70 PHYSIOLOGY 



digestion or inversion of cane sugar. It is difficultly crystallisable. 

 Its watery solution is laevo-rotatory, and reduces Fehling's solution 

 somewhat less strongly than glucose, its reducing power being 92, 

 if we take that of glucose as 100. It ferments readily with yeast ; 

 with phenyl hydrazine it gives the same osazone as is formed from 

 glucose. 



GALACTOSE is formed by the digestion or hydrolysis of milk 

 sugar or lactose. It is also obtained on hydrolysing cerebrin, a con- 

 stituent of the brain, with dilute mineral acids, and by the hydrolysis 

 of certain vegetable gums. It is much less soluble in water 

 than glucose. It is dextro-rotatory and shows marked bi-rotation. 

 With ordinary yeast it ferments but extremely slowly. One species 

 of yeast is known, namely, saccharomyces apiculatus, which, while 

 fermenting d-fructose and glucose, has no effect on galactose. This 

 yeast can therefore be used to isolate galactose from a mixture of the 

 monosaccharides. It reduces Fehling's solution, its reducing power 

 being somewhat less than that of glucose. Yeasts can be trained to 

 ferment galactose. 



MANNOSE. Mannose, though an assimilable sugar, is of such rare occur- 

 rence in our food-stuffs that it plays practically no part in animal physiology. 

 It is dextro-rotatory, reduces Fehling's solution, ferments easily with ordinary 

 yeast, and gives an osazone which is identical with that derived from glucose. 



DERIVATIVES OF THE HEXOSES 



Two derivatives of glucose are of considerable physiological 

 importance, namely, glucosamine and glycuronic acid. 

 Glucosamine, C 6 H 13 N0 5 , has the structural formula : 



CH 2 OH 



(CH.OH) 3 



CH.NH 2 



CHO 



It is obtained from chitin, which forms the exoskeleton of large 

 numbers of the invertebrata, by boiling this with concentrated hydro- 

 chloric acid. It is stated to have been obtained as a decomposi- 

 tion product of certain proteins and their derivatives, such as the 

 mucins. It is of special interest as affording an intermediate product 

 between the carbohydrates and the oxy-amino acids which can be 

 obtained by the disintegration of proteins. In solution it is dextro- 



