226 THE HUMAN MECHANISM 



Experiments have shown that sugar is absorbed from the 

 alimentary canal entirely by the intestinal blood vessels. It 

 must pass, therefore, through the liver by the portal vein 

 (see Fig. 62) before going to the rest of the body. The 

 liver, thus standing at this great gateway to the circulation, 

 would seem to act as the carbohydrate storehouse, or savings 

 bank, of the body. Any excess of sugar in the portal blood 

 is there transformed into glycogen and deposited, saved until 

 it is needed, and then " paid out," as sugar, when the ready 

 supply is exhausted. Other tissues doubtless aid in prevent- 

 ing an undue richness of sugar in the blood, acting likewise 

 as temporary storehouses for this form of food. 



12. Protein a source of sugar to the body. It has been 

 stated that glycogen may be formed from protein. This is 

 because the body can and does constantly form sugar 

 (dextrose) from protein. Experiments have shown that 

 about half of the protein may in this way be transformed 

 into sugar, the greater part of which is ordinarily oxidized 

 as fuel; but in case there is an excess over and above fuel 

 needs, this excess of sugar is stored as glyccgen by the liver 

 and other organs, just as the excess of sugar absorbed from 

 the alimentary canal is so stored. The formation of sugar, 

 and consequently of glycogen from fat, on the other hand, 

 is negligible. 



In this formation of sugar from protein the body obviously 

 has an additional means of supplying its carbohydrate needs 

 when the sugar delivered to the blood by absorption from 

 the alimentary canal is inadequate. 



13. The protein reserve. Provision is thus made for a 

 continuous supply of fat and carbohydrate (sugar) between 

 periods of absorption of these foods and even during starva- 

 tion. How are the protein needs of the body met under 

 similar conditions? There is no visible supply of inactive 

 protein in the body similar to fat or glycogen. It is true 

 that analysis of the lifeless cell shows that proteins make 



