CALORIFIC VALUE OF FOODS 193 



of food, just in the same way as a steam engine will require 

 a certain amount of coal to keep up the steam pressiu'e, 

 though doing no work, and any work required from it would 

 necessitate a further allowance of coal, a portion only of 

 which would be accounted for in the work done. The amount 

 of energy required for the utilization of food materials depends 

 upon the way in which the food materials are presented 

 to the animal. Pure foods and sugars can be digested with 

 the least exertion, but when these substances occur in food 

 among hay or straw, then the animal will have to do much 

 chewing, and other work, before the fats, carbohydrates, 

 and proteins are acted on by the enzymes in the digestive 

 tracts. Moreover, a far larger quantity of enzyme will have 

 to be produced, because a great many enzymes are condensed 

 on the surface of the fibrous matter in the alimentary tract, 

 and most rates of decomposition depending upon enzymes are 

 considerably retarded by the presence of cellulose in the 

 digestive tracts ; bulky food will also need a greater amount 

 of fluid, which has to be produced by the animal, at some 

 expenditure of energy. Under very extreme circumstances, 

 energy expended in the effort to digest food may exceed 

 the energy obtained from the digested part of the food. 

 Ruminants swallow much of their food with only partial 

 mastication but regurgitate it, '* chew the cud," and 

 again swallow. The finely comminated material is filtered 

 out by the third stomach and the insufficiently chewed fibres 

 again regurgitated. In this way a ruminant can make much 

 more effective use of fibrous food than a non-ruminant 

 herbivorous animal. A horse is quite incapable of living upon 

 straw alone, although an ox may just manage to keep itself 

 alive. If, however, part of the work of digestion be done 

 beforehand, much better results can be obtained. Kellner 

 found that straw pulp, as used for papermaking, was far more 

 digestible than straw itself. Of the straw pulp as much as 

 88 per cent, could be digested by an ox, and, after allowing 

 for the work of digestion, the straw pulp was worth rather 

 more than one-half its weight of starch as a food material. 

 As, however, in the process of turning straw into paper pulp, 



D. 13 



