GLYCOGEN 215 



besides forming bile, the hepatic cells are concerned in 

 other labours, the result of which can hardly be con- 

 sidered either as a loss or as a gain, since these labours 

 simply consist in manufacturing from the blood and 

 storing up in the hepatic cells substances which, sooner 

 or later, are returned, generally in a changed condition, 

 back into the blood. For instance we have already seen 

 that salts of ammonium carried by the blood to the liver 

 are there converted into urea and are returned as such to 

 the blood. 



Again, as we shall presently see, the portal blood is, 

 after a meal, heavily laden with substances, the result 

 of the digestive changes in tlie alimentary canal. When 

 these substances, carried along in the portal blood, reach 

 the hepatic cells, in the meshes of the lobules, some of 

 them appear to be taken up by those cells and to be 

 stored up in them in a changed condition. In fact, the 

 products of digestion passing along tl,e portal veins 

 suffer (in the liver) a further change, which has been 

 called a secondary digestion. Thus the liver produces 

 a powerful effect on the quality of the blood passing 

 through it, so that the blood in the hepatic vein is very 

 different, especially after a meal, from the blood in the 

 portal vein. 



The changes thus effected by the hepatic cells are 

 probably numerous, but they have not been fully worked 

 out, except in one particular case, which is very interesting 

 and deserves special attention. ° 



It is found that the liver of an animal which has been 

 well and regularly fed, when examined immediately after 

 death, contains a considerable quantity of a substance 

 which IS very closely allied to starch, consisting of carbon 

 hydrogen, and oxygen in proportions the same as in 

 starch. This substance, which may by proper methods 

 be extracted and preserved as a white powder, is in fact 

 an animal starch, and is called glycogen. As we 

 shall see, common starch is readily changed by certain 



