443 A MANUAL OF PHYSIOLOGY 



in the shrunken liver-cells ; if the diet has been wholly fatty, 

 no glycogen at all may be found. 



In the liver-cells of the frog in winter-time, a great deal of this 

 hyaline material this glycogen, or perhaps loose glycogen com- 

 pound is present ; in summer, much less. The difference is very 

 remarkable if we consider that in winter frogs have no food for 

 months, while summer is their feeding-time ; and at first seems 

 inconsistent with the doctrine that the hepatic glycogen is a store 

 laid up from surplus sugar, which might otherwise be swept into the 

 general circulation and excreted by the kidneys. It has been found, 

 however, that the quantity of glycogen is greatest in autumn at the 

 beginning of the winter-sleep, and slowly diminishes as the winter 

 passes on, to fall abruptly with the renewal of the activity of the 

 animal in the spring. The glycogen present at any moment is, 

 therefore, believed to be a residue, which represents the excess of 

 glycogen formed over glycogen used up ; and the amount is larger in 

 winter, not because more is manufactured than in summer, but because 

 less is consumed. It is possible, indeed, to produce the ' summer ' 

 condition of the hepatic cells merely by raising the temperature of 

 the air in which a winter frog lives ; at 20 or 25 C., glycogen 

 disappears from its liver. Conversely, if a summer frog is artificially 

 cooled, a certain amount of glycogen accumulates in the liver. The 

 meaning of this seems to be that at a low temperature, when the 

 wheels of life are clogged and metabolism is slow, some substance, 

 possibly dextrose, is produced in the body in greater amount than 

 can be used up, and that the surplus is stored as glycogen ; just as 

 in plants starch is put by as a reserve which can be drawn upon 

 which can be converted into sugar when the need arises. 



When a fasting dog is made to do severe muscular work, the 

 greater part of the glycogen soon disappears from its liver. When a 

 dog is starved, but allowed to remain at rest, the glycogen still 

 markedly diminishes, although it takes a longer time ; and at a 

 period when there is still plenty of fat in the body, there may be 

 only a trace of hepatic glycogen left. The 

 glycogen which is usually contained in the 

 muscles also diminishes very rapidly in the 

 first days of hunger. These facts have been 

 taken to indicate that glycogen and the sugar 

 formed from it are the readiest resources of 

 the starving and working organism. The fat 

 of the body is a good security, which, how- 

 ever, can only be gradually realized; its 

 organ-proteids are long-date bills, which will 



^PLACI^ CONTAIN" be discounted sparingly and almost with a 



ING GLYCOGEN. grudge ; its glycogen, its carbo-hydrate 



reserves, are consols, which can be turned 



into money at an hour's warning. Glycogen is drawn upon for a 

 sudden demand, fat for a steady drain, proteid for a life-and-death 

 struggle. 



