CHAP, v.] NUTRITION. 421 



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 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. When on the other hand sugar ceases to 

 pass into the blood from the alimentary canal, we may suppose 

 that the average percentage in the blood is maintained by the 

 glycogen previously stored up becoming reconverted into sugar, 

 and slowly discharged into the hepatic blood. 



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

 fund of carbohydrate material, is strongly supported by the analogy 

 of the migration of starch in the vegetable kingdom. We know 

 that the starch of the leaves of a plant, whether itself having 

 previously passed through a glucose stage or not, is normally 

 converted into sugar, and carried down to the roots or other parts, 

 where it frequently becomes once more changed back again into 

 starch. 



A similar argument may be drawn from the relations of 

 glycogen to muscle; that is to say, the glycogen in the muscle 

 may be regarded as a subsidiary store of carbohydrate material 

 laid up for the private use so to speak of the muscle. So fre- 

 quently is glycogen found in muscle that it may be regarded as 

 an ordinary though not an invariable constituent of that tissue ; 

 indeed it may almost be considered as a constituent of all con- 

 tractile tissues. The quantity varies very largely both in the dif- 

 ferent muscles of the same animal and in corresponding muscles of 

 different animals. It disappears readily upon starvation, even 



