830 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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, 

 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 and condensation, 

 such as may be represented in substance by the formula 



n(C 6 H 12 6 ) '- n(H 2 0) = (C 6 H 10 O 5 )n. 



There is no doubt that both dextrose and levulose increase markedly 

 the amount of glycogen in the liver; and, since cane-sugar is inverted 

 in the intestine before absorption, it also must be a true glycogen- 

 former, a fact that has been abundantly demonstrated by direct 

 experiment. Lusk* has shown, however, that, if cane-sugar is in- 

 jected under the skin, it has a very feeble effect in the way of increas- 

 ing the amount of glycogen in the liver, since under these conditions 

 it is probably absorbed into the blood without undergoing inversion. 



* Voit, "Zeitschrift f. Biologie," 28, 285, 18C1. 



