122 NUTRITION OF FARM ANIMALS 



of these two groups and to designate the sum as the " digestible 

 carbohydrates." The practice dates from the early experi- 

 ments of HennebergandStohmann, but in the light of our present 

 knowledge has little justification. 



In the first place, as just stated, the agreement in composition 

 is but approximate and variable. The essential point, however, 

 is that a digestion experiment can show simply that a certain 

 amount of material of a certain ultimate composition has failed 

 to reappear in the feces of the animal, and by itself affords no 

 information as to the changes which it has undergone nor as 

 to the nature of the products actually resorbed. As a matter 

 of fact, a large share of the " digested " portion of these two 

 groups, especially in the case of ruminants, has been fermented 

 rather than digested. A considerable proportion of it has been 

 excreted in gaseous form as carbon dioxid and methane and 

 only a residue of organic acids has been resorbed. Such being 

 the case, the term digestible carbohydrates is a palpable mis- 

 nomer. 



171. Digested ether extract. No determinations of the 

 composition of the digested ether extract similar to those on 

 crude fiber have been made, but a few determinations of the 

 heat of combustion of the digested extract are reported by 

 Kellner. 1 The presence in the feces of ether soluble excretory 

 products (165) interferes with the accuracy of such a comparison 

 and its results must be regarded as approximations. The ether 

 extract of the feces was found to have a higher heat of combus- 

 tion than that of the hay fed, doubtless on account of the 

 presence in the former of the indigestible waxes, etc., while 

 the computed heat of combustion of the digested portion was 

 distinctly lower than that for pure fats, which average about 

 9.5 Cals. per gram. The heats of combustion per gram on the 

 average of five trials were : 



Ether extract of hay .... 9.194 Cals. 

 Ether extract of feces .... 9.824 Cals. 

 Digested ether extract . . . . 8.322 Cals. 



1 Landw. Vers. Stat., 47 (1896), 301. 



