98 CARBOHYDRATES. 



precipitated by 60 per cent, alcohol, while the dextrins are not 

 precipitated. Watery solutions are dextrorotatory. 



In general it may be said that the action of the enzymes and 

 of boiling acids upon glycogen is the same as upon starch. The 

 glycogen of the liver becomes converted, by physiologic processes, 

 into liver-sugar, which is regarded as identical with dextrose. In 

 this process probably no maltose is formed, such as occurs in the 

 artificial hydrolysis already described. This difference would 

 seem to indicate that in the liver-cells there is no enzyme to 

 which this action can be attributed ; for, so far as can be judged, 

 most enzymes produce maltose, and not dextrose, and up to the 

 present time no dextrose-producing enzyme has been obtained 

 from the liver. 



Cellulose. Nowhere in the animal body is cellulose found, but 

 it exists in many of the vegetable alimentary principles upon 

 which man relies for his nutrition. As has already been stated, 

 it is a constituent of the starch-granule, and so covers the granulose 

 that the digestive fluids cannot reach it. When starch is boiled 

 the granules burst, and thus access to the granulose is given. It 

 has recently been suggested that there is in the intestinal canal, 

 formed by the epithelial cells, an enzyme which has the power 

 of causing a digestion of the cellulose. But the evidence of the 

 existence of such an enzyme is very meagre. The disappearance 

 of the cellulose is probably due to the action of bacteria, all the 

 products being unknown, though marsh-gas, acetic and butyric 

 acids are among them. This change takes place especially when 

 vegetables, such as celery and lettuce, and fruits are eaten whose 

 cell-walls are tender and have not yet become lignified or woody 

 in character. Lignin is the name applied to cellulose in this 

 advanced stage. In the human intestine from 4 to 60 per cent, 

 of the cellulose taken in is dissolved. It doubtless has very little 

 nutritive value, but is regarded as increasing by its local action 

 intestinal peristalsis and keeping the bowels free. In the rabbit 

 its absence from the food results in death, inflammation of the 

 intestine being caused thereby ; but if horn-shavings, which are 

 excreted unchanged, are substituted for cellulose, the animal 

 maintains its health. The cellulose of some plants, such as 

 the date, is regarded as a reserve material to be made use of in 

 germination. 



The presence of cellulose is recognized by the fact that when 

 treated with strong sulphuric acid it becomes converted into a sub- 

 stance that is colored blue by iodin. Schulze's reagent is another 

 test for its presence. This test consists in the production of a blue 

 color when the substance is treated with iodin dissolved to satura- 

 tion in a solution of chlorid of zinc to which potassium iodid has 

 been added. 



