enous_, _non-nitrogenous_, and _mineral_.
All of these constituents, or substances capable of being converted
into them, must exist in the food. Certain articles, for example, milk,
contains all of them; but in others, for instance, butter, only one of
these substances is found. The nitrogenous part of the body embraces the
muscles, or lean flesh, the gelatine of the bones, and the skin and its
appendages--such as hair and horns; the non-nitrogenous constituents are
its fat and oil; and its mineral matter is found chiefly in the bony
framework. These constituents are not, however, isolated: the mineral
matter, no doubt, accumulates in certain parts, but in small quantities
it is found in every portion of the body; and although the fat forms a
distinct tissue, the muscles of the leanest animal are never free from
a sensible proportion of it.

Albumen, fibrine, and caseine are the principal nitrogenous constituents
of food, and as they are employed in the reparation of the nitrogenous
tissues of the animal body, they have been termed _flesh-formers_.

The fat and oil of animals are derived either from vegetable oil and
fat, or from some such substance as starch or sugar. The constituents
of food which form fat are termed _fat-formers_, and sometimes
_heat-givers_ or _respiratory elements_, from the notion that their
slow combustion in the animal body is the chief cause of its high
temperature.

The mineral elements of the body are furnished principally by the
varieties of food which contain nitrogen. The whey of milk is rich in
them; but they do not exist in pure butter, in starch, or in sugar.

Fat is a much more abundant constituent of the animal body than is
generally supposed, That this substance should constitute the greater
portion of the weight of an obese pig seems probable enough; but few
are aware that even in a lean sheep there is 50 per cent. more fat than
lean.

For a very accurate knowledge of the relative proportions of the fatty,
nitrogenous, and mineral constituents of the carcasses of animals used
as human food, we are indebted to Messrs. Lawes and Gilbert. Before
these investigators turned their attention to this subject, it had
scarcely attracted the notice of scientific men; but a notion appears to
have been current, amongst non-scientific people, at least, that in all,
save the fattest animals, the lean flesh greatly preponderated over the
fat. That this idea was unsustained by a foundation of f