THE LIVER AND PANCREAS 247 



kingdom ; the starch in the leaves of plants may pass down the 

 stem as sugar for the purpose of nourishment, and be again formed 

 into starch. Similarly in the animal the starch must be first con- 

 verted into sugar before the bloodvessels of the bowel can take it 

 up, then in the liver once more converted into glycogen, and 

 lastly, again, into sugar before being finally used by the tissues. 

 The sugar formed from starch in the bowel is maltose, while that 

 formed in the liver from glycogen is dextrose. This conversion of 

 glycogen into dextrose is due to the presence of a ferment in the 

 liver cells. 



The total amount of glycogen obtained from a given quantity 

 of food is not wholly stored in the liver ; the latter organ can only 

 hold a limited amount, which in the dog, by a rich carbohydrate 

 diet, does not exceed 17 per cent., and in the rabbit 27 per cent., 

 of its weight, and in other animals is less. We know, as a fact, 

 that the liver, having taken up all the sugar it can from the portal 

 vessels and converted it into stored-up glycogen, allows the 

 balance to pass through the hepatic veins into the general 

 circulation as sugar, and that it is deposited in other organs, 

 principally the muscles, as glycogen for future use. The muscles 

 of well-fed animals contain in this way a considerable quantity of 

 glycogen ; even after nine days' starvation in the horse from 

 1 to 2 4 per cent, has been found. Ordinarily it may be stated 

 that the muscles hold as much glycogen as the liver, but it takes 

 longer by means of work and starvation to free the muscles from 

 glycogen than to clear the liver. The presence of glycogen in 

 muscle is not essential to contraction, for there are muscles in 

 which no glycogen is found, and yet in which active contraction 

 takes place. In the muscles of the embryo, before striation has 

 occurred, the amount of glycogen existing is something consider- 

 able ; as much as 40 per cent, of the dry material of the embryo 

 muscle may consist of this substance. As striation appears the 

 glycogen leaves the muscles to a great extent, and the liver takes 

 on the process of production. 



The Use of Glycogen . — The muscles and liver are not the 

 exclusive seats of glycogen deposits ; traces may be found practi- 

 cally everywhere in the body, but none can be found in the blood 

 plasma. The existence of glycogen in the embryonic muscle 

 points to its use in active nutrition and rapid growth ; further, 

 it is found in the placenta, where it is used for the nourishment of 

 the foetus. In the adult the chief use of glycogen is to facilitate 

 the metabolic production of muscular energy and animal heat, 

 and this is effected by the oxidation of dextrose to carbon dioxide 

 and water. This oxidation does not occur in the blood ; the 

 destruction of sugar (glycolysis) occurs in all active tissues, 

 especially muscles and glands. It can be shown that there is 



