110 NUTRITION OF FARM ANIMALS 



as wholly indigestible. Of more importance, however, are 

 such carbohydrates as cellulose and the various hemicelluloses, 

 the levulans, galactans, mannans, pentosans, etc., which 

 may be said to be practically only partially digestible. 

 By this is not meant, of course, that one molecule of cellulose, 

 e.g., is any less digestible per se than another, but only that 

 part of the cellulose of ordinary feeds does as a matter of fact 

 escape digestion, largely because the length of time during 

 which it is exposed to the action of the organisms which attack 

 it is insufficient to allow of its complete solution. The feces of 

 herbivorous animals, therefore, contain amounts of these di- 

 verse carbohydrates varying with the character of the feed and 

 the activity of the fermentation processes in the digestive tract. 

 Since these compounds are especially abundant in the roughages, 

 the feces from these feeds are bulky and especially rich in un- 

 digested " crude fiber." 



Other ingredients, particularly of vegetable feeding stuffs, 

 partially escape digestion not on account of any lack of the ap- 

 propriate digestive enzyms but because they are mechanically 

 protected from the action of the latter. If granules of starch, 

 e.g., are contained within a cell which has not been ruptured 

 during the mastication of the feed, the cell wall tends to protect 

 them from the action of the digestive juices, and they may escape 

 digestion although per se entirely digestible. The extent to 

 which such a nutrient will actually be digested, therefore, will 

 depend to a considerable degree upon whether the cellulose of 

 the cell wall is attacked and destroyed by the organisms of the 

 alimentary canal. What is true of starch in this respect is 

 obviously true of all cell enclosures, and explains why more or 

 less matter intrinsically digestible may be rejected in the feces. 

 For a like reason, seeds which escape mastication are but im- 

 perfectly digested, being protected by the relatively insoluble 

 seed coats. Similarly, cellulose itself may be so impregnated 

 with lignin and cutin substances that the " crude fiber " may 

 be attacked only with difficulty or not at all by the methane 

 fermentation. 



Finally, there is to be considered the possibility of a mis- 

 proportion between digestion and resorption. In heavy rations, 

 especially, substances which are actually digested may per- 

 haps escape resorption through insufficient contact with the 



