THE FOOD AND DIGESTION 367 



starvation there are only two possible sources of this glucose 

 the fats and the proteids of the tissues. There is no con- 

 clusive evidence that fats can be changed to sugar in the 

 liver, although it is difficult to explain the large amount of 

 sugar which is excreted in phloridzin poisoning, unless it is 

 formed from fats. That proteids are a source of sugary 

 substances is shown by the accumulation of glycogen in the 

 liver in animals fed upon proteids, and by continued excre- 

 tion of glucose in the urine of a dog without its pancreas, 

 and fed exclusively on proteids. It is therefore probable 

 that in starvation the proteids of the body are broken down 

 and their non-nitrogenous part changed to sugar. But, not 

 only does the liver manufacture sugar for the tissues in star- 

 vation, but, when the supply of sugar is in excess of the 

 demands of the tissues, it stores it as a form of starch 

 glycogen (see p. 318) and gives it out as sugar as that 

 substance is required. On a carbohydrate diet the accumu- 

 lation of glycogen in the liver is very great ; but even on a 

 proteid diet, in dogs at least, a smaller accumulation takes 

 place. The observation that various monosaccharids are 

 stored as the same form of glycogen shows that they must 

 first be assimilated by the liver protoplasm and then con- 

 verted to glycogen, the process being one of synthesis. 



The way in which glycogen is again changed to sugar is 

 more doubtful. The fact that the liver after treatment with 

 alcohol can change glycogen to glucose, has induced some 

 physiologists to believe that it is by an enzyme that this 

 conversion goes on during life. But it has been shown (1) 

 that the injection of methylene blue, which poisons proto- 

 plasm but does not interfere with the action of enzymes 

 checks the conversion, and (2) that stimulating the splanchnic 

 nerves going to the liver increases the conversion without 

 increasing the amylolytic enzyme in the liver or blood. 

 It is therefore probable that the conversion results from 

 chemical changes in the protoplasm which are controlled by 

 the nerves of the liver. 



If more sugar is taken than the liver can deal with, it then 

 passes on into the general circulation, and is excreted in the 

 urine. Every individual has a certain power of oxidising 

 and using sugar, and most persons can dispose of about 200 



