CHAPTER LXXVI 

 THE METABOLISM OF THE CARBOHYDRATES (Cont'd) 



FATE OF GLYCOGEN 



Having become familiar with the sources from which glycogen may 

 be derived, we may now proceed to study the fate of the glycogen found 

 in the liver cells and in the muscles. For the present we shall confine our 

 attention to the glycogen of the liver. If a portion of liver removed 

 from 'a well-fed animal is examined microscopically after staining either 

 with iodine or with carmine by Best's method, it will be found that the 

 cells of the lobules are filled with glycogen except for the nuclei, which 

 are free from this substance. If, on the other hand, the liver is from an 

 animal that has not been recently fed, the lobules will contain no glyco- 

 gen except for an area bordering on the central vein and perhaps a 

 narrow strip at the periphery of the lobule. When it is present the rela- 

 tive amount of glycogen in different lobules, as determined chemically, 

 is the same over the entire liver that is to say, no one lobe is richer in 

 this substance than another. Nothing definite is known as to how the 

 glycogen is held in the protoplasm of the cells, although some histolo- 

 gists suggest that it is combined with a sustentacular material especially 

 provided for this purpose. 



The glycogen stored in the liver is gradually given up to the blood of 

 the hepatic vein at such a rate as to maintain in the blood of the sys- 

 temic circulation a more or less constant percentage of glucose. Under 

 ordinary conditions this process of glycogenolysis is relatively slow, but 

 when the requirements of the organism for fuel become increased, as 

 during muscular exercise, it becomes very rapid. The glycogenic func- 

 tion of the liver appears therefore to exist, in part at least, for the 

 purpose of preventing the flooding of the blood of the systemic circu- 

 lation with excess of sugar during absorption from the intestine and of 

 maintaining the normal percentage at other times. This function is 

 analogous to that occurring in plants, in which the sugar produced in 

 the leaves, if not immediately required, is transported to various parts 

 of the plant and there converted into starch, which, when the plant 

 requires it, as during new growth, may again become transformed into 

 glucose. 



The agency converting the glycogen into glucose is the diastatic 



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