444 THE METABOLIC PROCESSES OF THE BODY. 



general considerations lead us to the same conclusion, that the processes 

 taking place in an hepatic cell are very complex. In the first place, the con- 

 stituents of bile are being formed and discharged into the bile passages after 

 the fashion of ordinary secreting glands. In the second place, a formation 

 of glycogen is also taking place, and we shall have presently to consider 

 briefly the relations of the one process to the other. In the third place, as 

 is especially indicated by the somewhat peculiar effects on the hepatic cell of 

 food exclusively proteid in nature, other processes, similar perhaps to the 

 formation of glycogen, but not resulting in the storage of any carbohydrate 

 material, and 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 obvi- 

 ous that the hepatic cell manufactures the glycogen in some way or other 

 and lodges it in its own substance for the time very much in the same way 

 that a secreting cell manufactures 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. 



380. We say " manufacture 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 favored 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 

 appearance in the liver after an amylaceous meal arises from a direct con- 

 version 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 a 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 substances; 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 assump- 

 tion to suppose that the continually changing living substance, which in its 

 changes is continually giving out energy, has the power of acting on mole- 

 cules of starch or of sugar in contact with or even only near to itself, and 

 so of hydrating starch into 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 living substance. We may fairly suppose that a 

 quantity of sugar in solution present in a vacuole, for instance, of the hepatic 

 cell substance can be, bv some action of the cell substance, converted into 



