722 STORAGE OF GLYCOGEN. [BOOK n. 



material a,nd dealing possibly with proteid substances, also take 

 place. Hence the exact interpretation of all the changes which 

 may be observed becomes exceedingly difficult. 



Leaving the processes of the first and third kind wholly on one 

 side for the present, and confining our attention entirely to the 

 glycogen, it is obvious that the hepatic cell manufactures the 



flycogen in some way or other, and lodges it in its own substance 

 >r the time very much in the way that a secreting cell manufac- 

 tures and lodges in itself for a time material for the secretion which 

 it is about to pour forth. There is this difference, that in the one 

 case the material of the secretion, after undergoing as we have seen 

 more or less change, is cast out into the lumen of the alveolus, 

 whereas in the other case the glycogen, which must undergo 

 change since it may be made to disappear rapidly from the hepatic 

 cell, is not when changed cast out into the bile passages ; it must 

 therefore be sent back again to the blood. 



460. We say " manufactures the glycogen in some way or 

 other," and we have now to inquire what we know concerning the 

 nature and the several steps of this manufacture. 



We have already seen that the presence of glycogen in the 

 liver is especially favoured by a carbohydrate diet ; and in our 

 studies on digestion we have seen reason to think that a very 

 large part at all events of the carbohydrate material of a meal is 

 absorbed as sugar by the capillaries of the intestine and carried as 

 sugar to the liver in the portal blood. Hence, it seems only 

 reasonable to conclude that the glycogen which makes its appear- 

 ance in the liver after an amylaceous 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 cell- substance 

 dehydrated into glycogen, or animal starch as it has been called, 

 the process being a reverse of that by which in the alimentary canal 

 starch is hydrated into sugar through the action of the salivary 

 and pancreatic ferments. Vegetable cells can undoubtedly convert 

 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 living substance cannot 

 accomplish the latter as well as the former of these changes. We 

 are quite ignorant it is true of the exact way in which either the 

 hydration or the dehydration is effected by living substance ; but 

 we are equally ignorant of the exact way in which an amylolytic 

 ferment effects the hydration of starch into sugar, which it carries 

 out with so much apparent ease. It is not a great assumption to 

 suppose that the continually changing living substance, which in 

 its changes is continually giving out energy, has the power of 

 acting on molecules of starch or of sugar in contact with, or even 

 only near to itself, and so of hydrating starch into the sugar or of 

 dehydrating sugar into starch. The latter process may be a more 

 difficult one than the former, but not one beyond the power of the 



