FORMATION OF GLYCOGEN IN THE LIVER. 409 



Glycogen (C 6 H 10 5 ) belongs to the class of carbohydrates and is iso- 

 meric with starch. It is readily converted into glucose (C 6 H 12 6 ). In nearly 

 all regards it has the properties of starch, but it gives a deep red color with 

 iodine instead of a blue. In the liver-cells it exists in the form of amor- 

 phous granules surrounding the nuclei. It may be extracted from a decoc- 

 tion of the liver-substance, by precipitating the albuminoids by adding alter- 

 nately dilute hydrochloric acid and potassio-mercuric iodide, filtering and 

 treating the filtrate with an excess of alcohol. The alcoholic precipitate, 

 washed with alcohol and dried rapidly, is in the form of a white powder, 

 which will keep indefinitely. In the adult, glycogen is most abundant in 

 the liver ; but it has been found in small quantity in the muscular substance, 

 in cartilage and in certain cells in process of development. In the early 

 months of foetal life it exists in nearly all the tissues. It is found, also, in 

 cells attached to the villi of the placenta. 



The most important of the conditions which influence the quantity of 

 glycogen in the liver relate to alimentation and digestion. The liver always 

 contains more glycogen during digestion than in fasting animals. After a 

 few days of starvation, glycogen may almost or quite disappear from the 

 liver. This also occurs in animals fed for a time exclusively with fats, and 

 the quantity is diminished by a purely albuminous diet as contrasted with a 

 mixed diet. Still, as was shown by Bernard, glycogen is invariably present 

 in the livers of healthy carnivorous animals that have always been fed with 

 meat alone. 



A very great increase in the quantity of glycogen in the liver is produced 

 by feeding animals largely with carbohydrates. Not only are the starches 

 apparently stored up for a time in the form of glycogen in the liver, but 

 sugars seem to undergo a change into glycogen which accumulates in the 

 liver. This is to be expected, as the starches are changed into sugar before 

 they are absorbed, and all the carbohydrates behave in the same way as 

 regards general nutrition. Very abundant alimentation with carbohydrates 

 sometimes produces a temporary diabetes, the quantity of sugar in the blood 

 increasing to such an extent that sugar is discharged in the urine. This is 

 due either to the passage of a certain quantity of sugar unchanged through 

 the liver or to an excessive formation of glycogen, which is more actively 

 changed into sugar than under normal conditions. 



As far as regards the influence of alimentation upon the formation of 

 glycogen, it seems probable that in the herbivora and in man the chief source 

 of hepatic glycogen is the class of alimentary substances called carbohydrates ; 

 but the fact that glycogen exists in the livers of the carnivora, and probably 

 in man, under a nitrogenized diet, shows that the liver is capable of forming 

 glycogen from the albuminoids. 



Change of Glycogen into Sugar. It is almost certain that the liver does 

 not contain sugar during life. Many years ago (1858) this fact was recog- 

 nized by Pavy, and it has since been confirmed by other physiologists. Pavy, 

 however, assumed that there was no such thing as sugar-formation by the 

 liver, under absolutely normal conditions. He regarded the sugar found in 



