510 EXPERIMENT STATION RECORD. 



writer's investigations made some 16 years ago, to from 0.3 to 0.5 gm. 

 and averages 0.4 gm. of nitrogen per 100 gm. of digestible dry matter. 

 Approximately the same figures were obtained later by T. Pfeiffer and 

 E. von Wolff; and a calculation of 20 experiments made by G. Kiilm 

 with steers, in which a method of artificial digestion worked out by 

 himself was used, shows that by this method of artificial digestion 

 from 0.36 to 0.58 gm. (average 0.40 gm.) more nitrogen was dissolved 

 (digested) than was indicated by the difference between the nitrogen in 

 the food and in the feces. 



The explanation of this discrepancy between the results of natural 

 and artificial digestion is near at hand. Since, on the one hand, the 

 digestion of the protein of a given feeding stuff is very constant for 

 the same class of animals, it is fair to assume that there is a quite 

 marked distinction between the digested and undigested part of the 

 feeding stuff. On the other hand, mucin and biliary products can be 

 recognized in the feces. It seems very probable, therefore, that the 

 excess of nitrogen in the feces is to be ascribed to the nitrogenous 

 digestive fluids. Experiments made by T. Pfeiffer with swine have 

 confirmed this opinion. He fed artificial mixtures of starch, sugar, 

 oil, salts, paper fiber, and pure conglutin to pigs, and found 0.426, 

 0.364, 0.384, and 0.401 gm. (average 0.394 gm.) of nitrogen in the form 

 of metabolism products per 100 gm. of digested dry matter, i. e., nearly 

 the same amount as found by the writer with sheep. 



It can not be expected that every individual experiment will show 

 exactly the above amount (0.4 gm.), for there are many sources of error 

 in this indirect determination. Principal among these are inaccuracies 

 in fixing the limits of the feces and the small errors in the determina- 

 tion of nitrogen in the food and feces ; a difference of ± 0.03 per cent 

 of nitrogen here will materially affect the results. It is strange, there- 

 fore, that some authors, on the basis of experiments which agree in the 

 average with the above value but show variations of 0.2 to 0.3 gm. of 

 nitrogen, refuse to recognize a regular relation between the digested 

 dry matter and the nitrogen of the feces derived from the digestive 

 secretions. Where a method of investigation contains unavoidable 

 errors in both directions, only the average of a large number of investi- 

 gations can be used. The data obtained by such means have as yet not 

 disputed the rule worked out by the writer that for each 100 gm. of 

 digested dry matter the feces contain 0.4 gm. of nitrogen from meta- 

 bolism products. This rule, as we shall see presently, is of importance 

 in judging of the so-called "depression in digestibility" of crude pro- 

 tein, believed to have been observed when substances poor in nitrogen 

 or entirely free from it were added to the ration. 



It would be very interesting in this connection if in all digestion experi- 

 ments with animals the feeding stuffs were also treated by G. Kiihn's 

 method of artificial digestion. This method, which is simpler and more 

 accurate than Stutzer's pepsin-pancreas method, prescribes that 2 gm. 



