158 STORRS AGRICULTURAL EXPERIMENT STATION. 
or protecting it from waste. They are, therefore, unable to 
' perform the principal functions of food. In the sense that 
they are not available for either building or repair of body 
material or for the yielding of energy, it is not improper to 
class them with the undigested residue of the food. ‘This is 
equivalent to saying that the difference between the feces and 
food may be taken as measuring the amount which is actually 
‘available to the body for the main purposes of nutrition. 
Viewing the subject in this light and using the term digest- 
ibility in the sense of availability, the proportion of the food 
which is digested will be its total amount, less the sum of the 
undigested residues and the metabolic products which are 
involved in the digestive process. This is practically equiv- 
alent to the current method and is the plan followed in the 
experiments here described. 
The subject demands much fuller discussion than can be given 
here, especially in the light of recent inquiry by Prausnitz and 
others. It will suffice here to insist upon the general fact 
that what is commonly called the digestibility of foods in expe- 
riments of this sort might be more properly designated as 
availability, reserving the detailed discussion for another place. 
Collection of urine. Nitrogen lag.—In every instance the 
urine was collected in 24-hour periods, beginning and ending 
with 7 A.M., the amount passed at 7 a. M. being included in 
that of the twenty-four hours then ending. Of course the 
urine thus collected during a period of twenty-four hours does 
not represent the nitrogen metabolized in the body during that 
period, since a longer or shorter time elapses between the 
breaking down of nitrogenous compounds in the body and the 
excretion of the final nitrogenous products. ‘This so-called 
nitrogen lag was referred to in a previous discussion.* For the 
main purpose of these experiments, namely, the determination 
of the income and outgo of matter and energy in the second 
period, the nitrogen lag is believed to be mainly if not entirely 
eliminated by the maintaining of a uniform diet through both 
periods. At the same time it is to be observed that the amount 
of nitrogen eliminated during the first day after the subject 
entered the calorimeter was sometimes larger or smaller than 
during the preceding or succeeding days. 
* See Report of this Station for 1896, p. 102. Also Bulletin No. 44 of the Office of 
Experiment Stations of the U. 8. Department of Agriculture, on Preliminary Ex peri- 
ments on the Metabolism of Nitrogen and Carbon in the Human Organism, p. 35. 
. 

