vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 159 



forming at times over 8 per cent of the weight of that organ, 

 it is found also in many other organs of the body. The 

 muscles contain a considerably less per cent of glycogen than 

 the liver ; but owing to their much greater bulk their total 

 glycogen content may exceed that of the liver, although it is 

 usually less. Smaller quantities of glycogen are found in 

 the ovaries, central nervous system, and skeleton. 



Periodic Changes in the Liver. — The liver of the frog 

 undergoes important changes in relation to food and tem- 

 perature. There is a regular seasonal change which affects 

 not only the size and general appearance of the organ, but 

 also the amount of pigment contained in it, and the contents 

 of the secreting cells. In the summer the liver is usually 

 large, comparatively light in color, and furnished with little 

 pigment (Weber, Eberth, Leonard). In the winter and 

 early spring, before the feeding period, the liver becomes 

 relatively small in size and dark in color, the number of 

 pigment cells increases, and there are more pigment granules 

 contained in the secreting cells. Miss Leonard, who has 

 made a study of the percentage of pigment in relation to the 

 whole mass of the organ in different times of year, arrives at 

 the following result : — 



November, .7 per cent June, 2.77 per cent 



December, 4.13 per cent July, .68 per cent 

 April, 1 1.1 2 per cent 



It may thus be seen that the relative amount of pigment 

 contained in the liver increases through the winter, then 

 diminishes in the spring after the period of feeding. 



The same observer found that in winter and early spring 

 the average size of the secreting cells and also their nuclei 

 was smallest in early spring, and increased during the 

 summer as is shown in the following table : — 



