vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 161 



vessels lo the whole mass of the liver varies in different sea- 

 sons. The following table represents the percentage of 

 area of cross sections of blood vessels in relation to the 

 whole areas of the sections studied, during different seasons 

 of the year : 



November, 17.23 per cent June, 9.82 per cent 

 December, 10.105 per cent July, 6.58 per cent 

 April, 7.47 per cent 



Comparing this with the previous tables, it will be seen that 

 as the size of the cells of the liver increases, the relative pro- 

 portion of blood vessels and pigment to the whole mass of 

 the liver decreases. 



Variations in glycogen contents of the liver at different 

 times of year have been studied by several investigators 

 (Langley, Luchsinger, Von Wittich, Barfurth, Langendorffand 

 Mozeik, Athanasiu). In the spring during the breeding 

 season the amount of glycogen is at its minimum, there being 

 often scarcely a trace of this substance in the liver cells. 

 After the frog begins to take food glycogen slowly accumu- 

 lates, but during the active life of the animal in summer it 

 is not stored in the liver in any great quantity. In the fall, 

 when the weather becomes cooler and the frogs less active, the 

 glycogen becomes much increased in amount. During the win- 

 ter sleep glycogen is used up only to a slight extent, but as the 

 temperature rises on the approach of spring, and the sexual 

 products are maturing, the store of glycogen is rapidly dimin- 

 ished. Athanasiu, who has investigated the amount of glyco- 

 gen in the whole body of the frog (R. esculenta) at different sea- 

 sons, finds that the minimum quantity (slightly over one tenth 

 per cent of the body weight) occurs in June, then there is a slow 

 accumulation until September, when there is a rapid increase 

 to the maximum (1.43 per cent of the body weight) followed 

 M 



