60 THE CARBOHYDRATES. 



tion, as also the character and number of the intermediary products, 

 is but imperfectly understood. 



In their physical and chemical properties considerable differences 

 exist between the polysaccharides and the other carbohydrates 

 which have so far been described. They are thus non-crystal lizable 

 substances and devoid of a sweet taste. In alcohol and ether they 

 are insoluble. In water most of them are more or less soluble, but 

 as a class they are incapable of diffusing through animal membranes. 

 From their solutions they can be precipitated by saturation with 

 neutral salts, and notably with ammonium sulphate. Like the 

 monosaccharides and disaccharides, they are optically active, but 

 with the exception of the dextrins they do not reduce metallic 

 oxides in alkaline solution, and none of them combine with phenyl- 

 hydrazin to form osazons. As such, they are incapable of under- 

 going fermentation ; but, like the disaccharides, they may be inverted 

 to monosaccharides through various ferments or acids, and can then 

 be further decomposed. 



Especially important is their behavior toward iodine, with which 

 most of the polysaccharides combine to form colored compounds that 

 are quite characteristic. Starch is thus colored blue, glycogen a 

 mahogany brown, and erythrodextrin red. 



The polysaccharides which are used as food-stuffs are conveni- 

 ently divided into starches, vegetable gums and dextrins, and cellu- 

 loses. Of these, the starches are by far the most important, as they 

 include not only vegetable starches and glycogen, but also give rise 

 to formation of the dextrins. 



Like the disaccharides, all these substances finally give rise to the 

 formation of glycogen, but it appears that they are previously trans- 

 formed into glucose, and that this transformation takes place in the 

 epithelial lining of the intestinal canal. 



Starch occurs widely distributed in the vegetable world, and con- 

 stitutes the most important reserve food of most of the higher 

 plants. It is found in the form of distinct granules, which, on 

 microscopic examination exhibit a marked concentric striation, and 

 which differ in size and form in different plants. The individual 

 granules are enclosed in a capsule of so-called starch cellulose, which 

 is insoluble in water, but which can be made to open by heating in 

 the presence of much water. The contained starch-granulose can 

 thus be obtained, and constitutes the so-called soluble starch, amylum 

 or amylodextrin. During this process no doubt a still more complex 

 molecular group of monosaccharine anhydrides is decomposed, but 

 of the intermediary products which are thereby formed, if any, 

 nothing is known. In the alimentary canal this change is effected 

 through the activity of certain ferments, which then further give 

 rise to the formation of dextrin, maltose, isomaltose, and a certain 

 amount of glucose. On boiling with dilute acids glucose is formed, 

 with various dextrins as intermediary products. Among these, as 

 has been shown, erythrodextrin is apparently the first to develop, 



