THE METABOLISM OF CARBOHYDRATES 797 



tasteless and odourless, with a formula identical with that of starch, viz. 

 C 6 H 10 5 . Like starch, it is hydrolysed by the action of acids and super- 

 heated water, or of amylolytic ferments, into dextrins, maltose, and finally 

 glucose. It gives with iodine a mahogany-red colour, which disappears on 

 boiling, but returns again on cooling. 



It is not possible to extract the whole of the glycogen from a tissue by merely boil- 

 ing it with water. Kiilz introduced on this account the method of dissolving 

 the tissues in caustic alkali, then throwing down the protein with phosphotungstic 

 acid, and in the nitrate precipitating the glycogen with alcohol. This method has 

 been modified by Pfliiger as follows : 100 grm. of the tissue (liver or muscle) are heated 

 with 100 c.c. caustic potash containing 60 to 70 per cent. KILO for twenty-four hours 

 in the water bath. The solution is then cooled, diluted with 200 c.c. of water, and 

 treated with 800 c.c. alcohol of 96 per cent. The precipitate of glycogen is filtered off 

 and washed several times with 66 per cent, alcohol. The precipitate of glycogen is 

 now washed with a little water into a small beaker, neutralised carefully with acetic 

 acid, and then introduced into a 100 c.c. flask. To the solution 5 c.c. of hydrochloric 

 acid of 1*19 sp. gr. are added, and the mixture is made up to 85 c.c. The flask is then 

 heated in the water bath for three hours. By this means the whole of the glycogen 

 is converted into glucose, which can be estimated by Fehling's method or by Allihn's 

 method. In practice it is more accurate to estimate the glycogen in the form of sugar 

 than to weigh it directly. If large quantities of glycogen are expected in the tissue, 

 the inversion of the glycogen must be carried out in a larger beaker, and only an 

 aliquot portion taken for titration. 



The large amount of sugar found in the liver which has been left in the 

 body is due to the conversion of glycogen into glucose. This conversion has 

 been variously ascribed to the activity of the surviving liver-cells, or to the 

 action of an amylase ferment present in the liver-cells. That it is really a 

 ferment action is proved by the fact that the liver may be dehydrated with 

 alcohol, dried and powdered, and kept for months in this condition without 

 any alteration occurring in the glycogen. If, however, the coagulated liver 

 be mixed with water and allowed to remain at the temperature of the body 

 for some hours, the glycogen is found to disappear and give place to glucose. 



FORMATION OF GLYCOGEN 



Glycogen is most readily formed from the carbohydrates of the food. 

 In order to obtain a large amount from the liver, the animal is fed twelve to 

 twenty-four hours previously on a meal which is rich in carbohydrates. 

 Not all carbohydrates will give rise to the formation of glycogen. Only 

 those which we have mentioned as directly assimilable, i.e. which will give 

 rise in the alimentary tract to mannose, glucose, fructose, or galactose, will 

 cause an increased formation of 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 

 monosaccharides . 



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

 products of their disintegration, the amino-acids. 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 



