536 METABOLISM, NUTRITION AND DIETETICS 



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 dis- 

 appears 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 formed from proteins 

 (or their constituent amino-acids) has been shown in various ways 

 for example, by feeding dogs with almost pure protein (casein) after 

 the production of permanent glycosuria by removal of the pancreas 

 (p. 636). 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 kilo- 

 gramme of body-weight, according to Pfliiger), 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 (see also p. 538)- 



Glycogen-Formers. As true glycogen-formeis in the higher 

 animals that is, compounds whose elements (particularly the 

 carbon) actually enter into the composition of the glycogen mole- 

 cule may be mentioned such substances as proteins (including 

 gelatin), the fermentable sugars, and glycerin. In the case of 

 proteins it is, of course, not the entire molecule which is transformed 

 bodily into glycogen, but amino-acids yielded by them, or dextrose 

 derived from the amino-acids. The liver is of itself capable of 

 dealing only with the dextrose, and not with the amino-acids. At 

 least, when the isolated liver (of the tortoise) is perfused with 

 blood containing amino-acids no increase in the glycogen of the 

 liver occurs. When glycerin is added to the blood the glycogen 

 content of the liver is very distinctly increased. Glycerin is a tri- 

 valent alcohol (C 3 H 8 O 3 ) whose aldehyde, obtained from it by gentle 



