68 THE CARBOHYDRATES. 



fermentable, and yields an osazon which melts at 193 C. It 

 reduces alkaline solutions of cupric oxide, but to a less marked 

 degree than glucose. On oxidation it yields first galactonic acid and 

 later mucinio acid. 



The Pentoses. The pentoses occur widely distributed in nature. 

 In the animal body they have been demonstrated in the pancreas, 

 the liver, thymus, thyroid, spleen, kidneys, salivary glands, brain, 

 and muscle. The total amount which can be obtained from the adult 

 human body probably represents about 10 grammes. The largest 

 amount is found in the pancreas, where it is present to the extent 

 of about 2.48 per cent, of the moist organ. In the tissues the 

 pentoses do not exist in the free state, but form an integral part of 

 the nucleoproteids, of which they are in a measure characteristic. 

 Traces of pentoses occur in the urine under normal conditions ; 

 abnormally much larger amounts may be encountered (see Urine). 



The pentoses which have been found in nature are arabinose, 

 xylose, and rhamnose. Neuberg has shown that of these /-xylose 

 is the common pentose of the nucleoproteids of the organs. It is 

 an aldose and has the structural formula : 



OH H OH 



The optically inactive arabinose occurs in the urine, where d-rham- 

 nose has also been found. 



On prolonged boiling with dilute hydrochloric acid the pentoses 

 lose water and form furfurol, viz., methyl-furfurol, according to 

 the equation : 



C 5 H 10 5 -3H 2 = QH.A 



This fact is utilized in their quantitative estimation. On heat- 

 ing with hydrochloric acid and phloroglucin, or orcin, characteristic 

 color-reactions result (see Urine). Like the hexoses they form 

 compounds with phenylhydrazin. They are not fermentable. 



Of special interest is the observation that on fermentative decom- 

 position glucuronic acid passes over into the aldopentose /-xylose : 



COH(CHOH) 4 .COOHe-^COH(CHOH) 8 .CH,.OH 

 Glucuronic acid. Z-xylose. 



This demonstrates the possible transformation of a sugar of the 

 d- into one of the /-series, which is quite analogous to what occurs 

 in the animal body. 



THE DISACCHARIDES. 



The disaccharides result from the monosaccharides through a con- 

 densation of the anhydrides of two monosaccharine molecules, analo- 



