114 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



another meal. This storing up brings about a kind of conserva- 

 tion of energy and an economical regulation of its distribution. 

 The amount of sugar in the circulation at any time is very small, 

 and a single carbohydrate meal may, by the action of the liver, 

 be made to supply the carbohydrate demands of the tissues for 

 a considerable period. 



Now, it was just said that the sugar of the blood is dextrose; if 

 the dextrose of the portal blood is converted into glycogen to be 

 stored up, it must be reconverted into dextrose before it can leave 

 the liver, since it leaves by the blood. The cells do effect the 

 second conversion, and this is the second part of the glycogenic 

 function. It may be that the liver cells produce an enzyme 

 corresponding to ptyalin, which converts the glycogen. Dex- 

 trose does not normally exist in the liver cells. At the very 

 moment of its formation it is carried away by the blood. 



The fact that the liver can form glycogen out of proteids shows, 

 of course, that the nitrogen is eliminated from the proteid mole- 

 cule in some way. A carbohydrate molecule is left to be oxidized 

 in the usual manner. This is thought to be the initial step in 

 the final consumption of proteids in nutrition. The fats have no 

 influence on glycogen formation. 



Glycogen also exists in other parts of the body, particularly 

 in the voluntary muscular substance. The cells of the tissue in 

 which it is found must also have a glycogenic function. 



Urea Formation. But the liver has another function besides 

 the production of bile and glycogen, and that is to form urea. 

 It will be seen later that the chief end product of proteid metab- 

 olism is urea, and that it is eliminated almost entirely by the 

 kidneys. The liver is much more active in the production of 

 this substance when the portal blood is charged with digested 

 materials, but it also forms urea in fasting animals. The liver 

 must, therefore, be capable of forming urea from some of the 

 products of digested foods. With reference to its formation in 

 fasting animals, suffice it to say here that it seems that as long as 



