306 HUMAN PHYSIOLOGY 



Carbohydrate Metabolism. We have seen that in 

 the course of digestion starches and complex sugars are 

 changed to sugars of the simplest order. By far the 

 most abundant of these simple sugars is the one which we 

 call dextrose. It is identical with the sugar found in 

 grapes. Our account of carbohydrates in the body 

 resolves itself into a history of the use made of dextrose 

 by the tissues. Within a few hours after a meal 4 or 5 

 ounces of this sugar may enter the portal circulation. 

 This sugar will be offered to the liver cells before it is 

 presented to any other organ. The clue is an important 

 one and was long ago followed by Bernard with fruitful 

 results. 



The liver of a well-nourished animal contains carbo- 

 hydrate to an extent that other organs do not approach. 

 The particular representative of the carbohydrate class 

 is more like a starch than a sugar : it is of high molecular 

 weight and limited solubility. It is found within the 

 cells of the liver in small solid clumps. We sometimes 

 call it animal starch but more often glycogen. The word 

 means " sugar former" and the reference is to the readi- 

 ness with which glycogen undergoes digestion and is 

 resolved into dextrose. The human liver may contain 

 as much as 6 ounces of glycogen ; in herbivorous animals, 

 such as the rabbit, the proportion may be higher than in 

 man. 



Since we know that the liver stands in the path of the 

 incoming carbohydrate, and since we find this organ rich 

 in a starch-like substance, it is natural to infer that some 

 portion of the absorbed sugar may be arrested and 

 retained by the liver. It is turned into glycogen by a 

 change which is a condensation, the reverse of digestion. 

 It remains subject to reconversion to dextrose, in which 

 form it is dealt out to the blood in times of fasting. 

 There is a definite resemblance between the liver, so 

 far as this function is concerned, and a tuber like a potato. 

 The liver and the potato are both repositories of surplus 

 carbohydrate and in both cases the material enters and 



