PHYSIOLOGY OF THE LIVER AND SPLEEN. 757 



It is probable, therefore, that the normal conversion of glycogen to 

 dextrose is effected by a special enzyme produced in the liver cells. 

 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 protein foods, 

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

 fulfill. 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. 



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 throughout 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 mammals 

 generally the most significant occurrence of glycogen, outside 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. Muscular tissue, as 

 well as liver tissue, has a glycogenetic function that is, it is cap- 

 able of laying up a supply of glycogen from the sugar brought 

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

 demonstrated directly by Kulz,* who has shown that an isolated 

 muscle irrigated with an artificial supply of blood to which dextrose 

 is added is capable of changing the dextrose to glycogen, as shown 

 by the increase in the latter substance in the muscle after irriga- 

 tion. Muscle glycogen is to be looked upon 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, considering 

 its bulk, is the most active tissue of the body from the standpoint 

 of energy production, 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 cells of embryos indicates 

 that this glycogenetic function 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 

 * "Zeitschrift f. Biologie," 27, 237, 1890. 



