THE METABOLISM OF THE CARBOHYDRATES 695 



conditions under which glycogen may be formed in the liver; that is, 

 whether it is formed exclusively from absorbed sugar, or whether other 

 substances, such as protein and fat may also form it. The importance of 

 such knowledge rests in the fact that in severe diabetes, sugar continues 

 to be added to the blood, although no sugar is being taken with the food. 

 To check the hyperglycemia in such cases it becomes necessary, therefore, 

 to curtail the diet not only with regard to its carbohydrate content, but 

 also with regard to whatever other foodstuffs may be capable of causing 

 glycogen formation. The practical question therefore is, What are these 

 foodstuffs? There are two methods by which the problem may be investi- 

 gated. The first, which we may call the direct method, consists in rendering 

 the liver free of glycogen and then feeding the animal with the food- 

 stuff in question, afterward killing it and immediately examining the liver 

 for glycogen. The other, which we may call the indirect method, con- 

 sists in first of all rendering the animal incapable of oxidizing glucose 

 that is, making it diabetic and then proceeding to see whether the in- 

 gestion of a given foodstuff causes an increase in the sugar excretion in 

 the urine. The methods for rendering an animal experimentally diabetic 

 will be considered later; for the present it is important to note that, if 

 a diabetic animal excretes more glucose while fed on a given foodstuff, 

 we may infer that the normal animal would convert it into glycogen. 



The results of the direct method are much less reliable than those of 

 the indirect for the reason that it is extremely difficult to remove all 

 traces of glycogen from the liver. The methods employed for this pur- 

 pose have-xjonsisted in: (1) starvation of the animal; (2) muscular ex- 

 ercise; (3) exercise and starvation combined; and (4) the production of 

 certain forms of experimental diabetes for example, that produced by 

 phlorhizin. Starvation alone is unsatisfactory, for it has been found 

 that, although at certain stages of this condition the liver may become al- 

 most entirely free from any trace of glycogen, at a later stage glycogen 

 may again make its appearance. It is therefore most difficult to decide 

 at what stage in starvation the animal should be considered as glycogen- 

 free. 



If the starving animal is made to perform muscular exercise, complete 

 removal of glycogen from the liver can be depended upon. The exercise 

 may be produced by the administration of strychnine in such dosage as 

 just to produce convulsions of the voluntary muscles without permanent 

 contraction of those of respiration. The most useful method, however, 

 consists in starving the animal for a few days and then placing it in a 

 cold, damp room, after giving it a cold bath. The evaporation of mois- 

 ture from the surface so cools the body down that the stores of glycogen 

 a.ll become used up in the attempt to supply fuel for the production of 



