16 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



and as a general rule nm ; that is, the number of oxygen and carbon 

 atoms is 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, &c., and 

 these are called trioses, tetroses, pentoses, heptoses, &c. The majority of 

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

 a pentose has been obtained from the nucleoprotein of the pancreas, of the 

 liver, and of yeast. If the pentoses 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 prin- 

 cipal ones are dextrose, levulose, and galactose. These are called 

 monosaccharides . 



Another important group of sugars is that of the disaccharides : 

 these are formed by the combination of two molecules of monosac- 

 charide together with the loss of a molecule of water, thus : 



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. 



The polysaccharides are starch, glycogen, various dextrins, cellu- 

 lose, gums, &c. We may therefore arrange the important carbo- 

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



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

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

 as regards polarised light. 1 The formulae given above 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. 



1 For a description of polarised light and polarimeters see Appendix. This 

 and the other matter in the Appendix are placed there for convenience, not because 

 they are unimportant. Students are therefore urged to refer to and carefully study 

 these subjects. 



