248 A MANUAL OF VETERINARY PHYSIOLOGY 



less sugar in the veins of an active muscle than in the arterial 

 blood supplying it. The muscles of the upper lip of the horse 

 used up in a state of activity 3-5 times more dextrose than during 

 rest, and the actively secreting parotid gland of the same animal 

 used more sugar than the resting gland. 



The sources of glycogen have been a fertile cause of discussion 

 and object of experimental inquiry. It was natural to consider, 

 as we have so far done, carbohydrate material as the chief con- 

 tributing agent ; it was less certain whether proteins contributed, 

 while the consensus of opinion was against fat taking any share 

 in the process. We must examine each of these in a little more 

 detail. 



We have learnt that starch is not absorbed as starch, but, 

 depending upon the nature of the diastatic ferment, is converted 

 into maltose, or maltose and some dextrin, and subsequently 

 dextrose. These sugars are readily converted into glycogen by 

 the liver cells by the process of dehydration. Cane sugar and 

 milk sugar are not readily converted into glycogen, but since 

 these double sugars undergo inversion in the intestinal canal 

 before absorption — cane sugar into dextrose and levulose, and 

 milk sugar into dextrose and galactose — they may in this form 

 be readily converted into glycogen. All carbohydrates, then 

 (with the exception of lactose), which are capable of being 

 changed into dextrose or levulose, may be converted into glycogen, 

 provided they pass through the laboratory of the intestinal canal. 

 For example, cane sugar, if injected subcutaneously, passes un- 

 changed into the urine ; in order to be converted into glycogen 

 it must pass through the intestine. 



The effect of protein on glycogen formation is not so easily 

 settled. It is observed that in diabetes, though all carbohydrate 

 food be withheld, yet sugar may appear in the urine on an 

 exclusively protein diet ; the same thing is observed in the 

 experimental glycosuria which may be produced by the adminis- 

 tration of a substance known as ' phloridzin,'* and, furthermore, 

 that sugar may be produced even when the animal is starved. 

 The conclusion appears irresistible that protein can produce 

 sugar, and this is explained by saying that certain proteins split 

 into a nitrogenous and non-nitrogenous portion, the former being 

 converted into urea, while the non-nitrogenous residue is con- 

 verted into sugar, and may thus give rise to glycogen. It is now 

 known, however, that proteins which do not contain a carbo- 

 hydrate group, such as casein, may take a share in the production 

 of glycogen ; this strengthens the belief that protein may give rise 

 to sugar. But apart from the carbohydrate portion of the protein 

 molecule as a source of glycogen, it is now known that sugar can 

 * Obtained from the roots of the apple-tree. 



