DISACCHARIDS OR SACCHAROSES. 93 



Inosit or muscle-sugar has been found in the muscles, lungs, 

 liver, spleen, kidneys, and brain, and pathologically in urine. It 

 occurs also in beans and grape-juice. Because of its sweet taste 

 and its chemical composition it was formerly regarded as a carbo- 

 hydrate, but as it has no rotatory action on polarized light, does not 

 reduce metallic salts, and does not undergo the alcoholic fermen- 

 tation, it is now regarded as belonging to the aromatic series, and 

 not as being a carbohydrate. 



Disaccharids or Saccharoses. The chemical formula 

 representing this group is C 12 H 22 O n . They are regarded as a con- 

 densation-product of two molecules of the monosaccharids, in which 

 a molecule of water is lost. This may be expressed as follows : 



C 6 H 12 6 + C 6 H 12 6 - C^HsA, + H 2 



Mono- Mono- Disaccharid. Water, 



saccharid. saccharid. 



This process is known as reversion. 



The members of this group which are of physiologic impor- 

 tance are Cane-sugar, Lactose, Maltose, and Isomaltose. 



Cane-sugar or Saccharose. This sugar is not found in the human 

 body, but it nevertheless plays an important part in the food of 

 .man. It occurs in sugar-cane, beet-root, and sugar-maple. It 

 does not reduce metallic salts, is soluble in water, dextrorotatory, 

 and does not undergo alcoholic, but does readily undergo lactic 

 fermentation in presence of sour milk to which zinc oxid is added 

 to fix the acid as formed. One of the interesting facts connected 

 with cane-sugar is its property of " inversion," which consists in 

 its decomposition into equal parts of dextrose and leyulose, and 

 to this mixture the name of "invert-sugar" has been given. This 

 change is represented chemically as follows : 



C I2 H 22 U + H 2 = C 6 H 12 6 + C 6 H 12 6 



Cane-sugar. Water. Dextrose. Levulose. 



and may be produced by the action of acid, as has been described 

 under Levulose. It takes place also in the small intestine 

 under the influence of an enzyme of the intestinal juice, namely, 

 invertin. A similar inversion takes place in lactose and maltose ; 

 thus maltose + water = dextrose + dextrose ; and lactose + water 

 = dextrose + galactose. Invertin exists also in yeast, in which it 

 has the same power as in the intestinal juice. 



Cane-sugar cannot be taken up as such by the blood, and when 

 injected into an animal it is eliminated unaltered in the urine. 

 When taken in as food it is absorbed, not as cane-sugar, but as 

 invert-sugar, into which it has been changed. This inversion is 

 most pronounced in the small intestine ; it is claimed that it may 

 take place also in the stomach, and that there exists in the gastric 

 juice an enzyme which has this power. 



