28 UNIV. OF N. H. AGR. EXPERIMENT STATION [Bulletin 240 



sent unutilized food nitrogen, that is, hippurie acid nitrogen and amino 

 acid nitrogen. The quantities, however, are small so that for general 

 purposes of study the urinary nitrogen may be accepted as a satisfactory 

 measure of metabolized body protein or nitrogen. 



The nitrogen balance is essential to the assessment of the energy metab- 

 olism either when the animal is being fed or is fasting, since in the former 

 case food nitrogen suppUes a material source of energy to the body, and 

 in the latter case the nitrogen output is a direct measure of the protein dis- 

 integration of the body. 



Insensible Perspiration 



During all our experimental work we have aimed to determine the daily 

 insensible perspiration of the experimental animals. The insensible per- 

 spiration represents the daily loss in weight due to the invisible carbon and 

 water vapor expired through lungs and skin. It is thus a measure of the 

 rate of metabolism because both carbon and water vapor are the by- 

 products of metabolism. This insensible perspiration represents specific 

 daily losses in weight (as shown in Table III) which vary with feed level, 

 exercise, and other causes that influence the rate of metabolism. 



It consists of the weight with which an animal starts the day plus what 

 it eats and drinks minus the weight of the animal at the end of the day 

 plus the visible excreta {i.e. urine and feces). Except for the first day, it 

 requires simply one daily weighing of the animal with a careful record of 

 the weight of food and water consumed and of the urine and feces voided. 

 The insensible perspiration forms a good general measure of the rate of 

 metabolism, but its accuracy depends entirely on the accuracy and de- 

 pendabihty of the weighings, particularly of the animal. Unless scales 

 for weighing the animal are dependable to within two or three hundred 

 grams (i.e. half a pound) the insensible perspiration loses significance. 

 Even then such scales must be tested frequently for their degree of ac- 

 curacy. 



Recent researches on humans^ indicate a reasonably close correlation 

 between the insensible loss and general metabolism. On basis of our 

 investigations along this line extending over a period of ten years the 

 same may be said with regard to steers and cows.^ 



Careful daily records of the insensible loss are of experimental value 

 quite apart from its relation to metabolism in that they offer a basis for 

 calculation of those changes in body weight from day to day which are 

 due to irregularities in the replenishment and in the losses of the so-called 

 "fill" which must always form part of the live body weight although it 

 does not represent body material. 



1 Benedict, Carnegie Inst. Wash. Pub. No. 203, 1915, p. 84; Benedict and Hendry, 

 Boston Med. and Surg. Journ., 1921, 184, pp. 217, 257, 282, 297, and 329; Benedict, Bos- 

 ton Med. and Surg. Journ., 1923, 188, p. 127; Benedict, Bull. Soc. Sci. d'Hygiene Alimen., 

 1923, 11, p. 343; Benedict, Schweiz. med. Wochenschr., 1923, 53, p. 1101; Benedict, 

 The correlation between perspiratio insensibilis and total metabolism. Collection of 

 articles dedicated to the seventy-fifth birthday of Professor I. P. Pawlow, published 

 from the Institution of Experimental Medicine in Leningrad, 1924, p. 193; Benedict and 

 Root, Arch. Intern. Med., 1926, 38, p. 1. 



2 Benedict, F. G., and E. G. Ritzman, Carnegie Inst, of Wash. Pub. No. 377, 1927, 

 p. 63. 



