320 THE METABOLISM OF THE CARBOHYDRATES 



With regard to the method of investigation of this glycogenic 

 function, we must, in the first place, see what carbohydrates and 

 other food- stuffs cause glycogen to be deposited. To do this it 

 is necessary that we render the organ as nearly as possible free of 

 glycogen, and then, if we find that feeding with a certain 

 food-stuff should cause glycogen to be deposited, we can state 

 that this food-stuff is a glycogen former. A positive result of 

 this nature cannot, however, without further investigation, be 

 taken to show that the food -stuff in question has itself, in whole 

 or in part, become transformed into glycogen ; for it is always 

 possible that the glycogen may have been derived from the tissues 

 of the animal, the food-stuff having favoured such a process by 

 replacing these tissues in their usual break-down, i.e. by becoming 

 oxidised in place of them. As we shall see later, proteid may 

 form glycogen, and it is therefore always possible that the glycogen 

 apparently derived from some ingested non-proteid substance 

 may really be derived from the tissue proteids. In other words, 

 we must carefully distinguish between true and pseudo-gtycogen 

 formers. 



Having ascertained the possible sources of glycogen, we must 

 next study what becomes of this substance in the general economy 

 of the animal, and what the conditions are which, apart from 

 food, influence its amount. 1 



The questions we will at present consider are therefore the 

 following : 



How is glycogen distributed quantitatively in the body ? 



How may the liver and other tissues be rendered glycogen- 

 Iree? 



What food-stuffs lead to glycogen formation ? 



W^hat becomes of the glycogen in the organism ? 



How is the glycogenic function controlled ? 



1 These questions depend for their satisfactory solution on reliable and 

 accurate methods for the estimation of glycogen. It is beyond the scope of this 

 article to describe these. A critical survey of all the more usual methods has 

 recently been given by Pfluger, who also gives in detail a new method of great 

 accuracy. (A detailed account of this method will be found in "Practical 

 Physiology," 2nd ed., London, 1905.) 



