CHEMICAL BASIS OF THE ANIMAL BODY. 1217 



either aldehydes (aldoses) or ketones (ketoses) of polyatomic 

 alcohols. 



The Pentose Group (C 5 H 10 O 5 ). 



Typical members of this group are arabinose and xylose, 

 obtained by the hydrolytic action of mineral acids on vegeta- 

 ble gums. They are crystalline, dextrorotatory, and reduce 

 Fehling's fluid (see note 2, p. 1222), but are not fermentable. 

 Their osazones melt at characteristically low temperatures, 

 155 — 160°. When heated with hydrochloric acid they yield 

 furfuraldehyde, the aldehyde (C 5 H 4 2 ) of pyromucic acid, 

 which serves for both the detection and estimation of these 

 sugars. When warmed with phloroglucin (trihydroxy benzene, 

 C 6 H 3 (OH) 3 ) in presence of strong hydrochloric acid, they give 

 rise to a brilliant red colour, which is characteristic and shows 

 an absorption band between D and E. 



Pentoses have recently been found in the urine of an opium 

 eater and in several cases of diabetes. It also appears probable 

 that the carbohydrate obtained during the decomposition of a 

 nucleo-proteid prepared from the pancreas is a pentose. When 

 pentoses are administered to animals (rabbits and fowls), they 

 are absorbed and assimilated and lead to an accumulation of 

 glycogen, which is however of the ordinary kind. There is 

 some difference of opinion as to their fate when introduced into 

 the human body, but the balance of evidence seems to shew 

 that they largely reappear unaltered in the urine. 



The Hexose or Grlucose Group. 

 1. Dextrose (Glucose, Grape-sugar). 



C 6 H i2° 6 - [COH-(CH.OH) 4 -CH 2 .OH]. 



Is found in minute but fairly constant quantities as a nor- 

 mal constituent of blood, lymph and chyle. Its occurrence in 

 the liver has been already referred to (§ 369) in connection 

 with diabetes, a disease which is characterized by the large 

 amount of dextrose which is present in the blood and the still 

 larger amount in the urine. The question whether dextrose is 

 a normal constituent of urine has led to much dispute, but it 

 now appears probable that it is present in minute amounts. 

 The experimental difficulties of detecting traces of sugar in this 

 excretion are considerable. There is no dextrose normally in 

 bile. 



The probability that it is as dextrose that the carbohydrates 

 are finally absorbed from the alimentary canal has already been 

 referred to (p. 1191). This corresponds with the fact that dex- 

 trose is the most readily assimilable sugar, as is known from 

 comparative injections of the various sugars into the blood-ves- 



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