824 PHYSIOLOGY OF DIGESTION AND SECRETION. 



charge into the blood is designated sometimes as the process of 

 glycogenolysis. It has been shown that the process may be facil- 

 itated by a number of conditions, such as acidosis or duninution in 

 the alkali reserve of the blood, strong emotions, or an increased 

 secretion of epinephrin. These reactions may be considered as 

 adaptations suited to increase the efficiency of the organism. Thus, 

 under the influence of strong emotions (Cannon), the glycogen, to 

 use the current expression, is mobilized, that is to say, is converted 

 to sugar so that it may be utilized as a source of energy in increased 

 muscular work. In this description of the origin and meaning of 

 the liver glycogen reference has been made only to the glycogen de- 

 rived directly from digested carbohydrates. The glycogen derived 

 from protein 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, when an excess is eaten beyond the energy 

 needs of the tissues, may 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 

 * Kulz, "Zeitschrift f. Biologie," 72, 237, 1890. 



