DIGESTION AND RESORPTION 1 21 



pentosans, but lignin appears to be much less readily digested 

 and some of the other incrusting materials not at all. As a 

 consequence, a computation based on the elementary composi- 

 tion of the crude fiber of the feed and of the feces respectively 

 and on the percentage of the former which is digestible shows 

 the digested portion to have approximately the ultimate compo- 

 sition and heat of combustion of cellulose. 



This is by no means equivalent to saying that the digested 

 crude fiber consists only of cellulose. The variations between 

 the results in individual experiments show clearly that this 

 cannot be the case and doubtless more or less of the pentosans 

 and other ingredients of the crude fiber are attacked to some 

 extent, but it is nevertheless evident that the cellulose is the 

 chief constituent digested. Neither is the heat of combustion 

 of the digested portion in any sense a measure of the energy 

 which it can supply for the bodily activities, as will appear 

 more clearly later. 



169. Composition of digested nitrogen-free extract. By 

 a difference calculation identical in principle with that em- 

 ployed for crude fiber but somewhat more complicated in its 

 details and involving certain assumptions, it has been shown 

 that the digested portion of the nitrogen-free extract has also 

 approximately the composition and heat of combustion of 

 starch or cellulose. Even less than in the case of crude fiber 

 does this fact serve to fix with any definiteness the nutritive 

 value of the digested portion. We know that the nitrogen- 

 free extract of feeding stuffs includes a great variety of sub- 

 stances (110), some of which, like starch, are digested in the 

 narrower sense of the word while many others, like the hemi- 

 celluloses, pentosans, etc., are fermented rather than digested. 

 The data as to the composition of the digested portion indicate, 

 it is true, that it consists chiefly of carbohydrates, but on ac- 

 count of the small range of ultimate composition shown by these 

 substances no indications are afforded of the specific carbohy- 

 drates present. 



170. Digestible carbohydrates. Since both the digested 

 crude fiber and the digested nitrogen-free extract have approxi- 

 mately an ultimate composition corresponding to the formula 

 C 6 HioO 5 , it has become customary in estimating the nutritive 

 values of feeding stuffs to add together the digestible portions 



