other solids and in the fluids of the body. The special place
of iron is in the coloring matter of the blood. Its various salts are
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
forms of proteids as they exist in muscle (myosin), coagulated blood
(fibrin), and bones (gelatin).

The Carbohydrates are formed of carbon, hydrogen, and oxygen, the
last two in the proportion to form water. Thus we have animal starch, or
glycogen, stored up in the liver. Sugar, as grape sugar, is also found in
the liver. The body of an average man contains about 10 per cent of
Fats. These are formed of carbon, hydrogen, and oxygen, in which the
latter two are not in the proportion to form water. The fat of the body
consists of a mixture which is liquid at the ordinary temperature.

Now it must not for one moment be supposed that the various chemical
elements, as the proteids, the salts, the fats, etc., exist in the body in
a con