572 GLYCOGEN IN THE MUSCLES. [BOOK n. 



Another view, which has already been suggested while we 

 were dealing with the manner of formation of glycogen, makes 

 use of the formation of fat for the purposes of analogy only. 

 Seeing that adipose tissue serves as a storehouse of fat which 

 is not wanted by the body at the moment but may be wanted 

 presently, the question readily presents itself, May not the 

 hepatic glycogen 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 amyla- 

 ceous 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 be- 

 comes 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 ( 290) 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 condition. 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 



