THE METABOLISM OF CARBOHYDRATES 841 



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 after- 

 wards, 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 

 interpretation 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. 



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 starva- 

 tion 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 sugar to the blood is withdrawn, the tissues continue to use the sugar of the 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 ferment formed in the liver-cells. Which of 

 these two processes preponderates depends, in the normal animal, on the 

 percentage amount of sugar in the blood which is circulating through the 

 organ. 



On account of the importance of glycogen as a reserve material it is 

 produced and stored up in almost all growing tissues, to be utilised in their 

 subsequent development. Thus it is found in large quantities in the placenta 

 during a certain period, in foetal muscles, and in various other situations. It 

 is found in yeast, in oysters, and in the muscles of the body generally. In 

 foetal muscles it may amount to as much as 40 per cent, of the total dried 

 solids. The glycogen of the adult muscle is apparently utilised during 

 muscular work, and diminishes in amount with activity of the muscle. In 

 adult muscles it never reaches anything like the percentage which is found in 

 the liver. The average ameunts found by Schondorf in the different tissues 

 were as follows : 



