330 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



the estimation of glycogen in the liver it is necessary to mince the organ and to 

 throw it into boiling water as quickly as possible, since by this means the liver- 

 eel].- are killed and the conversion of the glycogen is stopped. How the 

 glycogen is changed to dextrose by the liver is a matter not fully explained. 

 According to some authors, the conversion is due to an enzyme produced in 

 the liver. Extracts of liver, as of some other tissues, do yield an anxiolytic 

 enzyme that changes glycogen to dextrose. 1 It is possible, therefore, that 

 the 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 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 

 firmed 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 iu 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 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 glycogenetie func- 

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

 brought to it by the blood. The glycogenetie 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 had been added is capable 

 of changing the dextrose to glycogen, 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, 80 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 glycogenetie func- 

 tion may at times be exercised by any tissue. 



1 Tebb: Journal <;/" Physiology, 1897 98, voL xxii. }>. 423. 

 '- /..it. thrift fiir Biologic, 1890, 8. 237. 



