EXCHANGE OF MATERIAL. 439 



If the question to be decided is, how much of the nitrogenized 

 bodies is absorbed in certain modes of nutrition or after taking a 

 certain quantity of food, then naturally the quantity of nitrogen 

 originating from the food and leaving the body with the excre- 

 ments must be subtracted from the total quantity of nitrogen taken 

 up with the food. To obtain the quantity of nitrogen leaving the 

 body with the excrements it is necessary to subtract from the total 

 quantity of nitrogen of the excrements the quantity of nitrogen 

 coming from the digestive tract itself and its secretions, and the 

 amount of this last must be known. 



It is obvious that exact results which answer for all times can- 

 not be given for that part of the nitrogen which has its origin in 

 the digestive canal and fluids. It may not only vary in different 

 individuals, but also in the same individual after more or less active 

 secretion and absorption. In the attempts made to determine this 

 part of the nitrogen of the excrements it has been found that in 

 man, on non-nitrogenized or nearly nitrogen-free food, it amounts 

 in round numbers to about 1 grm. per 24 hours (RiEDER; RUBNER). 

 During starvation, in which a smaller quantity of digestive secre- 

 tions is eliminated, it is less. MULLER found in his observations 

 on the faster CETTI that only 0.2 grm. nitrogen was derived from 

 the intestinal canal. 



Nitrogen also leaves the body through the horn formation. The 

 quantity which is lost in this manner is, though it cannot be ex- 

 actly determined, insignificant. Man loses only about 0.3 grm. 

 nitrogen daily by means of the hair and nails (MOLESCHOTT) and 

 about 0.3-0.5 grm. by the scaling-off of the skin. The quantity of 

 nitrogen which leaves the body under ordinary circumstances by 

 the perspiration must be so small that, like the loss by the horny 

 structure, it need not be considered in experiments on the exchange 

 of material. The elimination of nitrogen by the perspiration need 

 only be considered in cases of profuse sweating. 



The view was formerly held that in man and carnivora an elim- 

 ination of gaseous nitrogen took place through the skin and lungs, 

 and because of this, on comparing the nitrogen of the food with 

 that of the urine and faeces, a nitrogen deficit occurred in the vis- 

 ible elimination. 



This question has been the subject of much discussion and of 



