I90 UTILISATION OF FOOD BY THE ANIMAL [chap. 



body, we must make a distinction between the work 

 done in carrying on such movements as breathing, the 

 circulation of the blood, etc., which are bound up with 

 the life of the animal, and the other work of digestion — 

 mastication, swallowing, the motions of the stomach and 

 intestines — which will vary in amount with the nature 

 of the food supplied. Pure fats and carbohydrates like 

 sugar can be digested with a minimum of exertion, but 

 when the same substances are found in food-stuffs 

 entangled among the fibres of tough materials like hay 

 and straw, the animal may have to do a good deal of 

 work in breaking down the food before the enzymes can 

 get at the digestible constituents, and this work has to 

 be derived from the combustion of some previously 

 digested food stored up in the body, i.e. eventually 

 from the energy contained in the food itself. If, then, 

 for any food-stuff we begin by estimating a certain 

 number of calories as its heat value, which heat value 

 is the measure of the energy it can liberate in the body, 

 we shall have to make a deduction for the energy used 

 up in digestion before we can get at the energy remain- 

 ing that is available for work or for such purposes as 

 putting on fat. If, indeed, the work spent in the 

 digestion of a given food is very large, it may approach 

 or even exceed its total heat value, and so leave no 

 margin for either the internal or external work of the 

 body. Of course, the energy thus spent either in 

 digestion or internal work is still transformed into heat, 

 whereby the animal is kept warm ; hence an animal at 

 rest on a maintenance diet, the heat value of which just 

 supplies enough energy for both digestion and internal 

 work, will still maintain its animal heat, and will 

 even be able to increase it by a greater consumption of 

 food if it is forced to make up for greater losses of 

 heat by being put to live under colder conditions. But 



