GLYCOGEN. 371 



death of the animal, and is minimal, if not nil, if the liver or 

 hepatic blood be taken for examination while the tissue elements 

 are still alive. 



The peculiar blood of the great portal vein coming from the 

 stomach, intestines, and the spleen, has then to pass through a 

 second set of capillaries in the liver, and undergoes such important 

 changes that this organ must be regarded as occupying a foremost 

 position among the blood-glands. Differences of the utmost im- 

 portance have long been thought to exist between the blood going 

 to and that coming from the liver, and to it has even been attrib- 

 uted paramount utility as a blood elaborator; but the scientific 

 knowledge of its power in this respect must date from the dis- 

 covery of its glycogeuic function. 



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 glycogeu, 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 filtrate 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- 



