THE HISTORY OF GLYCOGEN. 447 



the blood becoming loaded with far more sugar than is needed for the time 

 being ; and it may be incideutly noted that a larger quantity of sugar may 

 be injected into the portal than into the jugular vein without any reappear- 

 ing in the urine, apparently because a large portion of it is in such a case 

 retained in the liver as glycogen. At those times, on the other hand, when 

 we may suppose that sugar ceases to pass into the blood from the alimentary 

 canal, the average percentage in the blood is maintained by the glycogen 

 previously stored up becoming reconverted into sugar, and being slowly dis- 

 charged into the hepatic blood. 



Moreover, this view, that the glycogen of the liver is a reserve fund of 

 carbohydrate material, is strongly supported by the analogy of the migra- 

 tion of starch in the vegetable kingdom. We know that the starch of the 

 leaves of a plant, whether itself having previously passed through a glucose 

 stage or not, is normally converted into sugar, and carried down to the roots 

 or other parts, where it frequently becomes once more changed back again 

 into starch. 



383. 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 purposes 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 contractile tissues. The 

 quantity varies very largely both in the different 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 dex- 

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

 immediate supply of the energy set free in the contraction. But even grant- 

 ing that the glycogen in a muscle may be diminished during prolonged 

 labor, it cannot be admitted that the oxidation or other chemical change of 

 glycogen is a necessary part of the ordinary metabolism of a muscular con- 

 traction, since many muscles wholly free from glycogen are perfectly well 

 able to carry on long-continued contractions. 



Another view of the use of glycogen in muscle is suggested by the fact 

 that undeveloped embryonic muscles are peculiarly rich in glycogen. In a 

 young embryo, at the time when the muscular substance, though undergoing 

 striation, is still largely " protoplasmic" in nature, the quantity of glycogen 

 present is enormous ; it frequently amounts to 40 per cent, of the dry mate- 

 rial. At this period the hepatic cells are immature and very little glycogen 

 is present in them. Later on, as the muscles become more wholly striated, 

 the glycogen largely disappears from the muscle, and very soon afterward 

 begins to be stored up in the liver. 



The meaning of this can hardly be mistaken. The glycogen in the imma- 

 ture muscle is a store of carbohydrate material, laid down on the spot, and 

 ready at once to be used in what we may probably call the fierce metabolic 

 struggle by which the simple protoplasmic cell substance of the rudiment 

 of the muscular fibre is transformed into the highly differentiated striated 

 contractile substance. And we shall probably not err in considering the 



