CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 763 



converted into sugar, and carried down to the roots or other parts, 

 where it frequently becomes once more changed back again into 

 starch. 



But in thus putting prominently forward the value of the 

 hepatic glycogen as a storehouse of carbohydrate material, we 

 must not forget that the whole of the store is not necessarily 

 destined for other tissues than the liver ; it may be made use of 

 in part by the hepatic cell itself. The storing-up of glycogen is 

 only one of the many functions of the hepatic cell. We shall 

 presently bring forward evidence as to the occurrence in the 

 hepatic cell of metabolic processes, in addition to those more 

 directly concerned with the secretion of bile and the deposition of 

 glycogen. It may be that part of the hepatic glycogen is in 

 and by means of the hepatic cell under certain circumstances 

 converted into fat; and this would explain the frequent abundance 

 of fat in the hepatic cells. But it will be observed that this is a 

 very different thing from maintaining that the glycogen is wholly 

 destined to become fat. The position which we are expounding 

 now is that the primary purpose of the glycogenic function is to 

 provide a store of glycogen for the needs of the body ; by virtue 

 of this the liver holds the balance as it were between the carbo- 

 hydrate supply and demand of all parts of the body, whatever be 

 the purpose served by the carbohydrate in this or that tissue; 

 and all we are adding is that some of that material it may destine 

 for itself, and that the use which it may make of it is to 

 manufacture fat. 



463. Glycogen is found in other parts of the body than the 

 liver, and a study of the facts relating to the presence of 

 glycogen in other tissues will help us to a true conception of the 

 purpose of the hepatic glycogen. Next to the liver, the skeletal 

 muscles are perhaps the most conspicuous glycogen holders. So 

 frequently is glycogen found in muscle that it may be regarded as 

 an ordinary though not an invariable constituent of that tissue; 

 indeed it may almost be considered as a constituent of all con- 

 tractile tissues. The quantity varies very largely both in the dif- 

 ferent muscles of the same animal and corresponding muscles of 

 different animals. It disappears, according to some observers, 

 readily upon starvation, even before the hepatic glycogen is 

 exhausted ; but all observers are not agreed on this point, and in 

 some muscles, at least, it appears to be retained for a very long 

 time. It is said to be increased in quantity when the nerve of 

 the muscle is divided, and the muscle thus brought into a state of 

 quiescence. On the other hand it diminishes or even disappears, 

 being apparently converted into dextrose, when the muscle enters 

 into rigor mortis. Some observers have found that it diminishes 

 during tetanus, and maintain that it, after conversion into dextrose, 

 is used up in the act of contraction, forming through its oxidation 

 the immediate supply of the energy set free in the contraction. 



492 



