traced in the ash of bones, in muscles, and in many other tissues and
fluids. These compounds, forming salts or mineral matters that exist in
the body, are estimated to amount to about 6 per cent of the entire
weight.

9. Organic Compounds. Besides the inorganic materials, there exists
in the human body a series of compound substances formed of the union of
the elements just described, but which require the agency of living
structures. They are built up from the elements by plants, and are called
organic. Human beings and the lower animals take the organized
materials they require, and build them up in their own bodies into still
more highly organized forms.

The organic compounds found in the body are usually divided into three
great classes:

1. Proteids, or albuminous substances.
2. Carbohydrates (starches, sugars, and gums).
3. Fats.

The extent to which these three great classes of organic materials of the
body exist in the animal and vegetable kingdoms, and are utilized for the
food of man, will be discussed in the chapter on food (Chapter V.). The
Proteids, because they contain the element nitrogen and the others do
not, are frequently called nitrogenous, and the other two are known
as non-nitrogenous substances. The proteids, the type of which is egg
albumen, or the white of egg, are found in muscle and nerve, in glands, in
blood, and in nearly all the fluids of the body. A human body is estimated
to yield on an average about 18 per cent of albuminous substances. In the
succeeding chapters we shall have occasion to refer to various and allied