DIGESTION. 121 



namely: separation from the portal blood by its cells of a substance 

 known as glycogen, or animal starch. 



Glycogen exists constantly, though in very small proportions, in 

 protoplasm and animal membranes in general; also, in white blood- 

 corpuscles and pus. It occurs in more considerable quantities in liver, 

 muscle, and embryonic tissues after the third month. Glycogen is a 

 white, tasteless powder, soluble in water, but producing an opaque 

 solution. Glycogen possesses the property of being readily trans- 

 formed into glucose, to be ready for easy oxidation. Glycogen with 

 iodine in solution with iodide of potassium gives a port-wine color, 

 which disappears upon heating. 



Naturally during absorptive processes following active digestion, 

 portal blood contains more than the normal quantity 1 per 1000. 

 At the very same time the blood in the hepatic vein during the in- 





Fig. 34. A, Liver-cells during fasting. B, Cells filled with Glycogen. 



(HEIDENHAIN.) 



tervals of absorption of carbohydrates contains 2 parts per 1000. 

 Within the hepatic-cell protoplasm glycogen is deposited. When an 

 excess of carbohydrates is taken, not all of the glycogen can be 

 absorbed, but passes through into the general circulation, to be depos- 

 ited in the muscles and other tissues. Muscles may contain as much 

 as 1 or 2 per cent. 



That sugar should appear in both portal and hepatic blood is not 

 to be wondered at. when carbohydrates are fed, but that it should still 

 be present when but meats are given, or when the portal vein is ligated 

 at the transverse fissure, goes far to prove that glycogen, or sugar- 

 forming animal starch, must be manufactured within the parenchyma 

 of the liver. Even when an animal is made to fast, and at the same 

 time perform very severe muscular work, so that glycogen disappears 

 in muscles and liver, its presence in the liver is soon ascertained again 

 though the animal be fed but gelatin. 



Since neither glycogen nor sugar appears in the bile, it follows 

 that it, or some transformed product of it, must be absorbed into the 



