388 VETERINAHY PHYSIOLOGY 



That proteins are a source of sugary substances is shown by 

 the amount of sugar which is produced by an animal rendered 

 diabetic by removal of its pancreas and fed exclusively on 

 proteins. It is therefore probable that in starvation the 

 proteins 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 starvation, but, when the supply of sugar is in excess 

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

 glyeogen and gives it out as sugar as that substance is re- 

 quired. On a carbohydrate diet the accumulation of glycogen 

 in the liver is very great ; but even on a protein diet, in dogs 

 at least, a smaller accumulation takes place. The observation 

 that the various monosaccharids are all stored as the same 

 form of glycogen shows that they must first be assimilated by 

 the liver protoplasm and then converted to glycogen, the process 

 being one of synthesis. 



The way in which glycogen is again changed to sugar is 

 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 protoplasm 

 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 of glycogen without 

 increasing the amylolytic enzyme in the liver and 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 passes 

 on into the general circulation, and is excreted in the urine. 

 Every individual has a certain power of oxidising and storing 

 sugar, and most persons can dispose of about 200 grms. at a 

 time. But the carbohydrate capacity varies greatly, and even 

 in the same individual it is different under different conditions. 

 When the glycogen stored in the liver is changed to glucose 

 more quickly than is required by the tissues, the glucose may 

 be to a certain extent again stored in the muscles as glycogen, 

 or it may accumulate in the blood and be excreted in the 



