CARBOHYDRATES. 29 



time not been found in nature, yet a native alcohol adonitol, obtained in an 

 optically inactive form from Adonis vernalis, corresponds to rhibose, a 

 sugar of the pentose series and which has only been prepared synthetically. 



In the case of the sugars containing more than six atoms of carbon and 

 which up to the present time have only been obtained artificially, the 

 seven-carbon alcohols perseitol and volemitol are found in nature. The 

 former is present in the unripe seeds, leaves, and pericarp of Persea gratis- 

 sima, while the latter is contained in Lactarius volemus, and has recently 

 been prepared from the rhizomes of several species of Primula. 



Of the four above-mentioned hexoses, glucose, galactose, fructose and 

 mannose, only the first three are found in the animal organism. d-Mannose 

 is found only in the vegetable kingdom partly as such (for example, in 

 the sap of Japanese Amorphophallus Konjako), to some extent as glucoside- 

 like compounds (thus, strophantobiose decomposes into d-mannose and 

 rhamnose), and finally very extensively in anhydride-like, condensation 

 products known as mannans. 



Fructose occurs similarly in the vegetable kingdom, and likewise either 

 free or combined. In the former state it is seldom found pure, but usually 

 is mixed with other sugars as a component of many fruits. Fructose is 

 formed, furthermore, by the hydrolysis of many vegetable substances ; 

 thus, of inulin, the reserve-substance in the tubers of dahlia, helianthus, 

 sweet potato, elecampane, etc. 



Its most important occurrence is in cane sugar, by the hydrolysis of 

 which one molecule of d-glucose and one molecule of d-fructose are 

 obtained. This mixture is known as invert sugar. 



In the products of the animal kingdom, fructose is not often found. In 

 honey it occurs together with glucose. It is sometimes to be found in the 

 urine after one has eaten considerable fruit. In rare cases it is found in 

 larger amounts in the urine of a diabetic. That fructose occurs normally 

 in animal tissues is extremely doubtful. 1 



Grape sugar, d-glucose, glucose or dextrose, as it is variously called, 

 plays without question the most important role of all the monosaccharides 

 in the animal system. It is this form which carbohydrates in general 

 assume before absorption and assimilation. As d-glucose the greater part 

 of the carbohydrate is conducted from organ to organ, from the place of 

 storage to the place of consumption. Glucose is always present in the 

 blood, and the amount varies only within narrow limits, averaging from 

 0.05 to' 0.1 per cent in different animals. 2 These figures, however, are not 

 accurate, because glucose is not the only sugar that is found in blood. On 



1 Cf. Adler and Adler: Pfliiger's Arch. 110, 99 (1905); Neuberg and Strauss: Z. 

 physiol. Chem. 36, 233 (1902); Rudolf Ofner: Monatsh. 25, 1153 (1904); 26, 1165 

 (1905); and Z. physiol. Chem. 45, 359 (1905). 



8 See Lecture XXIII. 



