THE METABOLISM OF THE BODY. 455 
kidneys, of salts and water from the skin and kidneys, and of 
nitrogen, chiefly as urea almost wholly from the kidneys. Usu- 
ally in experimental determinations the total quantity of the 
nitrogen of the urine is estimated. If free nitrogen plays any 
part in the metabolic processes it is unknown. 
A large number of feeding experiments have been made by 
different investigators, chiefly, though not exclusively, on the 
lower animals. Some such method as the following has usu- 
ally been pursued: 1. The food used is carefully weighed and a 
sample of it analyzed, so that more exact data may be obtained. 
2. The amount of oxygen used and carbonic anhydride exhaled, 
as well as the amount of water given off in any form, is esti- 
mated. 3. The amount of the nitrogenous excreta is calculated, 
chiefly from an analysis of the urine, though any loss by hair, 
etc., is also to be taken into account. 
It has been generally assumed that the nitrogen of the ex- 
creta represents practically the whole of that element entering 
the body. This has been denied by some investigators. 
The respiratory products have been estimated in various 
ways. One consists in measuring the quantity of oxygen sup- 
plied to the chamber in which the animal under observation is 
inclosed, and analyzing from time to time samples of the air as 
it is drawn through the chamber; and on these results the total 
estimates are based. 
It will appear that even errors in calculating the composi- 
tion of the food—and this is very variable in different samples, 
e. g., of flesh; or any errors in the analysis of the urine, or in 
the more difficult task of estimating the respiratory products, 
may, when multiplying to get the totals, amount to serious de- 
partures from accuracy in the end; so that all conclusions in 
such a complicated case must be drawn with the greatest cau- 
tion. But it can not be doubted that such investigations have 
proved of much practical and some scientific value. The labor 
they entail is enormous. 
Proteid Metabolism.—If we conceive of a structural unit or 
cell as made up of a genuine protoplasm constituting its mesh- 
work and holding in the interstices certain substances that are 
not part of itself, strictly speaking, the question arises, Are 
these latter used up in the metabolic process as such, or do they 
become a part of the true protoplasm before they undergo the 
changes referred to above? Some writers speak of “organ 
albumin ” and “ circulating albumin,” and they believe that the 
latter, by which is meant the proteid material found every- 
