy than the same sized lump of coagulated albumin, and 

 the longer an egg is boiled the more energy is required to digest it 

 back to a liquid form. But the raw egg has a tendency to collect 

 in masses which the gastric juice cannot penetrate. 



An egg boiled for less than five minutes is usually masticated only 

 to medium-sized particles, which, however, owing to their smaller size, 

 may be digested sooner than the large masses of raw egg. But the egg 

 boiled for ten minutes is easily chewed fine, and, owing to the still 

 smaller size of its particles, is digested much sooner than small lumps 

 of soft-boiled eggs or the masses of raw eggs. Thus an egg boiled 

 for at least ten minutes is ordinarily the most available for digestion. 

 When mixed with a considerable quantity of milk, the raw egg is pre- 

 vented from forming a lump, and in this form it may digest more easily 

 than a cooked egg. 



168. Quality of eggs. Fresh eggs vary but little in composi- 

 tion. In time they lose a little water by evaporation through the shell, 

 which is porous. A fresh egg appears clear and pink when held in 

 front of a strong light, while an old egg appears dark-colored, even if it 

 has not begun to decay. It will first show a dark spot where the yolk 

 settles to the side of the shell, and later will be dark all through. This 

 test is reliable and is often applied in markets. The shell of a fresh 

 egg is bright in color and slightly rough like common newspaper, but 

 an old egg becomes duller in color and shiny in appearance like writing 

 paper. Ducks' eggs are nearly like hens 1 eggs, except that sometimes 

 they acquire a peculiar taste from the ducks' food. Nearly all kinds of 

 birds' eggs, as well as the eggs of turtles, are used as food. They 

 differ but little from hens' eggs. 



169. Meat. The flesh of oxen, sheep, and hogs is the 

 common form of meat. All kinds of game, fowl, fish, and 



ov. PHYSIOL. 8 



