670 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



more glycogen will be found in the liver than would be obtainable from 

 this organ in a starving animal. It would, therefore, appear that while 

 the amount of glycogen in the liver is dependent upon the food, it is 

 especially the carbohydrates which are the sources of this substance. 



As already stated, the digestible carbohydrates in the alimentary 

 canal are converted into some form of sugar, are absorbed by the 

 intestinal walls, and enter the blood of the portal vein, and are thus carried 

 to the liver. As is well known, the food of herbivora is constituted 

 largely of carbohydrates, and these substances can only be absorbed after 

 being converted into sugar. On the other hand, it is well known that the 

 presence of sugar in the blood above a very small percentage at once 

 leads to its elimination through the kidneys, constituting glycosuria. 

 Admitting, therefore, that large quantities of sugar in these animals 

 enter the blood through the walls of the alimentary canal, two possibil- 

 ities arise — either it is eliminated as rapidly as absorbed, or it at once, 

 through combustion, serves in the development of heat. 



Neither of these possibilities are, however, actually the case, since 

 even after the richest carbohydrate diet but traces of sugar are to 

 be found in the urine, and it is not conceivable that the large amount of 

 sugar which may be absorbed in the food at once is converted into 

 carbon dioxide and water. 



The only remaining conclusion is that the sugar at once, after its 

 absorption, is carried by the portal vein to the liver, and is there con- 

 verted into some less diffusible form by which its immediate excretion by 

 the kidneys is avoided. Such a substance is evidently found in glycogen, 

 and the liver may, therefore, be regarded as a storehouse for one of the 

 most important food-stuffs, which is again reconverted into sugar, as the 

 needs of the economy demand. 



Bernard believed that there is a continual conversion of glycogen 

 into sugar going on in the liver, and that the sugar so formed is carried 

 by the hepatic vein to the general circulation to be oxidized in the lungs 

 and muscles. 



It is evident that this hypothesis necessitates the presence of a 

 larger amount of sugar in the hepatic than in the portal vein, and such 

 a state of affairs is claimed by Bernard to be a fact, although his state- 

 ments have met with a certain amount of contradiction. It may, however, 

 be concluded that even if the estimates made by Bernard as to the 

 comparative amount of sugar in the hepatic and portal veins are not 

 absolutely conclusive, the statements of his opponents are no more 

 trustworthy. 



As to the ultimate end of the sugar derived from the conversion of 

 the glycogen, but little can be accurately stated. It is known that 

 normal blood contains always a definite amount of sugar. If this 



