CHEMISTRY OF DIGESTION AND NUTRITION. 327 



gen not only in its solutions, but also in the liver-cells. Glycogen is readily 

 soluble in water, and the solutions have a characteristic opalescent appearance. 

 Like starch, glycogen is acted upon by ptyalin and araylopsin, and the end- 

 products are apparently the sam< — namely, maltose, or maltose and some 

 dextrin. For a more complete account of the chemical relations of glycogen 

 reference must be made to the Chemical section. 



Occurrence of Glycogen in the Liver. — Glycogen can be detected in 

 the liver-cells microscopically. It' the liver of a dog is removed twelve or 

 fourteen hours after a hearty meal, hardened in alcohol, and sectioned, the 

 liver-cells will be found to contain clumps of clear material which give the 

 iodine reaction for glycogen. Even when distinct aggregations of the glycogen 

 cannot be made out, its presence in the cells is shown by the red reaction with 

 iodine. 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 sufficieutly prolonged it may dis- 

 appear altogether. This fact is, however, shown more satisfactorily by quanti- 

 tative determinations, by chemical means, of the total glycogen present. The 

 amount of glycogen present in the liver is quite variable, being influenced by 

 such conditions as the character and amount of the food, muscular exercise, 

 bodv-temperature, 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 increased great ly 

 bv feeding upon a diet largely made up of carbohydrates. 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 

 (Xeumeister) that the quantity may be increased to at least 10 per cent. It 

 is usually believed that glycogen exists as such in the liver-cells, being depos- 

 ited in the substance of the cytoplasm. Reasons have been brought forward 

 recently to show that possibly this is not strictly true, but that the glycogen is 

 held in some sort of weak chemical combination. It has been shown, for 

 instance, that although glycogen is easily soluble in cold water, it cannot be 

 extracted readily from the liver-cells by this agent. One musl use hoi water, 

 salts of the heavy metals, and other similar means 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, jusl as 

 we speak of haemoglobin 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 will be necessary to describe briefly the effect of 

 the different food-stulf- upon its formation. 



Effect of Carbohydrates <>n the Amount of Glycogen. — The amount of 

 glycogen in the liver is affected very quickly by the quantity of carbohydrates 

 in the food. It' the carbohydrates are given in exec--, the supply of glycogen 

 maybe increased largely beyond the average amount present, as ha- been stated 

 above. Investigation of the different sugars has shown that dextrose, levulose, 



