METABOLISM. 349 



sand, and returned to the boiling water, which is made slightly acid 

 with acetic acid. The mixture is boiled for a minute or two and is 

 then filtered the coagulated proteins remain behind, and the filtrate, 

 which is free from protein, forms an opalescent solution containing 

 glycogen. 



Glycogen, like dextrin, gives a mahogany-brown colour with iodine, and 

 does not reduce Fehling's solution ; when boiled with dilute mineral acids it is 

 converted into dextrose. It differs from dextrin, first, in forming an opalescent 

 solution, whereas a solution of dextrin is clear, and, secondly, in being pre- 

 cipitated more readily by alcohol. Further, glycogen is precipitated by basic 

 lead acetate, which does not precipitate dextrin. 



If the liver is left for some hours before being treated in the 

 manner just described, the filtrate contains an abundance of dextrose, 

 but no glycogen. The amount of glycogen present in the fresh liver 

 varies with the previous condition of the animal and, if it has been well 

 fed, may form 10 per cent, of the total weight of the liver. When an 

 animal has been starved for a few days, and particularly if during this 

 period it has been made to take exercise, the liver may be almost free 

 from glycogen. Both in the well-fed and in the starved animal the 

 percentage of sugar in the arterial blood remains unaltered. These 

 observations can be confirmed by histological examination of the liver. 

 When the liver of a well-fed animal is hardened in alcohol and examined 

 microscopically, the cells are seen to be full of glycogen, which can 

 be stained with iodine ; and the protoplasm may be reduced to a 

 network in the meshes of which the glycogen lies. An' examination of 

 the liver of a starved animal shows that glycogen is almost or quite 

 absent. 



From these and other observations Claude Bernard, who discovered 

 glycogen, concluded, first, that the sugar absorbed from the digestive 

 tract entered the portal blood, and that, as it passed through the liver, 

 this organ removed the sugar from the blood arid stored it as glycogen. 

 Secondly, he believed that the percentage of sugar in the systemic blood 

 normally remained constant, and that as the sugar was removed from 

 the blood by the tissues for their metabolism some glycogen was made 

 into sugar by the liver : this passed into the general blood stream, 

 thereby keeping the percentage of sugar in arterial blood at the normal 

 level. He regarded the rapid conversion of glycogen into sugar after 

 death as being due to a ferment, the activity of which was no longer 

 controlled, as it had been during the life of the animal. This view has 

 been generally accepted, and glycogen may be regarded as a store of 

 carbohydrate which is increased at each meal, and which is continuously 

 being drawn upon by the tissues. In all probability the conversion of 

 glycogen into sugar, and of sugar into glycogen,' is Qarried out by a 



