372 MANUAL OF PHYSIOLOGY. 



cent solution in water, which becomes clear on the addition of 

 caustic alkalies. It is insoluble in alcohol and ether. With a 

 solution of iodine it gives a wine-red color, and not blue, like 

 starch, which it otherwise much resembles in chemical relation- 

 ship. 



Glycogen has been found in many other parts besides the liver, 

 namely, in all the tissues of the embryo, and in the muscles, tes- 

 ticles, inflamed organs, and pus of adults ; in short, where any 

 very active tissue change or growth is going on, some traces of 

 glycogen can be found. 



The amount of glycogen in the liver depends in a great mea- 

 sure on the kind and quantity of food used. It rapidly increases 

 with a full, and decreases with a spare diet, and it slowly falls to 

 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 glycogeu. 

 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- 

 sfactory means of determining the use or destiny of glycogen. 



