6. As milk is usually taken into the body in liquid form, the common
tendency is to regard it as a beverage, rather than as an important
source of nourishing food material. However, a knowledge of its
composition, as well as the fact that milk becomes a solid food in the
stomach and must then be dissolved in the process of digestion, will
serve to show that milk contains solids. That it possesses all the
elements required to sustain life and promote health is proved by the
fact that a child may live for months on milk alone and during this time
increase in weight.

7. The solids contained in milk are proteins, fat, carbohydrate in the
form of sugar, and mineral salts, besides which, of course, water occurs
in large quantities. The sugar and fat of milk serve as fuel; the
mineral salts are chiefly valuable for the growth of bones and teeth and
for their effect on the liquids of the body; and the proteins, like the
fat and sugar, serve as fuel, but they also make and repair the muscular
tissues of the body.

In considering the food substances of milk, it will be well to note also
that they vary according to the breed, feeding, and individual
characteristics of the cow. Jerseys and Guernseys give milk rich in fat
and total solids, and while Holstein cows give a greater quantity of
milk, such milk has a smaller proportion of fat and total solids. As a
rule, though, the composition of milk may be considered as approximately
3.3 per cent. protein, 4 per cent. fat, 5 per cent. carbohydrate, and
.7 per cent. mineral matter, making a total of 13 per cent. This
indicates the quantity of actual food material in milk, the remainder,
or 87 per cent., being water.

8. PROTEIN IN MILK.--Because of the double usefulness of protein--to