154 NUTRITION OF FARM ANIMALS 



mation of glycogen, while a deficiency of dextrose would have the 

 contrary effect. If it be supposed further that the glycogen 

 as soon as formed combines with the protoplasm of the liver 

 cells, forming compounds which withdraw a considerable por- 

 tion of it from the sphere of action of the enzym, after the anal- 

 ogy of the precipitation of an insoluble compound, we have a 

 plausible, even if chiefly hypothetical, scheme of the chemical 

 mechanism of the process. Whether or not it adequately rep- 

 resents the actual facts, it may at least serve as a concrete il- 

 lustration of the manner in which the conception of enzym ac- 

 tion may be applied to metabolic processes. 



215. Muscle glycogen. While the glycogenic function of 

 the liver has been the subject of very extensive investigation, 

 the presence of glycogen is by no means confined to this organ. 

 Indeed, glycogen seems to be a normal constituent of animal 

 protoplasm. It is found in greater or less amounts in practi- 

 cally all tissues, being particularly abundant where rapid cell 

 multiplication is taking place, as in embryonic tissues or in 

 rapidly growing tumors. It is estimated that in an animal in 

 normal condition roughly one-half of the glycogen of the body 

 is contained in the liver. Of the other half by far the larger 

 proportion is found in the muscles (96). 



The glycogen of the muscles (and other organs) is not simply 

 glycogen which has been formed in the liver and transported to 

 the muscles, but is produced independently from the dextrose 

 of the blood, apparently in much the same manner as in the 

 liver. That this is true is shown by the fact that glycogen is 

 still formed in the muscles when, by surgical interference 

 (Eck fistula), the blood is prevented from passing through the 

 liver. In fact, the formation of glycogen in the muscles, etc., 

 appears to be the primary process, while the liver serves rather 

 as a secondary reservoir which may be eliminated without 

 seriously affecting the general carbohydrate metabolism. With 

 the liver excluded from the circulation, the dextrose resorbed 

 from the digestive tract is still converted into glycogen and the 

 animal is still able to digest considerable quantities of carbohy- 

 drates without the appearance of sugar in the urine. 



Even in the normal animal, however, the power to dispose of sur- 

 plus sugar is not unlimited. If large quantities of sugar are consumed, 

 the conversion into glycogen, together with the normal katabolism, 



