METABOLISM 37 



is always going on to a less extent even in normal 

 metabolism. 



3. Carbohydrates. 



From the comparative simplicity of the chemistry of 

 the carbohydrates and the ease with which sugar can be 

 recognized, even in small amounts, it might have been 

 supposed that by this time we would have been well 

 informed as to the details of carbohydrate metabolism. 

 In spite, however, of the immense amount of work which 

 has been devoted to the subject, and the stimulus to 

 research which has been supplied by the ever-present 

 riddle of diabetes, we are still profoundly ignorant even 

 of the main outlines of the process. We know that 

 carbohydrates are all converted into glucoses by the 

 processes of digestion, but so soon as these disappear 

 into the wall of the intestine our uncertainties begin. 

 That sugar reaches the liver by the portal blood and 

 is there converted into glycogen * is well established, but 

 in what form it leaves the liver is still open to dispute- 

 According to the classical view, glycogen is reconverted 

 into sugar by the action of a ferment, and in that form 

 is transported to the cells. Opposed to this is the view 

 of Pavy, who strenuously denies that carbohydrates 

 leave the liver in the form of sugar, and maintains that 

 they are worked up into combination with nitrogenous 

 material to form proteins, and in that form are carried 

 to the cells. It might be thought that the dispute could 

 be settled by the experimentum crucis of estimating the 

 amount of sugar in the blood of the portal vein during 



* The liver contains about 10 ounces of glycogen, and the 

 muscles rather less. 



