THE METABOLISM OF CARBOHYDRATES 901 



glycogen. The conversion involves a direct polymerisation of the 

 glucose, produced either directly from the foods or by a molecular 

 rearrangement taking place in one of the other three of these mono- 

 saccharides. 



Glycogen can also be formed from the proteins of the food, or 

 from the products of their disintegration, the ammo-acide. By 

 means which we shall consider shortly, it is possible to free the 

 liver of animals entirely from glycogen : if such animals be fed on a 

 diet of washed fibrin or of pure caseinogen, or even on the ultimate 

 products of pancreatic digestion of proteins (containing therefore only 

 amino-acids), and be killed shortly afterwards, the liver is found to 

 contain glycogen. It does not seem to be possible for the liver to 

 manufacture glycogen out of fats. At any rate, that is the interpreta- 

 tion which is generally placed on experiments on feeding with fats. 

 In these experiments it is found that if fats be administered to 

 an animal after the liver has been freed from glycogen, although 

 the liver may store up fats it does not store up any glycogen. 

 The most important source of the glycogen of the liver is the 

 carbohydrates of the food. Indeed, during a meal rich in starches 

 or sugars the blood of the portal vein has been found to contain a much 

 larger proportion of sugar than the blood of the hepatic vein, or of the 

 rest of the body. 



If an animal be starved, the glycogen gradually disappears from 

 the liver, although even at the end of ten or twelve days' complete 

 deprivation of food small traces of glycogen may still be found in this 

 organ. If, however, starvation be combined with hard work ; if, for 

 instance, a dog be made to drag about a milk-cart on the second day 

 of the starvation period, its liver becomes quite free from glycogen. 

 The same disappearance of glycogen may be produced by any means 

 which evoke an increased muscular activity, e.g. poisoning with 

 strychnine. Of the various reserve materials which are available 

 the carbohydrate is the first to be called upon to meet the increased 

 needs of the tissues during functional activity, such as muscular work 

 or increased heat production. Thus the glycogen rapidly disappears 

 from the liver of a rabbit which has been immersed in a cold bath. 



The glycogen of the liver represents a reserve material analogous 

 to the reserve carbohydrates stored up as starch in different parts of 

 plants. When the blood is loaded with carbohydrates, a considerable 

 proportion is laid down as the inert polysaccharide glycogen. As soon 

 as the supply of sugar to the blood is withdrawn, the tissues continue 

 to use the sugar of the blood, which is made up at the expense of the 

 glycogen in the liver. In every liver-cell therefore a twofold process is 

 always going on, namely, a building up of glycogen by the activity of 

 the liver-cells,and a breaking down of glycogen under the action of the 



