GLYCOGEN AND GLUCOSE IN THE LIVER. 239 



Unless, therefore, a new supply of food be taken, all the glycogen of the 

 liver, as shown by experiments cited above, becomes after a time ex- 

 hausted ; having gradually undergone the saccharine transformation and 

 absorption by the bloodvessels. 



Owing to these processes going on in the hepatic tissue, the blood 

 beyond the liver, that is, the blood in the hepatic vein, the inferior vena 

 cava above the diaphragm, and the right side of the heart, contains 

 traces of glucose. The proportion of sugar in the blood, however, con- 

 stantly diminishes as it recedes from the point of its origin, since the 

 saccharine blood coming from the liver is diluted with that of the inferior 

 vena cava, and this mixture again with that of the superior vena cava, 

 before reaching the right cavities of the heart. Beyond -the pulmonary 

 circulation, under ordinary circumstances, it has disappeared altogether, 

 so that no sugar is to be found, as a rule, in the blood of the general 

 circulation. 



The changes, therefore, which take place in the liver, so far as regards 

 the carbohydrates, consist, first, in a deposit of glycogen, derived from 

 the ingredients of the blood. This glycogen acts as a reserve material, 

 which is afterward used for the purposes of nutrition. The vegetable 

 starch and sugar of the food, after digestion, are absorbed under the 

 form of glucose and taken up by the vessels of the portal system. This 

 glucose does not at once enter the general circulation, but on reaching 

 the liver undergoes, for the most part, a conversion and deposit as 

 glycogen, or animal starch. It then gradually again passes into the 

 form of glucose by a secondary transformation, and in this form is 

 carried away by the hepatic blood, to be finally decomposed or assimi- 

 lated in some unknown manner for the maintenance of the vital phe- 

 nomena. The final product of its metamorphosis or destruction in the 

 animal body is undoubtedly carbonic acid and the elements of water ; 

 but how far this is accomplished by direct oxidation, or what other 

 intermediate changes may occur in the act of nutrition, cannot as yet be 

 determined with certainty. 



Similar successive transformations of the starchy and saccharine 

 carbohydrates are already known to take place in vegetables. In the 

 growing plant, under various conditions, the starch, first formed in the 

 green leaves, passes into the condition of a saccharine fluid to be trans- 

 ported into organs of reserve, such as tuberous roots, grains, and fruits, 

 where it is again deposited as starch ; and from these it is subsequently 

 taken up at the requisite time as glucose, and carried by the vascular 

 channels into the growing organs for its final destruction or assimila- 

 tion. 1 Starch and sugar, therefore, in animals as well as in vegetables, 

 are to be regarded as two different forms of the same nutritive substance, 

 one of which is in the condition of temporary deposit, the other in that 

 of solution and activity. 



1 Mayer, Lehrbuch der Agrikultur Chemie. Heidelberg, 1871, Band i. pp. 76, 



78, 81. 



