132 PRINCIPLES OF ANIMAL NUTRITION. 
EXTENT oF Protein StoracE.—Whatever may be the expla- 
nation of the action of the non-nitrogenous nutrients, its effect is 
obvious. Attention has already been called (p. 102) to Gruber’s 
hypothesis that the transitory storage of nitrogen following an 
increase in the proteid supply is the result of a superposition of the 
daily curves of nitrogen excretion. The effect of the non-nitroge- 
nous nutrients appears to be to diminish the rate of nitrogen cleavage 
and to protract it, in the case of a single meal of proteids, over a 
longer time. Evidently, then, an increase of the proteid supply in 
a mixed diet, or the addition of non-nitrogenous nutrients to a pro- 
teid diet, will extend its effect over a considerably longer period 
than in case of an exclusive proteid diet—that is, nitrogen equi- 
librium will be reached more slowly, and there will be a longer or 
shorter time after the change during which the nitrogen excretion 
will be less than in the absence of the non-nitrogenous matters. 
This explanation also implies, however, that the storage of 
nitrogenous matter in the body of the mature animal is of limited 
duration and that no long-continued gain of protein can occur; in 
other words, that it is impossible to materially increase the proteid 
tissue (lean meat) of a mature animal. 
Numerous comparative fattening experiments with domestic 
animals, notably those of Henneberg, Kern, & Wattenberg * upon 
sheep, fully sustain this conclusion. On the other hand, metabo- 
lism experiments with domestic animals rarely show an equality 
between the income of nitrogen and its outgo in feces and urine, 
but almost always indicate a gain of nitrogenous matter by the 
body. As regards the significance of this fact, however, several 
considerations must be borne in mind. 
First, the normal growth of the epidermal tissues—hair or wool, 
hoofs, horns, etc.—as pointed out in Chapter III, consumes a por- 
tion of the nitrogen of the food and contributes its share to the 
storage of nitrogen in the body. , 
Second, the adipose tissue itself contains a small percentage of 
_ proteid matter, and a storage of fat in considerable amounts in- 
volves the production of new adipose tissue in which to store it. 
Third, in many cases the metabolism experiments which show 
astorage of nitrogen have been made within a rather short time 
* Jour. f. Landw., 26, 549. 
