METABOLISM AND FOOD 87 



As a third cause for the diminished utilisation 

 of the more indigestible feeding-stuffs, regard 

 must be paid to the processes of decomposition 

 which go on in the intestines. There many com- 

 ponents of the food are so far decomposed that 

 they can serve for the production of heat, but 

 not for the formation of flesh or fat. It is usual 

 to regard the difference between food and dung as 

 being digested, but in the case of difficultly digest- 

 ible food, a portion is not really digested, but has 

 undergone putrefaction. In certain cases food con- 

 stituents, such as organic acids, which have no value 

 for the formation of new tissue, may also partially 

 account for the inferiority of the digested substances. 



It is therefore quite clear that the amount of 

 work required for mastication and digestion, as 

 well as the extent of putrefaction in the partially 

 digested food, must be closely connected with the 

 hardness and digestibility of the food material. When 

 it is considered what components of the food deter- 

 mine its hardness and digestibility, there can be no 

 doubt that the quantity of crude fibre takes the first 

 place. The digestible crude fibre is of itself equal in 

 nutritive value to starch, but the work of mastication 

 and of digestion, and also the putrefactive changes in 

 the intestines, fall not upon this portion alone, but 

 upon the whole of the crude fibre consumed. The 

 richer the straw or hay is in crude fibre, the less 

 must be the amount of the digested substances of 



