ANIMAL HEAT. 583 



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 ingested, 

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

 oxidation of each of the food-stuffs to their normal residual substances. As the 

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

 of any 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. 

 302). 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 2 and H 2 O ; 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 show that 

 each gram of urea contains potential energy equivalent to 2523 calories, and 

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

 841 calories, each gram of proteid yields to the organism only 4937 calories. 

 The available energy from the proteid would, therefore, be equivalent to the 

 total amount of energy derivable from the complete oxidation of the proteid 

 minus the amount represented in the urea. With these facts in view it is a 

 simple matter to determine the total income of energy, should the diet be 

 known. Thus, if the diet consists of 120 grams of proteids, 90 grams of fat, 

 and 330 of carbohydrates, the absolute and available amounts of energy 

 ingested are 



Grams. Calories. Calories. 



Proteids 120 x 5778 693,360 



Fats 90 x 9312 837,080 



Carbohydrates 330 x 4116 ^^i 28 ^ 



2,888,720 



Deduct the proteid energy in 40 grams of urea, 40 x 2523 = 100,920 



Total daily heat-production 2,787,800 



This is assuming that the entire quantity of proteids, fats, and carbohydrates 

 is digested, absorbed and ultimately broken down into CO 2 , H 2 O, and urea. 

 This assumption, however, is not justified by facts, since we know, for instance, 

 that more or less food escapes digestion. In practice, therefore, it is necessary 

 to ascertain from the excreta of the animal (see section on Nutrition) just how 

 much of the ingested food has been absorbed and completely or partially 

 destroyed in the body. 



Calorimetric investigations also afford us indirect information as to the 

 income of heat by showing the quantities of heat produced and dissipated. 

 Such data are of much value, since it is evident that should the energy of the 

 body be maintained in a condition of equilibrium from day to day, and should 

 the energy resulting from the transformation of potential energy be manifested 



