204 FUNCTIONS OF NUTRITION. 



only a moderately opaline tinge. But if the same thing be done 

 within a few hours after feeding, the watery decoction of the liver 

 will be strongly opalescent ; containing in considerable quantity a 

 matter which communicates to the solution a partial turbidity. This 

 matter is glycogen, which is present in varying quantity under these 

 two conditions. 



Origin and Formation of Glycogen. As this substance is present 

 in the liver tissue of both carnivorous and herbivorous animals, it may 

 be derived from the materials of either kind of food. In the carnivora, 

 at least, there is evidence that it is supplied from nitrogenous materials, 

 by the nutritive changes which they undergo in the liver. Under some 

 circumstances a material resembling glycogen, or identical with it, may 

 be present in the muscles of the herbivora. Bernard has found it in 

 the muscular tissue in rabbits, and especially in pigeons, when fed on 

 the cereal grains, and in horses kept on oats and barley ; but in all 

 these animals it disappears when the food is changed, or after some 

 days' fasting. Luchsinger* has also found it absent from the muscles 

 of the rabbit after several days' fasting, but more persistent in the 

 pectoral muscles of the fowl under similar conditions. 



It is accordingly not a constant but only an occasional ingredient of 

 muscular flesh, and when present is usually in very small quantity. 

 Poggiale,f in many experiments instituted for this purpose by a 

 Commission of the French Academy of Sciences, found glycogen in 

 ordinary butcher's meat only once. We have also found it absent 

 from the fresh meat of the bullock's heart, when examined in the 

 manner above described. Nevertheless, in dogs fed exclusively for 

 eight days on this food, glycogen may be abundant in the liver, 

 while it does not exist in other internal organs, as the spleen, lungs, 

 and kidneys. 



Glycogen is produced in the liver in especial abundance, after the in- 

 gestion of starchy and saccharine food. Bernard J found the decoction 

 of the liver tissue in dogs, after feeding for two days with bread and 

 starch paste, very turbid and milky in appearance. Subsequent ex- 

 periments by the same observer have shown that a starchy diet 

 augments notably the quantity of glycogen in the liver. This fact was 

 first demonstrated in a special manner by the observations of Pavy,|| 

 who, by comparative experiments on dogs fed with animal and vege- 

 table food, found that the influence of the latter was to increase 

 decidedly the weight of the liver, and also the percentage of glycogen 

 which it contained. The same effect was produced by a diet of animal 

 food and sugar. The following table gives the average results of three 

 series of observations by Pavy : 



* Archiv fur die gesammte Physiologie. Bonn, 1873, Band viii., p. 290. 



f Journal de la Physiologie. Paris, 1858, p. 558. 



J Lepons de Physiologie Experimentale. Paris, 1855, p. 159. 



$ Revue des Sciences Medicales. Paris, 1874, tome iii., p. 34. 



|| Nature and Treatment of Diabetes. London, 1862. 



