CARBOHYDRATES. 31 



the framework-substance in all varieties of cellulose. Finally it takes 

 part in the formation of a great number of glucosides. 



The last member of this series which we have to consider is galactose. 

 Up to the present time it has never been detected with certainty in the 

 free state. It is found as a constituent of certain vegetable glucosides, 

 for example, digitonin and sapotoxin. 



On the other hand, numerous polymers of galactose, the so-called galac- 

 tanes, are known, some of which yield galactose alone on hydrolysis, while 

 some give other sugars as well. 



In the animal organism, galactose is present chiefly in milk-sugar, but 

 similarly it has been obtained by the hydrolysis of cerebron (see page 20) . 

 We know very little concerning the formation of galactose or of milk- 

 sugar. There are, as has been mentioned, certain well-known higher 

 sugars in the vegetable kingdom which yield galactose, but it is very 

 questionable that there is any direct connection between the galactose in 

 the nourishment and that of milk-sugar. We shall come back to this 

 point in the discussion of the latter compound. 



At this place we will consider two compounds which are very closely 

 related to glucose, namely, glucuronic acid and glucosamine (chitosamine) . 



Glucuronic acid (also written glycuronic) is a derivative of glucose. 

 Schmiedeberg and Meyer 1 suspected this, for they realized that the com- 

 pound combined the properties of an acid, an aldehyde, and a polyvalent 

 alcohol. It was not proved, however, until Thierf elder 2 succeeded in 

 changing glucuronic acid into d-saccharic acid, and Fischer and Piloty 3 

 effected its synthesis from d-saccharic acid. By the work of the last 

 named, the configuration was established, as shown by the following 

 summary : 



CHO GO . OH CHO 



HCOH HCOH HCOH 



HOCH HOCH HOCH 



HCOH HCOH HCOH 



HCOH HCOH HCOH 



CH 2 OH CO .OH CO . OH 



d-Glucose d-Saccharic acid d-Glucuronic acid 



- Z. physiol. Chem. 3, 422 (1879). 



2 Ibid. 11, 389 (1887). 



3 Ber. 24, 521 (1891). 



