METABOLISM, NUTRITION AND DIETETICS 513 



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 glyco- 

 gen 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, 

 probably dextrose, is produced in the body from proteins 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. That carbo-hydrates may be produced 

 from proteins has been shown by feeding dogs with almost pure 

 protein (casein) after the production of permanent glycosuria by 

 removal of the pancreas (p. 518). To induce the animal to take 

 the casein it had to be mixed with a certain amount of butter, or 

 serum, or meat extract. The amount of sugar excreted was much 

 more than could possibly have come from the glycogen originally 

 present in the animal's body, computing it on the most generous 

 scale (41 grammes per kilogramme of body- weight, according to 

 Pniiger), or from free carbo-hydrate present in traces in the food, 

 or as prosthetic groups (p. 2) in the ingested protein. That the 

 source of the sugar was protein and not fat was indicated 

 by the fact that when the amount of protein food was 

 increased, the dextrose and the nitrogen excreted increased 

 proportionally. 



Glycogen-formers. As true glycogen-formers in the higher 

 animals, only a few substances have been demonstrated, such as 

 proteins (including gelatin), the fermentable sugars, and glycerin. 

 The most interesting demonstration of the transformation of 

 glycerin into glycogen, because the most direct, has been afforded 

 by perfusing the liver of the tortoise with blood to which glycerin 

 was added. The glycogen content of the liver was very distinctly 

 increased. The monosaccharides dextrose, levulose, and galac- 

 tose gave a similar result, while the disaccharides cane-sugar and 

 lactose caused no increase in the glycogen of the perfused liver, 

 since the liver contains no ferment capable of splitting them 

 into monosaccharides. It is said that even such small mole- 

 cules as those of formaldehyde (CH 2 O) can be condensed to glyco- 



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