METABOLISM OF CARBO-HYDRATES GLYCOGEN 533 



sugar into glycogen. Contrariwise, a diminution in the dextrose 

 content of the blood might be expected to lead to diffusion of 

 dextrose out of the liver-cells, and a consequent acceleration of the 

 hydrolysis of the glycogen. We have already learnt, however, that 

 in physiology above all, perhaps, in the physiology of the glands 

 ' simple ' explanations are usually suspect. And when we come to 

 study those conditions in which, as a consequence of the derange- 

 ment of the mechanisms which regulate the carbo-hydrate metabo- 

 lism, sugar appears in the urine, it will be seen that the matter is 

 more complicated. For one thing, the nervous system seems to 

 take a hand in the regulation, and where the nervous system takes 

 a hand things are generally doing which the experienced physiologist 

 does not expect to be simple. We may be certain, as in the case of 

 the intracellular proteolytic ferments, that the vital action of the 

 hepatic cells is a most important factor in controlling the rate of 

 production of the ferment, and therefore its concentration in rela- 

 tion to that of the substrate and the rate at which it works. 



(3) With the microscope, glycogeri, or at least a substance which 

 is very nearly akin to it, which very readily yields it, and which 

 gives the characteristic port-wine colour with iodine, can be actually 

 seen in the liver-cells. The liver of a rabbit or dog which has been 

 fed on a diet containing much carbo-hydrate is large, soft, and very 

 easily torn. Its large size is due to the loading of the cells with a 

 hyaline material, which gives the iodine reaction of glycogen, and 

 is dissolved out by water, leaving empty spaces in a network of cell- 

 substance. If the animal, after a period of starvation, has been 

 fed on protein alone, less glycogen is found in the shrunken liver- 

 cells; if the diet has been wholly fatty, little or no glycogen at all 

 may be found. Glycogen can even be formed by an excised liver 

 when blood containing dextrose is circulated through it. 



A fact of great interest recently demonstrated is that in animals 

 where the mobilization of the hepatic glycogeri is accelerated (as 

 in rabbits after puncture of the medulla), glycogen in the form of 

 granules and small masses can be seen in the blood capillaries 

 (or rather sinusoids) between the liver cells, and in the sublobular 

 veins (Huber and Macleod). Chemical evidence has also been 

 obtained of the presence of a glycogen-like polysaccharide in the 

 blood leaving the liver. 



Formation of Glycogen from Protein. In the liver-cells of the 

 frog in winter-time a great deal of this hyaline material this 

 glycogen, or perhaps loose glycogen compound is present; in 

 summer, much less. The difference is remarkable if we con- 

 sider that in winter frogs have no food for months, while summer 

 is their feeding-time; and at first it seems inconsistent with the 

 doctrine that the hepatic glycogen is a store laid up from surplus 

 sugar, which might otherwise be swept into the general circulation 

 and excreted by the kidneys. It has been found, however, that 



