658 METABOLISM 



from the portal vein without serious disturbance to the animal. Indeed, 

 the only way by which the problem has been accurately studied is by 

 comparing the blood of the portal circulation with that of the systemic 

 circulation during the injection of a solution of dextrose into one of the 

 smaller branches of the portal vein. 21 In such experiments it has been 

 found that the percentage of sugar is a little less in the blood of the 

 abdominal vena cava than in that of the portal vein, and is still less in 

 the blood of the systemic veins, such as the femoral. results which justify 

 the conclusion that the barriers responsible for taking out some of the 

 absorbed sugar from the blood exist in the liver and in the muscles. The 

 curve in Fig. 189 will illustrate to what extent the mechanism operates. 



./ 



.~ 



Cans tint i*/ectSen of /gXOextnst $ I ut/'ow. 



/ <cr to sf 30 if to y*~ &> ss- 60 tr TO rs so tr to is- /oo 



Fig. 189. Curves showing the percentage of glucose in blood after a constant injection of 

 an 18 per cent solution into a mesenteric vein. V.C., vena cava, continuous line; P.O., pan- 

 creaticoduodenal vein, broken line; /, iliac, dotted line. 



It will be observed that, so far as can be judged from changes in the 

 concentration of sugar in the blood, the sugar-retaining power of the 

 liver is about equal to that of the muscles a conclusion which is, how- 

 ever, contrary to the usually accepted one that the liver has such pro- 

 nounced sugar-retaining powers that under ordinary circumstances it 

 removes from the portal blood all the excess of sugar added to it by 

 absorption and which is not required by the organism. 



One objection which may properly be made to these observations is 

 that the animals on which they were made were under anesthesia, and 

 that the anesthetic had a paralyzing effect on the sugar-retaining power 

 of the liver. In view of this criticism it is important to examine the 

 results obtained on animals that are not under the influence of anes- 



