732 PHYSIOLOGY OF DIGESTION AND SECRETION. 



when distinct aggregations of the glycogen can not be made out, its 

 presence in the cells is shown by the red reaction with iodin. By 

 this simple method one can demonstrate the important fact that the 

 amount of glycogen in the liver increases after meals and decreases 

 again during the fasting hours, and if the fast is sufficiently prolonged 

 it may disappear altogether. This fact is, however, shown more 

 satisfactorily by quantitative determinations, by chemical means, 

 of the total glycogen present. The amount of glycogen in the 

 liver is quite variable, being influenced by such conditions as the 

 character and amount of the food, muscular exercise, body tem- 

 perature, drugs, etc. From determinations made upon various 

 animals it may be said that the average amount lies between 1 .5 and 

 4 per cent, of the weight of the liver. But this amount may be in- 

 creased greatly by feeding upon a diet largely made up of carbohy- 

 drates. It is said that in the dog the total amount of liver glycogen 

 may be raised to 17 per cent., and in the rabbit to 27 per cent., by 

 this means, while it is estimated for man (Neumeister) that the quan- 

 tity may be increased to at least 10 per cent. It is usually believed 

 that glycogen exists as such in the liver cells, being deposited in the 

 substance of the cytoplasm. Reasons have been brought forward 

 to show that this is not strictly true, and that the glycogen is prob- 

 ably held in some sort of weak chemical combination. It has been 

 shown, for instance, that although glycogen is easily soluble in cold 

 water, it can not be extracted readily from the liver cells by this agent. 

 One must use hot water, salts of the heavy metals, and other similar 

 agents that may be supposed to break up the combination in which 

 the glycogen exists. For practical purposes, however, we may speak 

 of the glycogen as lying free in the liver-cells, just as we speak of 

 hemoglobin existing as such in the red corpuscles, although it is 

 probably held in some sort of combination. 



Origin of Glycogen. To understand clearly the views held as 

 to the origin of liver glycogen, it is necessary to describe briefly the 

 effect of the different foodstuffs upon its formation. 



Effect of Carbohydrates on the Amount of Glycogen. The amount 

 of glycogen in the liver is affected very quickly by the quantity of car- 

 bohydrates in the food. If the carbohydrates are given in excess, the 

 supply of glycogen may be increased largely beyond the average 

 amount present, as has been stated above. Investigation of the differ- 

 ent sugars has shown that dextrose, levulose, saccharose (cane-sugar), 

 and maltose are unquestionably direct glycogen-formers, that is, 

 that glycogen is formed directly from them or from the products into 

 which they are converted during digestion. The bulk of our car- 

 bohydrate food reaches the liver as dextrose, or as dextrose and levu- 

 lose, and these forms of sugar may be converted into glycogen in the 

 liver cells by a simple process of dehydration, such as may be repre- 



