70 THE FEEDING OF ANIMALS 



starches, celluloses, gums, and so on. Chemically we dis- 

 tinguish them by their structure and by their relation 

 to one another. 



90. Classification of carbohydrates according to struc- 

 ture. The structure of certain sugars is such that their 

 molecules cannot be divided into simpler compounds that 

 retain the carbohydrate character, and these are known 

 as mono-saccharides. To this class belong glucose (grape- 

 sugar) and fructose (fruit-sugar). On the other hand, 

 there are a large number of carbohydrates, one molecule 

 of which by treatment in certain ways may be converted 

 into two or more molecules of a mono- (simple) sugar. 

 For instance, one molecule of starch, when submitted to 

 the action of an acid or of certain ferments, breaks up 

 into several molecules of glucose, and so starch is known 

 as a poly-saccharide. Other poly-saccharides are sucrose 

 (cane-sugar), maltose (malt-sugar), lactose (milk-sugar), 

 cellulose, and the gums, all of which may be split up into 

 mono- or simple sugars. The poly-saccharides are sub- 

 divided into di, tri, and so on, according as they break up 

 into two, three, or more molecules of a simple sugar. 



There are subdivisions of the mono-sugars also, on 

 the basis of carbon atoms in their molecules and thus we 

 have the names diose, triose, tetrose, pentose, hexose, 

 heptose for sugars having two, three, four, five, six, or 

 seven carbon atoms in the molecule. It may be remarked 

 here that it is among the hexose (six-carbon) sugars or their 

 multiples that we find the carbohydrates most important 

 to human nutrition. 



91. The mono-saccharides or simple sugars. The 

 simple sugars that are most important in animal nutri- 

 tion are dextrose (grape-sugar), levulose (fruit-sugar), 

 and galactose (from milk-sugar). These are hexose (six- 



