X.] WORK EXPENDED IN DIGESTION 191 



if all the energy derivable from the food is spent in 

 effecting digestion and internal work, there will be no 

 margin left either for external work or the production of 

 increased weight, hence, however much food the animal 

 ate, it can never do any work nor grow any heavier. 

 This condition, when the energy derived from the food 

 is wholly or even more than balanced by the energy 

 required for digestion, is realised in the case of a horse 

 feeding upon straw, which is very fibrous, so that the 

 work required is great and a large proportion remains 

 undigested, while of the digested carbohydrate about 

 one-fifth is lost as methane. Zuntz found that the 

 horse actually consumes more energy in digesting 

 straw than is contained in the portion digested, and 

 this is confirmed by an experiment of Miintz, who fed a 

 horse on straw alone. Although the horse was allowed 

 an unlimited amount of straw it died at the end of about 

 two months, thoroughly exhausted because it had been 

 compelled to draw upon its body. Again, Kellner, in 

 experiments with oxen, which are better able than 

 horses to deal with foods like straw, found that more 

 than four-fifths of the energy contained in the digested 

 part of the straw was consumed in the digestion 

 processes, leaving less than one-fifth available for work 

 or increase. When, however, the straw was made into 

 a pulp by the processes employed by papermakers, 

 who disintegrate the cellulose by boiling with an alkali 

 under pressure, as much as 88 per cent, of the straw was 

 digested, and of this digestible matter a little more than 

 a third only was consumed in effecting digestion. 



So fundamental are these considerations regarding 

 the total and available energy of foods, that we may 

 recapitulate : the total energy resident in any food is 

 measured by the heat it will evolve on burning, and is 

 called its fuel value. From this fuel value must be 



