270 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



metabolic activity of the liver-cells, and so long as these cells are in a living 

 condition they can effect this change. In this description of the origin and 

 meaning of the liver glycogen reference has been made only to the glycogen 

 derived directly from digested carbohydrates. The glycogen derived from proteid 

 foods, once it is formed in the liver, has, of course, the same functions to fulfil. 

 It is converted into sugar, and eventually is oxidized in the tissues. For the 

 sake of completeness it may be well to add that some of the sugar of the blood 

 formed from the glycogen may under certain conditions be converted into fat in 

 the adipose tissues, instead of being burnt, and in this way it may be retained 

 in the body as a reserve supply of food of a more stable character than is the 

 glycogen. 



Glycogen in the Muscles and other Tissues. The history of glycogen is 

 not complete without some reference to its occurrence in the muscles. Glycogen 

 is, in fact, found in various places in the body, and is widely distributed through- 

 out the animal kingdom. It occurs, for example, in leucocytes, in the placenta, 

 in the rapidly-growing tissues of the embryo, and in considerable abundance in 

 the oyster and other molluscs. But in our bodies and in those of the mam- 

 mals generally the most significant occurrence of glycogen, outside of the liver, 

 is in the voluntary muscles, of which glycogen forms a normal constituent. It 

 has been estimated that the percentage of glycogen in resting muscle varies 

 from 0.5 to 0.9 per cent., and that in the musculature of the whole body there 

 may be contained an amount of glycogen equal to that in the liver itself. 

 Apparently muscular tissue, as well as liver-tissue, has a glycogenetic func- 

 tion that is, it is capable of laying up a supply of glycogen from the sugar 

 brought to it by the blood. The glycogeuetic function of muscle has been 

 demonstrated recently by Kulz, 1 who has shown that an isolated muscle irrigated 

 with an artificial supply of blood to which dextrose had been added is capable 

 of changing the dextrose to glycogeu, as shown by the increase in the latter sub- 

 stance in the muscle after irrigation. Muscle glycogen is to be looked upon, 

 probably, for reasons to be mentioned in the next paragraph, as a temporary 

 and local reserve supply of material, so that, while we have in the liver a large 

 general depot for the temporary storage of glycogen for the use of the body at 

 large, the muscular tissue, which is the most active tissue of the body from 

 a chemical standpoint, is also capable of laying up in the form of glycogen 

 any excess of sugar brought to it. The fact that glycogen occurs so widely in 

 the rapidly-growing tissues of embryos indicates that this glycogenetic func- 

 tion may at times be exercised by any tissue. 



Conditions Affecting the Supply of Glycogen in Muscle and Liver. 

 In accordance with the view given above of the general value of glycogen 

 namely, that it is a temporary reserve supply of carbohydrate material which 

 may be rapidly converted to sugar and oxidized with the liberation of energy 

 it is found that the supply of glycogen is greatly affected by conditions calling 

 for increased oxidations in the body. Muscular exercise will quickly exhaust the 

 supply of muscle and liver glycogen, provided it is not renewed by new food. 

 1 Zeitschrift fur Biologie, 1890, p. 237. 



