METABOLISM, NUTRITION AND DIETETICS 515 



after being put on a given diet for two or three days, are starved 

 for a time, and then put again on the original diet, the hepatic and 

 the muscular glycogen behave differently at first during the period 

 of re-alimentation. While glycogen accumulates in the liver in 

 greater quantity than under normal conditions of nutrition, in the 

 muscles it at first accumulates much less rapidly than normally. 



Function and Fate of the Glycogen. When a fasting dog 

 is made to do severe muscular work, the greater part of the 

 glycogen soon disappears from its liver. When a dog is starved, 

 but allowed to remain at rest, the glycogen still markedly 

 diminishes, although it takes a longer time ; and at a period when 

 there is still plenty of fat in the body, there may be only a trace 

 of hepatic glycogen left. The glycogen which is usually con- 

 tained in the skeletal muscles also diminishes very rapidly in 

 the first days of hunger, but the heart contains the normal 

 amount of glycogen at a time when the proportion in the skeletal 

 muscles has sunk to y 1 ^ to ^ of the normal. These facts have 

 been taken to indicate that glycogen and the sugar formed from 

 it are the readiest resources of the starving and working organism, 

 for the transformation of chemical energy into heat and mechani- 

 cal work. To borrow a financial simile, the fat of the body has 

 sometimes been compared to a good, but rather inactive security, 

 which can only be gradually realized ; its organ-proteins to long- 

 date bills, which will be discounted sparingly and almost with a 

 grudge ; its glycogen, its carbo-hydrate reserves, to consols, 

 which can be turned into money at an hour's warning. Glycogen, 

 on this view, is especially drawn upon for a sudden demand, fat 

 for a steady drain, tissue-protein for a life-and-death struggle. 



Although it cannot be doubted that much of the hepatic 

 glycogen leaves the liver as sugar, there is no proof that it all does 

 so. It is known that fat may be formed from carbo-hydrates 

 (p. 524) ; and globules of oil are often conspicuous among the 

 contents of liver-cells, side by side with glycogen. It is possible, 

 therefore, that some of the glycogen may represent a half-way 

 house between sugar and fat, or, since it is probable that fat 

 can also be formed from protein, and a purely protein diet pro- 

 duces some glycogen, a half-way house between protein and fat. 



Pavy has put forward the heterodox view that the glycogen 

 formed in the liver from the sugar of the portal blood is never 

 reconverted into sugar under normal conditions, but is changed 

 into some other substance or substances, and he denies that the 

 post-mortem formation of sugar in the hepatic tissue is a true 

 picture of what takes place during life. But in spite of the 

 brilliant manner in which he has defended this thesis, both by 

 argument and by experiment, it must be said that the older 

 doctrine of Bernard, which in, the main we have followed above, 



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