376 MANUAL OF PHYSIOLOGY. 



nil after prolonged starvation. The formation of glycogen is 

 much more dependent on the carbohydrate food than on the pro- 

 teid, for it rapidly rises with increase in the quantity of sugar 

 taken, and falls, as in starvation, when pure proteid (fibrin) with- 

 out any carbohydrate is used either with or without fat. Although 

 the large supply of glycogen normally manufactured in the liver 

 is probably derived from the sugar of the food, we must not con- 

 clude from this that the liver cells cannot make glycogen from 

 other materials. Possibly, anything that suffices for the nutrition 

 of their own protoplasm enables the cells to produce glycogen. 

 The slowness with which glycogen disappears in starvation would 

 seem to point to this. 



The ultimate destiny and uses of glycogen are still vexed ques- 

 tions. Much trouble has been taken to decide whether it is con- 

 verted into sugar, and as such carried off by the blood to the 

 tissues. One set. of observers deny the existence in the living 

 tissues of the amylolytic ferment necessary for its conversion into 

 sugar, and think that it is distributed simply as glycogen to the 

 tissues ; while others say that it is gradually changed to sugar 

 before it is carried off in the hepatic veins. 



The difficulty of determining the exact amount of sugar in the 

 blood with sufficient accuracy may account for the remarkable 

 disparity of opinion on this subject, and makes this a very unsat- 

 isfactory means of determining the use or destiny of glycogen. 

 In fact, the whole controversy seems idle, for the real questions 

 are, not whether the glycogen is distributed to the tissues as sugar 

 or not, but is the glycogen distributed to the tissues as a general 

 carbohydrate nutritive, or is it to be regarded as a step in the 

 manufacture of some other material by the liver cells ; in other 

 words, is the glycogen of the liver a store of special animal carbo- 

 hydrate to be kept till called for, or is it a stage in the formation 

 of fat? 



