ucts resulting
from the destruction of the foods and tissues containing nitrogen--of, for
instance, albumin, fibrin, gluten, casein, gelatin, woody tissue, etc.
While much of the waste material containing nitrogen leaves the body by the
bowels, this is virtually only such of the albuminoid food as has failed to
be fully digested and absorbed; this has never formed a true constituent
part of the body itself or of the blood, but is so much waste food, like
that which has come to the table and again carried away unused. If the
albuminoid food element has entered the blood, whether or not it has been
built up into a constituent part of the structure of the body, its waste
products, which contain nitrogen, are in the main expelled through the
kidneys, so that the latter become the principal channels for the expulsion
of all nitrogen-containing waste.

It would be an error, however, to infer that all nitrogenous food, when
once digested and absorbed into the blood, must necessarily leave the
system in the urine. On the contrary, in the young and growing animal, all
increase of the fibrous structures of the body is gained through the
building up of those flesh-forming constituents into their substance; in
the pregnant animal the growth of the offspring and its envelopes has a
similar origin, and in the dairy cow the casein or curd of the milk is a
means of constant elimination of these nitrogen-containing agents. Thus, in
the breeding cow and, above all, in the milking cow, the womb or udder
carries on a work in one sense equivalent to that otherwise performed by
the kidneys. Not only are these organs alike channels for the excretion of
albuminous products, but they are also related to each other structurally
and by nervous sympathy, so that suffering in the one is liable to induce
some measure of disorder in the other.

As in the case of other mammals, this nitrogenous waste matter is mainly
present in the urine of cattle in the form of urea, but also, to some
extent, as hippuric acid, a derivative of vegetable food which, in the
herbivora, replaces the uric acid found in the urine of man and carnivora.
Uric acid is, however, found in the urine of sucking calves which have
practically an animal diet, and it may also appear in the adult in case of
absolute, prolonged starvation, and in diseases attended with complete loss
of appetite and rapid wasting of the body. In such cases the animal lives
on its own substance, and the pr