422 GLYCOGEN. [BOOK n. 



before the hepatic glycogen is exhausted ; at least this is the case 

 with most muscles. It is said to be increased in quantity when 

 the nerve of the muscle is divided, and the muscle thus brought 

 into a state of quiescence. On the other hand it diminishes or 

 even disappears when the muscle enters into rigor mortis. Some 

 have maintained that it diminishes during tetanus, but this 

 appears doubtful; and certainly muscles may be fully alive and 

 contractile from which glycogen is wholly absent. From this we 

 may infer, not that glycogen is a necessary chemical factor of 

 muscular metabolism, but that it can furnish materials for that 

 metabolism, and hence is stored up in the muscle so as to be ready 

 at hand for use. 



Accepting then the view that the hepatic glycogen is simply 

 store glycogen, waiting to be converted into sugar little by little as 

 the needs of the economy demand, and not glycogen on its way to 

 take part, through the agency of the hepatic protoplasm, in the 

 formation of some more complex compound, such as fat, we have 

 next to deal with the question what is the exact origin of the 

 hepatic glycogen ? By what steps is it formed and what are 

 its immediate antecedents ? We have already seen that the 

 presence of glycogen in the liver is especially favoured by a 

 carbohydrate diet. Hence, if the use of the glycogen be such as 

 we have supposed, it seems only reasonable to conclude that the 

 glycogen which makes its appearance in the liver after an amy- 

 laceous meal arises from a direct conversion of the sugar carried 

 to the liver by the portal vein, the sugar becoming through some 

 action of the hepatic protoplasm dehydrated into starch, by a 

 process the reverse of that by which in the alimentary canal starch 

 is hydrated into sugar through the action of the salivary and 

 pancreatic ferments. Vegetable protoplasm can undoubtedly con- 

 vert both starch into sugar and sugar into starch ; and there are 

 no d priori arguments or positive facts which would lead us to 

 suppose that the activity of animal protoplasm cannot accomplish 

 the latter as well as the former of these changes. Again, as we 

 have incidentally mentioned, sugar injected into the jugular vein 

 readily gives rise to sugar in the urine; but a very considerable 

 quantity can be slowly injected into the portal vein without any 

 appearing in the urine. This suggests the idea that the liver, so 

 to speak, catches the sugar as it is passing through the hepatic 

 capillaries and at once dehydrates it into glycogen. 



Upon such a view, the carbohydrate taken as food would be 

 converted in glycogen by the agency of the hepatic cell, without at 

 any time becoming an integral part of the protoplasm of the cell. 

 Such a view may be the true one ; but it is open for us to look at 

 the matter in another light. We may conceive of the hepatic cells 

 as being continually engaged in giving rise to carbohydrate 

 material, in the form either of sugar or of some other body, as a 

 product of the metabolism of their own protoplasm ; and we may 



