METABOLISM 183 



muscles also may contain it. It appears that glycogen is a 

 reserve supply material which serves to store up sugar for the 

 organism. In case of need, the glycogen is broken down, and fur- 

 nishes the tissues with a supply of glucose for fuel. The amount 

 of glycogen which the liver and muscles can store is limited, 

 however. About 150 grams is the maximum amount which either 

 of these tissues can lay up. Since glycogen is a reserve fuel for 

 the body, it is called upon in case of need and conditions re- 

 quiring the body to call on its reserves will cause a diminution 

 in the glycogen. Liver glycogen appears to be particularly 

 available for immediate use. Hard work, starvation, exposure 

 to cold and various other conditions will greatly reduce the 

 amount of glycogen in the liver, and also in the muscles. The 

 sources from which glycogen may be built up will be discussed 

 at a later point in this chapter. 



The breaking down of liver glycogen has been shown to be 

 influenced by a center in the medulla. Injury to this center, 

 which may be brought about in rabbits by forcing a steel pencil 

 into the brain in front of the occipital prominence in such a 

 way that the floor of the fourth ventricle is pierced, causes, sugar 

 to appear in the urine. The percentage of sugar in the blood 

 rises much above normal. If the animal is killed and the liver 

 examined, it will be found to contain only a trace of glycogen. 

 Evidently impulses from this region of the medulla cause the 

 conversion of liver glycogen into glucose. Overstimulation of 

 this center results in flooding the blood with sugar. The kidneys 

 are so regulated that they keep a constant percentage of sugar 

 in the blood. Any excess over this amount is excreted in the 

 urine. It is evident that puncture of the medulla does not 

 destroy the sugar center, but only irritates it, for the glycosuria is 

 temporary, and passes off after a short time. The center is con- 

 sidered to be a true reflex center, and may be stimulated by 

 afferent impulses. Thus if the vagus is severed and the central 

 end stimulated, sugar appears in the urine. If the splanchnics 

 are cut, there is no glycosuria after puncture. Thus evidently, 

 it is only liver glycogen which is affected by puncture, and the 



