22 HUMAN PHYSIOLOGY 



formed by the activity of the milk-glands. It is found only 

 in milk, has but a slight sweet taste, and is somewhat less 

 soluble in water than the other sugars. Milk-sugar is 

 dextrorotatory (rotatory power 52.5). It reduces, but 

 does not ferment with yeast, even after previous action of 

 the invertin, which otherwise splits up the disaccharides and 

 renders the yeast fermentation possible. On the other 

 hand, it is split up by bacterium lactis (lactic-acid fermenta- 

 tion), also by the Kephir fungus, which also produces alco- 

 holic fermentation. Through the oxidation of milk-sugar 

 there is formed, among others, mucic acid, an oxidation 

 product of galactose. 



The polysaccharides are anhydrid compounds of several 

 molecules of the simple sugars. Their general formula is 

 (C 6 H 10 O 5 );r, in which x is the still unknown factor by which 

 the formula must be multiplied to obtain the real size of the 

 molecule. In this group belong: vegetable starch (amy- 

 loses), animal starch (glycogen), dextrins, gums, and cellu- 

 lose. Some of the polysaccharides are insoluble in water 

 (cellulose); some swell up in water, forming a sticky fluid 

 (starch, gums) ; some are soluble, but are not dialyzable, 

 and are precipitated by alcohol (glycogen, dextrin). They 

 are dextrorotatory, do not reduce (except a few dextrins), 

 and do not ferment with yeast. Many ferments (diastase, 

 ptyalin) and boiling with strong mineral acids change the 

 moftosaccharides, chiefly to grape-sugar. When gum is 

 oxidized, mucic acid^ is formed ; the oxidation of starch, 

 glycogen, dextrin, yields saccharic acid. Most of the poly- 

 saccharides give color reaction with iodine: Starch gives 

 blue; glycogen, brownish red; dextrin, blue or red; cellu- 

 lose, after being treated with concentrated sulphuric acid, 

 gives blue. 



Of the polysaccharides, only glycogen is found in the 

 animal body. 



Cellulose is the chief constituent of wood fibre. Starch and 

 dextrin are important foods. Gum has only a technical value. 

 A carbohydrate, closely related to cellulose, is found in the 



