CARBOHYDRATES. 21 



Later on, when we come to consider the proteins, we shall have to take up 

 these substances in detail. 



From the group of glucosides, furthermore, there are derived a great 

 many compounds, some of which have strong toxic properties and are very 

 important drugs, their pharmaceutical value having been discovered purely 

 empirically. 1 Of this large group of such compounds we shall mention 

 only the saponin substances, phloridzin, salicin, helleborin, and the digitalis 

 glucosides (digitalin, digitonin, digitoxin) 2 . Finally, it may be stated 

 that alizarin, the well-known red dye, likewise occurs in nature as a gluco- 

 side (ruberythric acid) in madder root (Rubia tinctoruiri). This gluco- 

 side has lost most of its practical importance on account of the famous 

 synthesis of alizarin by Graebe and Liebermann (1868). This synthesis 

 was considered a great triumph of chemical investigation, and awakened 

 many bright dreams for the future. Even then it was suggested that the 

 time was near at hand when foods could be produced practically by 

 synthetic methods. Although this hope has not yet been fully 

 realized, nevertheless, such syntheses as that of alizarin have an indirect 

 effect upon the production of foods because whenever a natural substance 

 is replaced by an artificial one a considerable amount of acreage is 

 released. 



As far as the animal organism is concerned, the hexoses are the most 

 important representatives of the monosaccharides, and for a long time they, 

 and the corresponding polysaccharides, were the only carbohydrates to be 

 considered at all. It was not until 1892 that a sugar with five molecules of 

 carbon corresponding to the formula CsHioOs was discovered. In that 

 year Jastrowitz 3 found a specimen of human urine showing strong reducing 

 properties but which fermented little if any, and was moreover optically 

 inactive. Salkowski 3 then showed that a pentose was present. In the 

 same year Kossel, 4 by the hydrolysis of yeast-nucleic-acid with hydrochloric 

 acid, obtained furfurol, and soon afterwards Hammarsten 5 made similar 

 observations in studying the nucleoproteid obtained from the pancreas; 

 later on Salkowski 6 followed the matter still further, and proved finally 

 that pentoses are present in the above-mentioned products. Sugars, 

 then, of the five carbon series, have been detected one after another in 

 various products found in the body, 7 especially in the nucleoproteids. The 



1 See text-books on pharmacology for the physiological and pharmacological action 

 of these substances. 



2 For other special cases, see van Rijn: Die Glykoside, Berlin, 1900. 



3 Zent. med. Wissensch. 19 and 35, pp. 337 and 593 (1892). 



4 "Ueber die Nucleinsaure, " Verhandl. physiol. Gesellsch, Berlin, and in Arch. 

 Physiol. Anat. 1893, 157. 



5 Z. physiol. Chem. 19, 19 (1894). 



6 Ibid. 27, 507 (1899). 



7 Z. klin. Med. 34, 160 (1898). 



