192 UTILISATION OF FOOD BY THE ANIMAL [chap. 



deducted the fuel value of the undigested excreta, and also 

 of the incompletely oxidised urea and the gases evolved in 

 the intestine, in order to obtain the energy available for 

 the animal — the heat value of the food. All this energy is 

 available for the maintenance of the heat of the animal, 

 but a certain proportion is spent in the work of digestion, 

 and it is only the surplus that is available for the internal 

 and external work performed by the animal or for the 

 increase of weight that it may be putting on. If the 

 food is being added to the maintenance ration, all the 

 surplus will be available for external work or for 

 increased weight. We can now define the surplus 

 energy which a food will give out over and above the 

 work required for its own digestion, as the " dynamic " 

 energy of the food, and it is this dynamic energy alone 

 which is available for performing work or for giving 

 rise to increased weight. Thus we distinguish the total 

 energy or fuel value of the food, the heat value or 

 thermal energy, and now the dynamic energy or value 

 for work and production. 



One or two examples may perhaps make this more 

 clear. 



Taking decorticated cotton cake we may on its 

 analysis calculate the " fuel value " from the experiment- 

 ally determined facts that i gramme of oil (in such seed 

 cakes) produces 9-2 calories on combustion, i gramme 

 of protein and i of carbohydrate producing 5-8 and 

 4- 1 calories respectively. From 100 grammes of decorti- 

 cated cotton cake we therefore obtain a fuel value of 

 460 calories as follows : — 



Oil, 9 grammes x 9*2 calories . . = 82*8 



Protein, 41 grammes x 5'8 calories . == 237*8 



Carbohydrates and Fibre, 34 grammes 



X 4-1 calories . . . . = 139-4 



460'0 



