158 NUTRITION OF FARM ANIMALS 



digestible, or at least disappear from the feed during its passage 

 through the alimentary canal. 



220. Pentose sugars. In general, it may be stated that the 

 pentose sugars (in particular arabinose and xylose), whether 

 administered by the stomach or injected into the blood, are at 

 least partially oxidized in the body. The pentoses differ from 

 the hexoses chiefly in the fact that the limit of tolerance in the 

 blood (215) is lower. Excessive amounts of hexose carbohy- 

 drates cause an excretion of sugar in the urine. The same 

 effect is produced by the pentoses, but much smaller quantities, 

 relatively, are required to bring it about. 



Most, although not all, investigators have found an increase 

 in the glycogen of the liver consequent upon the ingestion of 

 pentoses, but in every case it has been the ordinary Ce glycogen, 

 indicating that the effect is an indirect one. 



221. Pentosans. The investigations upon the soluble 

 pentose sugars or their derivatives just referred to have shown 

 that they are to a greater or less extent assimilable. The pen- 

 tose carbohydrates in the feed of herbivora, however, exist to a 

 very limited extent, if at all, in this form. They are chiefly 

 polysaccharids, being either pure pentosans or combinations 

 of pentosans and hexosans. In discussing the nutritive value 

 of these pentosans, it seems to have been frequently assumed that 

 they are converted into pentoses during digestion. As a matter 

 of fact, however, there is no direct evidence that such is the case, 

 while Kellner's results (129) afford reason to believe that they 

 are largely fermented along with cellulose, yielding, besides 

 gaseous products, chiefly organic acids. If this is the case, 

 farm animals do not acquire from their feed any considerable 

 amounts of pentoses and conclusions drawn from experiments 

 with the pentose sugars regarding the nutritive value of these 

 substances are inapplicable to ordinary stock feeds. Their 

 true value in the latter would be simply that of the products of 

 their fermentation. 



The organic acids 



222. Formation in digestion. As was shown in Chapter 

 IV (128-130, 132), a considerable proportion of both the hexose 

 and pentose carbohydrates contained in the feed of herbivora 

 undergoes fermentation in the digestive tract, giving rise, -in 



