GLYCOGEN. 375 



Glycogen is a substance nearly allied to starch in its chemical 

 composition, and is converted with great readiness into grape 

 sugar by the action of certain ferments and acids. Many of the 

 animal textures contain these ferments, among others the liver 

 itself, at least when its tissue is dying; and consequently the liver 

 with the blood coming from it (if examined in an animal some 

 time dead) does not contain glycogen, but sugar which has been 

 formed from it. If a piece of liver taken from an animal imme- 

 diately after it is killed be plunged into boiling water, so as to 

 check the action of the ferment, no trace of sugar is found in it, 

 but only glycogen. After the lapse of a little time, another piece 

 of the same liver, which has lain at the ordinary room tempera- 

 ture, will give abundance of sugar. 



The mode of preparation of glycogen depends upon the fore- 

 going facts. The perfectly fresh liver taken from an animal 

 killed during digestion is rapidly subdivided in boiling water. 

 When the ferment has been destroyed by heat the pieces of liver 

 are rubbed up to a pulp in a mortar, and then reboiled in the 

 same fluid. The liquor is then filtered, and from the nitrate the 

 albuminous substances are precipitated with potassio-mercuric 

 iodide and hydrochloric acid, and removed on a filter. From 

 this filtrate the glycogen may be precipitated by alcohol, caught 

 on a filter, washed with ether to remove fat, and dried. 



Glycogen thus prepared is a white powder, forming an opales- 

 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 rela- 

 tionship. 



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 meas- 

 ure 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 



