1010 EXPERIMENT STATION RECORD. 



warranted that the amount of water consumed with a ration should 

 b- regulated, unless it was believed to be desirable to furnish large 

 amounts of water. If this were done a correspondingly large amount 

 of nitrogen should also be supplied. If it should appear that the 

 amount of water drunk was without effect on the excretion of nitrogen 

 the conclusion would be warranted that there was no reason for regu- 

 lating the supply of water. 



The manurial value of a ration is an important consideration from 

 an economic standpoint, and this can not be known with certainty unless 

 the nitrogen in the urine and feces is determined. 



If feeding tests were made with the additional determinations 

 suggested the results obtained by different observers would be more 

 comparable than is now the case. There is so little proof of uniformity 

 in experimental conditions at present that many of the deductions 

 obtained are only of local or transient interest. 



Nitrogen metabolism in its relation to digestion experiments. — What 

 has been said of the value of experiments in which the balance of 

 income and outgo of nitrogen is determined as a control on the accu- 

 racy of feeding experiments applies with equal force to digestion 

 experiments. In the digestion experiment, as ordinarily conducted, 

 the animal is fed for a longer or shorter period a definite ration consist- 

 ing of a single food or combination of foods. The ration is fed until 

 it is assumed the animal is living on it, and then the feces are collected 

 and analyzed. Sometimes various substances, such as powdered cork, 

 bones, or charcoal are given with the last food consumed before and 

 the first food after the digestion experiment proper. These substances 

 impart either a definite consistency or color to the feces and permit the 

 separation of the feces due to the special food under consideration from 

 the feces due to the preceding and following ration. From the compo- 

 sition of the food and feces the coefficients of digestibility are cal- 

 culated. Unless the balance of income and outgo of nitrogen is 

 determined the time which should elapse after a given diet is consumed 

 before the feces should be collected for analysis is more or less a matter 

 of conjecture, as in experiments with herbivora the feces can not be 

 marked, for instance, with charcoal, as may be done in experiments with 

 man. 



It is not known with certainty that the digestibility of food is influ- 

 enced by the state of nutrition of the subject. It seems reasonable to 

 assume that digestion would be normal under normal conditions; that 

 is, when the body is in nitrogen equilibrium. 



It has been sometimes assumed that the feces consist principally of 

 undigested residue, but investigations — some of them comparatively 

 recent— have shown that this supposition is far from correct, and that 

 a considerable percentage of the nitrogen of the feces is metabolized 

 nitrogen derived from bile and other digestive secretions. As pre- 

 viously stated, the nitrogen assimilated from a given diet is largely 



