[Sidenote: Importance of proteid or nitrogenous foods]
Owing to the fact that the tissues of the normal body are constructed
chiefly from proteids, the metabolism of proteids or nitrogenous foods
is of very great importance. When we realize the fact that muscle,
blood, brain, nerves, cartilage, tendons, the various internal organs
and the tougher material of the skeleton are only various forms of
proteid material, and must contain their proportions of available
or organic nitrogen, we can understand why nitrogenous foods form a
distinct class that must be considered by themselves. Only the mineral
deposits of the bones and the teeth, and the globules of fat that are
deposited as a source of stored energy represent the nitrogen-free class
of substances within the animal body.