nimal bodies amids are formed during oxidation, digestion,
and disintegration of proteids. It is not definitely known whether or
not a protein in the animal body when broken down into amid form can
again be reconstructed into protein. The amids have a lower food value
than the proteids and albuminoids. It is generally held that, to a
certain extent, they are capable, when combined with proteids, of
preventing rapid conversion of the body proteid into soluble form. When
they are used in large amounts in a ration, they tend to hasten
oxidation rather than conservation of the proteids.

24. Alkaloids.--In some plant bodies there are small amounts of
nitrogenous compounds called alkaloids. They are not found to any
appreciable extent in food plants. The alkaloids, like ammonia, are
basic in character and unite with acids to form salts. Many medicinal
plants owe their value to the alkaloids which they contain. In animal
bodies alkaloids are formed when the tissue undergoes fermentation
changes, and also during disease, the products being known as ptomaines.
Alkaloids have no food value, but act physiologically as irritants on
the nerve centers, making them useful from a medicinal rather than from
a nutritive point of view. To medical and pharmaceutical students the
alkaloids form a very important group of compounds.

[Illustration: FIG. 5.--GRAPHIC COMPOSITION OF FLOUR.

1, flour; 2, starch; 3, gluten; 4, water; 5, fat; 6, ash.]

25. General Relationship of the Nitrogenous Compounds.--Among the
various subdivisions of the nitrogenous compounds there exists a
relationship similar to that among the non-nitrogenous compounds. From
proteids, amids and alkaloids may be formed, just as invert sugars and
their products are formed from sucrose. Although glucose products are
derived from sucrose, it is not possible to reverse the process and
obtain sucrose or cane sugar from starch. So it is with proteins, while
the amid may be obtained from the proteid in animal nutrition, as far
as known the process cannot be reversed and proteids be obtained from
amids. In the construction of the protein molecule of plants, nitrogen
is absorbed from the soil in soluble forms, as compounds of nitrates and
nitrites and ammonium salts. These are converted, first, into amids and
then into proteids. In the animal body just the reverse of this process
takes place,--the protein of the food undergoes a series of changes, and
is finally eliminated from