EXCRETION 323 



closely resembles it. If such a check as this has been 

 kept upon the diet of our imaginary subject it will 

 probably be found that the nitrogen of the income has 

 been very nearly the same as the nitrogen of the outgo. 

 Furthermore, if we were to continue the experiment for 

 several days and take pains to vary the nitrogen of 

 the food supply as widely as possible we should find that 

 the excretion would adjust itself to the changes in the 

 diet with but little lag. 



At one period of our investigation we might encourage 

 the eating of meat; eggs, and legumes to insure a high- 

 protein ration. We should find that if 30 grams of 

 nitrogen could be tolerated nearly as much would re- 

 appear in the urine. The other extreme would be 

 reached with nearly non-nitrogenous diet. This has 

 been closely approached by feeding nothing but corn- 

 starch pudding with cream and sugar. In this case we 

 should not have the customary equilibrium for there 

 would still be a definite output of nitrogen in the ab- 

 sence of income. But within very wide limits the 

 body excretes just about as much nitrogen as it receives. 



The facts ought to be intelligible in the light of what 

 has been said about the way in which the organism deals 

 with proteins. It has been pointed out that the require- 

 ment of amino-acids for synthetic service is a very 

 moderate one. No matter how greatly we exceed it 

 we find that the system responds continually in the 

 same fashion: it sets aside for excretion all the surplus 

 nitrogen. Hence the balance is generally struck be- 

 tween the food and the excreta unless the income is 

 distinctly deficient. Retention of nitrogen occurs dur- 

 ing growth and in the related conditions of recov- 

 ery from fasting, convalescence from illness, and in 

 pregnancy. 



As we can calculate the protein decomposed from 

 the nitrogen excreted, we can equally well calculate 

 the storage of protein in the body of growing animals 

 from the quantity of nitrogen retained. If, in the course 



