418 METABOLISM, NUTRITION, AND DIET 



of young animals is often considerable. The placenta is also a storehouse 

 of glycogen. 



The Destination of Glycogen. The chief theories concerning the 

 use of glycogen in the organism are advanced by Bernard and by Pavy. 

 The former considers glycogen as a reserve supply of carbohydrate. When- 

 ever the glycogen of the blood is reduced below the normal level, i.e., about 

 o.i to 0.15 per cent, there is a conversion of glycogen into sugar. The sugar 

 enters the blood and passes to the tissues where its oxidation is a source of 

 energy. Pavy considers glycogen to be a stage in the synthesis of carbohydrate 

 to fat and proteid. Bernard's theory is more generally accepted. It ex- 

 plains more satisfactorily why the sugar content of the blood is so constant. 

 The conversion of glycogen to sugar takes place by the action of an intracellu- 

 lar ferment in the glycogenic cells. Such an enzyme has been isolated for 

 the liver. It is this enzyme that converts the liver glycogen to dextrose after 

 death, and which is destroyed by boiling in the usual process of isolating 

 glycogen. 



Glycosuria. Sugar may be present to excess not only in the hepatic 

 veins, but in the systemic blood. When such is the case, the sugar is ex- 

 creted by the kidneys, and appears in variable quantities in the urine. This 

 condition is known as glycosuria. 



Glycosuria may be experimentally produced by puncture of the medulla 

 oblongata in the region of the vaso-motor center, puncture diabetes. The 

 better fed the animal, the larger is the amount of sugar found in the urine 

 following this operation. In the case of a starving animal no sugar appears. 

 It is, therefore, highly probable that the sugar comes from the hepatic glyco- 

 gen, since in the one case glycogen is in excess, and in the other it is almost 

 absent. The nature of the influence is uncertain. This influence may be 

 exercised in dilating the hepatic vessels, or possibly may be exerted on the 

 liver cells themselves. 



Many other circumstances will cause glycosuria. It has been observed 

 after the administration of various drugs, e.g., strychnine; phloridzin, a glu- 

 coside, and its derivative phloretin, which is not a glucoside; morphine; 

 adrenalin; nitrite of amyl, etc.; after the injection of curari, poisoning with 

 carbonic-oxide gas, the inhalation of ether, chloroform, etc., the injection of 

 oxygenated blood into the portal venous system. Glycosuria has been 

 observed in man after injuries to the head and in the course of various 

 diseases. In such cases the glycosuria appears to be due either to some ab- 

 normal activity of the liver cells themselves or to an interference with the 

 normal metabolism of the carbohydrate group. In this latter case it is pos- 

 sible that the usual complete oxidation of carbohydrate is interfered with. 



The well-known disease, diabetes mellitus, in which a large quantity of 

 sugar is persistently excreted daily with the urine, has, doubtless, some close 

 relation to the normal functions of the pancreas. The nature of the relation- 



