ANIMAL HEAT. 47.") 



Income of Heat. — Since the energy-yielding food-stuffs are essentially 

 proteids, fats, and carbohydrates, and composed of C, II, O, and X, and since 

 the products of their disintegration arc essentially urea, 0O 2 , and H 2 0, the 



amount of energy yielded by the oxidation of the food-stuffs can readily be 

 determined if we know the quantity and quality of the food and excreta. 

 Since the energy of the organism is manifested essentially in the form of 

 heat and work, and as under ordinary circumstances but a fraction of it is 

 manifested as work, we may in making this estimate, as a matter of con- 

 venience, consider that the total available energy of the food appears in the 

 form of heat. 



The income of energy has been estimated by determining — (1) the quan- 

 tity of oxygen consumed ; (2) the amounts of ( ' and II that are oxidized in 

 the body into C0 2 and H 2 ; (3) the quantity and quality of the food con- 

 sumed in the body and the products resulting from their decomposition, and 

 the energy yielded by the oxidation of the same substances outside the body 

 when they are decomposed into the same residual products as appear in the 

 body ; (4) the quantity of heat produced, by the aid of a calorimeter, the 

 individual being kept quiet so that as little as possible of the energy 

 expended appears as work. The third method is a method of indirect 

 calorimetrv, and the fourth method that of direct calorimetry, or, briefly, 

 calorimetry. 



The first two methods have fallen into disuse. According to the third 

 method, it is necessary that we know the kind and quantity of food consumed, 

 the final products of disintegration, and the quantity of energy evolved by the 

 conversion of each of the food-stuffs to its normal residual substances. As the 

 basis of these calculations we have the fact that during the complete oxidation 

 of anv given substance a definite amount of energy is given off, and that when 

 the oxidation is but partial only a portion of energy is evolved, the proportion 

 being in accordance with the stage of oxidation. The complete oxidation of 

 1 gram of proteid yields 5778 calories; of 1 gram of fat, 9312 calories ; and 

 of 1 gram of carbohydrate, 4116 calories (see Potential Energy of Food, p. 

 364). If these substances be completely oxidized in the body, the amount of 

 energy evolved will be the same as though the oxidation occurred outside 

 of the body, provided that the final products are the same in both cases. As 

 far as fats and carbohydrates are concerned, we are justified in assuming that 

 they are completely oxidized in the body into CO a and H 2 Oj but the proteids, 

 as already pointed out, undergo only partial oxidation, each gram yielding 

 about one-third of a gram of urea. The results of experiments -how that 

 each gram of urea contains potential energy equivalent to l!o'J"» calories, and 

 since each gram of proteid yields one-third of a gram of urea, representing 

 841 calories, each gram of proteid yields theoretically to the organism onl\ 

 4937 calories. The available energy from the proteid would, therefore, be 

 equivalent to the total amount of energy derivable from the complete oxida- 

 tion nf the proteid minus the amount represented in the urea. Practically, 



