388 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV. 



It therefore contains the group CO in the middle of the chain. 

 Some of the sugars are ketones of more complex alcohols ; these are 

 called ketoses. The only one of these which is of physiological interest 

 is levulose. 



The alcohols of which we have already spoken are called monatomic, 

 because they contain only one OH group. Those which contain two 

 OH groups (like glycol) are called diatomic; those which contain 

 three OH groups (like glycerin) are called triatomic ; and so on. The 

 hexatomic alcohols are those which contain six OH groups. Three 

 of these hexatomic alcohols with the formula C 6 H 8 (OH) 6 are of 

 physiological interest ; they are isomerides, and their names are sorbite, 

 mannite, and dulcite. By careful oxidation their aldehydes and 

 ketones can be obtained ; these are the simple sugars ; thus, dextrose 

 is the aldehyde of sorbite; mannose is the aldehyde of mannite; 

 levulose is the ketone of mannite ; and galactose is the aldehyde of 

 dulcite. These sugars all have the empirical formula C 6 H 12 6 . The 

 constitutional formula for dextrose is : 



OH OH OH OH OH 



By further oxidation, the sugars yield acids with various names. 

 If we take such a sugar as a typical specimen, we see that their general 

 formula is 



C n H 2w O m 



and as a general rule n = m ; that is, the number of oxygen and carbon 

 atoms are equal. This number in the case of the sugars already 

 mentioned is six. Hence they are called hexoses. 



Sugars are known to chemists, in which this number is 3, 4, 5, 7, etc., and 

 these are called trioses, tetroses, peutoses, heptoses, etc. The majority of these 

 have no physiological interest. It should, however, be mentioned that a peutose 

 has been obtained from the nucleoproteid of the pancreas, of the liver, and of yeast. 

 If the peutoses that are found in various plants are given to an animal, they are 

 excreted in great measure unchanged in the urine. 



The hexoses are of great physiological importance. The principal 

 ones are dextrose, levulose, and galactose. These are called mono- 

 saccharides. Another important group of sugars are called disac- 

 charides ; these are formed by what is called condensation ; that is, 

 two molecules of monosaccharide combine together with the loss of a 

 molecule of water, thus : 



C 6 H 12 6 + C 6 H 12 6 == C 12 H 22 O n + H 2 0. 



The principal members of the disaccharide group are cane-sugar, 

 lactose, and maltose. If more than two molecules of the mono- 



