CH. XXVIII.] THE CARBOHYDRATES 409 



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

 has been obtained from the certain nucleic acids presently to be described (see p. 

 431) which are contained in animal organs (pancreas, liver, etc.), and in plants 

 (for instance, yeast). If the pentoses which 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, laevulose, and galactose. These are called mono- 

 saccharides. 



Another important group of sugars are called disaccharides ; 

 these are formed by what is called condensation ; that is, two mole- 

 cules 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 monosaccharide group undergo 

 a corresponding condensation, we get what are called polysaccharides. 



nC 6 H 12 6 = (C 6 H 10 5 )n + "H 2 0. 



The principal polysaccharides are starch, glycogen, various dextrins, 

 and cellulose. We may, therefore, arrange the important carbo- 

 hydrates of the hexose family in a tabular form as follows : 



The + and signs in the above list indicate that the substances 

 to which they are prefixed are dextro- and Isevo-rotatory respectively 

 as regards polarised light. The formulae given in the table are merely 

 empirical ; the quantity n in the starch group is variable and often 

 large. The following are the chief facts in relation to each of the 

 principal carbohydrates. 



Dextrose, Glucose, or Grape Sugar. This carbohydrate is 

 found in many fruits, honey, and in minute quantities in the 

 blood and numerous tissues, organs, and fluids of the body. It is 

 the form of sugar found in large quantities in the blood and urine 

 in the disease known as diabetes. 



Dextrose is soluble in hot and cold water and in alcohol. It is 

 crystalline, but not so sweet as cane sugar. When heated with strong 

 potash certain complex acids are formed which have a yellow or 



