CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 751 



usually fed, contains in itself ( 62) a certain amount either of 

 glycogen or some form of sugar. Moreover when animals are fed 

 not on meat but on purified proteid, such as fibrin, casein or 

 albumin, the quantity of glycogen in the liver becomes still 

 smaller, though according to most observers remaining greater 

 than during starvation. We may infer therefore that part of the 

 glycogen which appears in the liver after a meat diet is really 

 due to carbohydrate materials present in the meat. Part, however, 

 would appear to be the result of the actual proteid food ; and we 

 have similar evidence that gelatin taken as food leads to the 

 formation of some glycogen in the liver. But in this respect 

 these nitrogenous substances fall far short of carbohydrate 

 material. 



With regard to fats, all observers are agreed that these lead to 

 no accumulation of glycogen in the liver ; an animal fed on an 

 exclusively fatty diet has no more glycogen in its liver than a 

 starving animal. 



Hence of the three great classes of food-stuffs, the carbohydrates 

 stand out prominently as the substances which taken as food lead 

 to an accumulation of glycogen in the liver. We may remark 

 that the greatest accumulation of glycogen is effected not by a 

 pure carbohydrate diet, but by a mixed diet rich in carbohydrates. 

 A quantity of carbohydrate mixed with a certain proportion of 

 proteid gives rise to a larger amount of glycogen in the liver than 

 the same quantity of carbohydrate given by itself; and it is possible 

 that the presence of an appropriate quantity of fat still further 

 assists the accumulation. But this result probably depends, in 

 part at least, on the fact that, though differences may be met with 

 in different animals, a mixture of the several classes of food-stuffs 

 is more readily digested, resulting in more nutritive material being 

 thrown upon the blood, than is a meal consisting exclusively of 

 one kind of food-stuff alone. 



So far as we know at present the glycogen which thus appears 

 in the liver as the result of feeding either with any of the various 

 forms of carbohydrates, or with proteids, or with other substances, 

 is of the same kind and presents the same characters ; at least we 

 have no evidence to the contrary. 



The storing-up of glycogen in the liver is also influenced by 

 other circumstances than the taking of food. For instance in the 

 frog an increase of glycogen takes place during the winter months. 

 In the summer months the liver of a frog will be found to con- 

 tain very little glycogen, Fig. 92 B, unless the animal has been 

 unusually well fed ; whereas a liver examined in mid winter, 

 Fig. 92 A, will be found to contain a considerable quantity, even 

 though no food has been taken for months. In such a case the 

 material for the formation of the glycogen in the liver must have 

 been furnished by some part of the body of the frog, and could 

 not, as may be the case when a meal leads immediately to an 



