ii THE THEKMIC ECONOMY OF THE ORGANISM 55 



oxygen absorbed by the lungs. This process transforms potential 

 into actual energy, which in the case of a man doing no external 

 work appears almost entirely as heat. 



When the quantity of heat which can be developed by the 

 complete combustion of 1 grm. of each food-stuff is determined 

 in calories and the amount of each substance taken by a man in 

 twenty-four hours is also known, a simple calculation will show 

 how many calories are developed by him daily. When making 

 this calculation we must be careful to distinguish between the 

 consumption of fat and carbohydrates, which are completely 

 consumed in the organism and transformed into water and 

 carbon dioxide, and that of proteins, which undergo a partial 

 process of oxidation and are transformed into urea and other 

 nitrogenous products, representing a considerable amount of 

 potential energy which is not used by the organism. For this 

 reason we must deduct from the heat ' of combustion of protein 

 the thermic value of the final nitrogenous products contained in 

 the urine and the faeces. 



Rubner determined the heat developed in the body by proteins 

 in the following way : He fed a small dog on proteins similar to 

 those which he used for the calorimetric determination of the 

 heat of combustion, and determined the heat of combustion of 

 the corresponding urine ; this he found to be T0945 cal. per 

 gramme of albumen consumed. In the next place the heat of 

 combustion of the faeces was found by experiment to be O1854 

 cal. per gramme of albumen, and 0'05 cal. was allowed for the 

 heat absorbed by the solution of the albumen and the urea. 

 The physiological value of 1 grm. of dry albumen, including the 

 ashes, would thus be represented by 5'754 cal. - (1*0945 + 01864 

 + 0-05) = 4-424 cal. 



A similar calculation showed the available physiological value 

 of 1 grm. of muscle free from water and fat but including the 

 ash to be equivalent to 4'001 cal., and in the case of muscle 

 from an animal which had died of inanition 3 '842 cal. 



We have already seen that in studying exchange of material 

 the amount of protein consumed is calculated from the quantity 

 of nitrogen eliminated. Expressing material exchange in terms 

 of energy, we find that 1 grm. of nitrogen discharged corresponds 

 to 26'66 cal. in the case of albumen, and 25'98 cal. for muscle. 

 The heat of combustion of human urine corresponds on an 

 average for 1 grm. of nitrogen to 8'0 cal. ; that of human faeces 

 per gramme of nitrogen varies greatly between 66 and 159 cal. ; 

 but the range is much less per gramme of organic substance 

 from 5'2 to 7'7 cal. ; and 6'5 may be taken as an average figure 

 (Rubner, Atwater, Loewy). 



When we desire to calculate from the intake and output the 

 calories developed daily by the organism, we must be satisfied 



