665. Two Kinds of Food. — All the varieties of food are
divided into two classes according to the purpose which
they serve, the one class serving to build up the tissues,
and the other to maintain the animal heat. To the former
class belong those substances that contain nitrogen, as the
gluten of bread, the fibrin of meat, the casein of milk, etc.
The other class comprises those substances that have no ni-
trogen in them, as starch, sugar, and oily substances. These
all serve to maintain the warmth of the body, and it is sup-
posed have little or nothing to do with building its struct-
ures. They are burned up, as we may express it, in create
ing heat, which is just as essential to the maintenance of
life as nutrition is. This food is called respiratory food,
because it is supposed that the oxygen introduced by the
respiration is employed in consuming or burning it. The
products of this flameless combustion are the same with
those of ordinary combustion with flame, water, and car-
bonic anhydride. The heat-making food is often called
carbonaceous from the predominance of carbon in it, while
the building food is called nitrogenous because it is dis-
tinguished from the other by containing nitrogen.
While the view thus presented is generally true, there



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is ground for doubt whether the distinction can be as
strictly carried out as is attempted by Liebig and others.
The reasons for this doubt will be noticed soon.
