i88 PLANT PRODUCTS 



Digestion. Attempts have been made to measure the 

 ultimate results of the digestive processes in animals. In 

 such experiments all the food consumed by the animal is 

 analysed, and, in addition, all the solid excreta are analysed 

 in the same way. The difference between the two is supposed 

 to represent the material which has been digested. There 

 are several errors, nevertheless, in this assumption. In the 

 process of digestion, portions of the food are first absorbed, 

 converted into intestinal mucus, etc., and are excreted. 

 The ultimate gain to the animal is quite correctly represented 

 by the difference between the two analyses named above, 

 but a more serious error is introduced by bacterial activity. 

 The bacteria are, all the time digestion is going on, struggling 

 to get a share of the food. Such bacteria as oxidize the food 

 materials will produce just the same amount of heat as 

 the oxidation would give under other circumstances. If 

 the animal requires this heat there would be no loss. If the 

 animal does not require the heat, as might be the case in 

 hot weather, then the heat produced is not merely useless, 

 but a nuisance. The bacteria, however, that flourish in 

 the intestinal tracts, are for the most part of a different type, 

 and much of their energy is devoted to the decomposition 

 of carbohydrates with the production of marsh gas and 

 carbon dioxide. As much as 700 litres of marsh gas from 

 one beast in one day has been observed, which is equivalent 

 to a waste of four pounds of carbohydrate. These carbo- 

 hydrates, of course, disappear, and are considered as digested, 

 although they have produced little heat, and no good of 

 any kind to the animal. Such fermentive changes depend 

 upon slow digestion, the quicker the animal can digest the 

 food the smaller is the share available for the bacteria. 

 As the result of such experiments, tables of digestibilities 

 have been constructed (see Kellner). Such tables will 

 allow one to calculate the probable amount of food actually 

 digested by the beasts from any particular food supplied. 

 The ordinary analysis can be carried out according to the 

 text-books (see Bibliography), and then the digestive 

 coefficients used to convert these figures into digestibilities. 



