CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 727 



wanted by the body at the moment but may be wanted presently, 

 tin- question readily presents itself, May not the hepatic glycogcn 

 have an analogous function ? May we not regard the presence of 

 glycogen in the liver as in large measure due to the fact that it is 

 deposited there simply as a store of carbohydrate material, being 

 accumulated whenever amylaceous material is abundant in the 

 alimentary canal, and being converted into sugar and so drawn 

 upon by the body at large to meet the general demands for 

 carbohydrate material during the intervals when food is not being 

 taken ? And we can accept this view without being able to say 

 definitely what becomes of the sugar thus thrown into the hepatic 

 blood. It was formerly believed that this sugar underwent an 

 immediate and direct oxidation as it was circulating in the blood 

 but we have already dwelt ( 359) on the objections to such a 

 view. It is sufficient for us at the present to admit that the 

 sugar is made use of in some way or other. 



Now, many considerations lead us to believe that a certain 

 average composition is necessary for that great internal medium 

 the blood, in order that the several tissues may thrive upon it 

 to the best advantage, one element of that composition being a -*- 

 certain percentage of sugar. It would appear that some at least if 

 not all of the tissues are continually drawing upon the blood for 

 sugar, and that hence a certain supply must be kept up to meet 

 this demand. On the other hand an excess of sugar in the blood 

 itself would be injurious to the tissues. And as a matter of fact 

 we find that the quantity of sugar in blood is small but constant ; 

 it remains about the same when food is being taken as in the 

 intervals between meals. If sugar be injected into the jugular 

 vein in too large quantities or too rapidly, a certain quantity 

 appears in the urine, indicating an effort of the system to throw 

 off the excess and so bring back the blood to its average con- 

 dition. The maintenance of such a constant percentage of sugar 

 would obviously be provided for or at least largely assisted by 

 the liver acting as a structure where the sugar might at once 

 and without much labour be packed away in the form of the 

 less soluble glycogen, at those times when, as during an amylaceous 

 meal, sugar is rapidly passing into the blood, and there is a 

 danger of the blood becoming loaded with far more sugar than 

 is needed for the time being; and it may be incidentally noted 

 that a larger quantity of sugar may be injected into the portal 

 than into the jugular vein without any reappearing in the urine, 

 apparently because a large portion of it is in such a case retained 

 in the liver as glycogen. At those times, on the other hand, when 

 we may suppose that sugar ceases to pass into the blood from 

 the alimentary canal, the average percentage in the blood is main- 

 tained by the glycogen previously stored up becoming reconverted 

 into sugar, and being slowly discharged into the hepatic blood. 



Moreover, this view, that the glycogen of the liver is a reserve 



