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165
[1898
nitrogenous. Even the non-cellular liquids passing out into the
alimentary canal at various points, which have so great an action in
preparing the food in different ways, are not only nitrogenous but the
constancy of this implies the necessity of the nitrogen in order
that these actions shall be performed ; and the same constancy of the
presence of nitrogen, when function is performed, is apparently
traceable through the whole world. Surely such constancy proves
necessity, and hence their great necessity in all dietries.
Proteids then are a most valuable food, and indeed, it is their
presence in milk which gives this article its great pre-eminence as an
article of diet. But proteids are also themselves a source of fats, and
probably of carbo-hydrates, so that they play two parts, 1st: that
of regulators of oxidation and of the transformation of energy; and
2nd: they may form a non-nitrogenous substance which is oxidised
and transformed. Indeed, that fats are formed from proteids is proved
by the following facts :—
J. Carnivora giving suck, when fed on plenty of flesh and
little fat, yield milk rich in fat.
2. A cow which produces one pound of butter daily does not
take nearly this amount of fatty matter in her food, so that the fat
would appear to be formed in this case from vegetable diets.
The above remarks, mainly drawn from the Firth & Notter's
"Theory and Practice of Hygiene," are sufficient to show the
inestimable value of the proteids which are present in milk as
casein and albumin, in which the proportion of nitrogen is nearly
as 2 to 7.
Now as regards the carbo-hydrates. These are found in plants
and animals, and are so called because they contain hydrogen and
oxygen in the proportion to which these occur in water, in addition to at
least six atoms of carbon. Their taste is sweet and they can be readily
changed into sugar by the action of diluted acids. In milk they take
the form of a peculiar sugar, like cane sugar, and called lactose or sugar
of milk.