. Lecithin is also found in the brain and nerve material of
animals, in the yolk of egg, and in several plants.

The nitrogenous constituents of milk--casein and albumen--are usually
estimated together, and they are reckoned as of equivalent food value.
The name protein is very commonly applied to the total of these bodies
in milk, or other animal and vegetable foods. They are composed of
different proportions of carbon, hydrogen, oxygen, and nitrogen, with
small quantities of sulphur, while casein contains phosphorus in
addition. Albumen exists to the extent of about 0.6 per cent. in milk.
It is very similar in properties to egg albumen. The coagulum which
forms on the surface of milk when boiled is largely composed of albumen.
Casein is combined with, and kept in solution by, lime, soda, and
calcium phosphate, and its amount averages a little over 3 per cent.

The remarkable property possessed by rennet, of curdling or coagulating
casein, is well known; rennet is an extract from the stomach of the
calf, and similar principles are present in the stomachs of man and
other animals, so that the coagulation of milk is the first process in
its digestion. If milk is gulped down in large quantities it is apt to
coagulate in lumps, and digestion is much interfered with, but if it is
taken hot and slowly, it coagulates in small pieces which are readily
attacked by the gastric juice, and milk is then one of the most
assimilable of foods.

Nature provides that the milk for young animals is supplied in finely
divided streams, so that coagulation takes place in the best possible
way.

The proteids are the most important constituents of food; they are
abundant in the blood, and build up the muscles, brain, nerves, and
other bodily structures.

Besides these mentioned, milk contains traces of another proteid of
similar composition called globulin.

The sugar of milk is not found anywhere else. It is a carbohydrate like
cane and grape sugar--that is to say, the hydrogen and oxygen they
contain are in the same relative proportions as in water. Milk sugar is
not so soluble or so sweet as the other sugars. It does not ferment with
ordinary yeast, but certain special yeasts which are made use of in the
preparation of keffir, koumiss, etc., have the power of transforming it
into alcohol. Its most remarkable property, however, is the facility
with which, under the influence of certain bacteria, it is changed into
lactic acid.

Ever