METABOLISM, NUTRITION, AND DIET. 437 



sugar be prevented by boiling, or other means calculated to interfere 

 with the action of a ferment. 



Instead of adopting the view that normally, during life, glycogen 

 acts as a store of carbo-hydrate material to be converted, little by little, 

 into sugar as occasion requires, and that it passes as sugar into the he- 

 patic venous blood, to be conveyed to the tissues to be further disposed 

 of, Pavy inclines to the belief that it may represent an intermediate 

 stage in the formation of fat from materials absorbed from the alimen- 

 tary canal. There is little evidence in favor of this view, and although 

 it is possible that the liver cells may, in some way or other (not at pres- 

 > ent understood) , be able to convert part of its store of glycogen into fat, 

 the consensus of opinion inclines to the belief that most of the glycogen 

 leaves the liver as sugar. 



Indeed, wherever glycogen is found, in the muscles, in the placenta, 

 or elsewhere, it must be looked upon as a store of carbo-hydrate material 

 which may be oxidized to furnish energy to the body. Whether the 

 glycogen which probably reaches the muscles as sugar is reconverted into 

 glycogen before it is built up as it were into the protoplasmic molecule 

 is not known. 



T lie relation of glycogen to the cell metabolism. It is not exactly known 

 whether the glycogen is formed simply by a process of dehydration of 

 the sugar which reaches the cells in the portal blood, or whether 

 the cells by their metabolism are usually in the habit of form- 

 ing glycogen or sugar which, during fasting and other similar conditions, 

 is at once discharged into the hepatic blood to be used up by the tissues, 

 but which is stored up in the cells as glycogen as long as there is suffic- 

 ient sugar in the blood without it, or as long as the tissues are so quiescent 

 as not to require more than a small quantity of the total amount of 

 carbo-hydrate secreted by the hepatic cells. 



Glycosuria. Sugar may be present not only in the hepatic veins, 

 but in the systemic blood to excess, and when such is the case, the sugar 

 is excreted by the kidneys, and appears in variable quantities in the 

 urine. This condition is known as glycosuria. 



Influence of the Nervous System. Glycosuria may be experimentally 

 produced by puncture of the medulla oblongata in the region of the 

 vaso-motor centre. The better fed the animal the larger is the amount 

 of sugar found in the urine; in the case of a starving animal no sugar 

 appears. It is, therefore, highly probable that the sugar comes from 

 the hepatic glycogen, since in the one case glycogen is in excess, and in 

 the other it is almost absent. The nature of the influence is uncertain. 

 It may be exercised in dilating the hepatic vessels, or possibly may be 

 exerted on the liver cells themselves. The whole course of the nervous 



