512 A MANUAL OF PHYSIOLOGY 



are destroyed at the temperature of boiling water. It has been 

 clearly shown that the action is brought about by a diastatic 

 enzyme, which some writers call glycogenase, for it readily occurs 

 when the minced liver is mixed with chloroform water, and chloro- 

 form kills all living tissues. Although blood contains a diastase 

 in small amount, the change does not depend essentially upon 

 this, since the glycogen also undergoes hydrolysis (glycogenolysis) 

 to dextrose when all the blood has been washed out of the organ. 

 Lymph also contains a diastase, but there is evidence that the 

 post-mortem glycogenolysis is chiefly due to an enzyme contained 

 in the hepatic cells (an endo-enzyme) (Macleod). The diastases 

 in the blood and lymph seem to be ' discards ' of the tissues 

 which are on the way to destruction or elimination (Carlson). 

 The post-mortem change is to be regarded as an index of a similar 

 action which goes on during life : sugar in the intact body is 

 changed into glycogen ; glycogen is constantly being changed 

 into sugar. There is no reason to doubt that here, too, the 

 hydrolysis is effected by the endo-enzyme. But, as in the case of 

 the intracellular proteolytic ferments, we may be certain that 

 the vital action of the hepatic cells is a most important factor in 

 controlling the rate of production of the ferment or the rate at 

 which it works. 



(3) With the microscope, glycogen, 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 

 'folood containing dextrose is circulated through it. 



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 pre- 

 sent ; in summer, much less. The difference is very remarkable 

 if we consider that in winter frogs have no food for months, 

 while summer is their feeding-time ; and at first it seems incon- 

 sistent 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 the quantity of glycogen is greatest in 



