66 



PHYSIOLOGY 



letter prefixed, not the optical qualities of the substance in question, 

 but its relation to other substances, especially glucose. Thus, d- 

 fructose means that fructose is the ketose corresponding to the dextro- 

 rotatory glucose, d-fructose itself being Ia3vo-rotatory, though its 

 active asymmetric C atoms are identically arranged with those in 

 glucose. With this limitation one may say that it is only the d-hexoses 

 of a particular form which are assimilable, and therefore of physio- 

 logical importance. The small differences in the configuration of the 

 four d-sugars can be readily seen if their graphic formulae be com- 

 pared : 



CHO CHO CH 9 OH CHO 



H.C.OH 



HO.C.H 



H.C.OH 



H.C.OH 



CH 2 OH 



d-glucose 



HO.C.H 



HO.C.H 



H.C.OH 



H.C.OH 



CH 2 OH 



d-mannose 



CO 



HO.C.H 



H.C.OH 



H.C.OH 



CH 2 OH 



d-fructose 



H.C.OH 



HO.C.H 



HO.C.H 



H.C.OH 



CH 2 OH 



d-galactose 



THE PENTOSES. C 5 H 10 5 



These bodies occur largely in plants in the form of complex polysaccharides, 

 the pentosanes, which give pentoses on hydrolysis with acids. Two forms 

 of pentose have been found in the animal body, namely, i-arabinose, which 

 has been isolated from the urine in cases of pentosuria, and 1-xylose, which 

 occurs built up into the nucleic acid molecule of the pancreas and perhaps 

 other organs. The pentoses can apparently be utilised by herbivora as food- 

 stuffs. We know nothing, however, as to the part they play in the animal body 

 or as to the causation of the rare condition of pentosuria. Since, however, they 

 are reducing substances and the presence of pentose in urine might therefore 

 lead to a suspicion of diabetes, it is necessary to mention the tests by which 

 the presence of pentoses may be detected. The two folldwing are the chief 

 tests for pentoses : 



(1) The solution supposed to contain a pentose is mixed with an equal volume 

 of concentrated hydrochloric acid. To the mixture is added a small quantity 

 of solid orcin and the whole is heated. If pentose is present the solution becomes 

 at first reddish-blue and later bluish-green. The colour can be extracted on 

 shaking the fluid with amyl alcohol, the solution, on spectroscopic examination, 

 showing an absorption band between C and D. 



(2) Instead of adding orcin, we may add phloroglucin to the mixture of 

 hydrochloric acid and pentose. The solution on heating becomes first cherry 

 red and then cloudy. On shaking with aniyl alcohol a red solution is obtained 

 which shows a band between D and E. 



