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EXCRETA OF THE ORGANISM. 879
Such knowledge can be attained only by a series of systematic and
thorough observations, in which the quantity of nutritive material, rela-
tive to the weight of the body, taken and absorbed in a given time is
compared with the quantity of final metabolic products which leave the
organism at the same time. Researches of this kind have been made by
investigators, but above nil should be mentioned those made by BlBCHOFF
and Voir, by Pettenkofer and Voit, and by Voit and his pupils, by
Rubner, Zintz and by Atwater.
It is absolutely necessary in researches on the exchange of material
to be able to collect, analyze, and quantitatively estimate the excreta
of the organism, so that they may be compared with the quantity and
composition of the nutritive bodies ingested. In the first place, one must
know what the habitual excreta of the body are and in what way these
bodies leave the organism. One must also have trustworthy methods
for their quantitative estimation.
The organism may, under physiological conditions, be exposed to
accidental or periodic losses of valuable material—such losses as occur
only in certain individuals, or in the same individual only at a certain
period; for instance, the secretion of milk, the production of eggs, the
ejection of semen or menstrual blood. It is therefore apparent that these
losses can be the subject of investigation and estimation only in special
cases.
The regular and constant excreta of the organism are of the very
greatest importance in the study of metabolism. To these belong, in
the first place, the true final metabolic products
—
carbon dioxide, urea
(uric acid, hippuric acid, creatinine, and other urinary constituents),
and a part of the water. The remainder of the water, the mineral bodies,
and those secretions or tissue constituents
—
mucus, digestive fluids, sebum,
perspiration, and epidermal formations—which are either poured into
the intestinal tract, or secreted from the surface of the body, or broken
off and thereby lost to the body, also belong to the constant excreta.
The remains of food, sometimes indigestible, sometimes digestible but not
acted upon, which are contained in the feces, and which vary considerably in
quantity and composition with the nature of the food, also belong to the excreta
of the organism. Even though these remains, which are never absorbed and
therefore are never constituents of the animal fluids or tissues, cannot be con-
sidered as excreta of the body in a strict sense, still their quantitative estimation
is absolutely necessary in certain experiments on the exchange of material.
The determination of the constant loss is in some cases accompanied with the
greatest difficulties. The loss from the detached epidermis, from the secretion
of the sebaceous glands, etc., cannot be determined with exactness without dif-
ficulty, and therefore— as they do not occasion any appreciable loss because of
their small quantity—they need not he considered in quantitative experiments
on metabolism. This also applies to the constituents of the mucus, bile, pancreatic
and intestinal juices, etc., occurring in the contents of the intestine, and which,
leaving the body with the feces, cannot be separated from the other contents of
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