900 PHYSIOLOGY 



From its solution in water it can be thrown down by the addition of 

 alcohol to 60 per cent. When collected and dried, it forms a snow- 

 white powder, 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 superheated water, or of amylolytic ferments, into 

 dextrines, 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 

 boiling it with water. Kiilz introduced on this account the method of dis- 

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

 phosphotungstic acid, and in the filtrate 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. KHO 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 hydro- 

 chloric 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 



