72 THE FEEDING OF ANIMALS 



95. The pentoses. There are several pentoses, none 

 of which occurs in nature, but which are prepared by chem- 

 ical methods from the gums. Thus, from gum arabic, 

 containing araban, arabinose may be obtained, and from 

 zylan (wood-gum), zylose may be prepared. Certain of 

 these sugars have been isolated from animal compounds. 

 They also have been found to appear in human urine. 

 They are important in the nutrition of herbivorous 

 animals. 



96. Di-saccharides. These carbohydrates are all 

 sugars which may be decomposed into two molecules of 

 a simple sugar, or one molecule of each of two simple 

 sugars. They are only three in number saccharose or 

 sucrose (cane-sugar), maltose (malt-sugar), and lactose 

 (milk-sugar). When acted on by weak acids or cer- 

 tain ferments, they break by cleavage (hydrolysis) 

 as follows : 



Saccharose-fwater=dextrose+levulose. 

 Maltose +water=dextrose-j-dextrose. 



Lactose +water=dextrose+galactose. 



These are the changes that occur during the digestion 

 of food. 



97. Saccharose. The most important of these, com- 

 mercially considered, is saccharose, which is the ordinary 

 crystallized sugar of the markets. As a human food it is 

 widely used, is especially valuable, and its manufacture 

 and sale constitute a prominent industry. This sugar is 

 obtained mostly from two plants, sugar-cane and the 

 sugar-beet. It also exists abundantly in sorghum, pine- 

 apples, carrots, and in considerable proportions in the 

 stalk of ordinary field corn. The first spring flow of sap 

 in one species of maple tree is richly charged with it. 



