5 H A MANUAL OF PHYSIOLOGY 



gen in the liver (Grube).* When given by the mouth both cane- 

 sugar and lactose form glycogen, being hydrolysed in digestion. 

 It has not hitherto been proved that the fatty acid component of 

 neutral fats can be converted into glycogen. Many other bodies 

 are known to influence the formation of glycogen by ' sparing ' 

 substances which are true glycogen producers, but their carbon 

 does not actually take its place in the glycogen molecule. Some 

 writers deny that proteins can directly form glycogen or sugar 

 apart from carbo-hydrate groups contained in the protein mole- 

 cule. But the proteins of meat, gelatin, and casein are capable 

 of forming 60 per cent, of their own weight of dextrose in diabetic 

 metabolism, and even the end products of pancreatic digestion of 

 meat yield so much sugar that the greater part of it must have 

 come from the amino-bodies, and not from a sugar -group in the 

 protein. When given to dogs with total phloridzin glycosuria 

 (p. 521), glycin and alanin are completely, glutamic and aspartic 

 acids in great part, converted into dextrose (Lusk, etc.). 



Extra-hepatic Glycogen. While the liver in the adult (con- 

 taining as it does from 2 to 10 per cent, of glycogen, or even, with 

 a diet rich in sugar or starch, more than 

 1 8 per cent.) may be looked upon as the 

 main storehouse of surplus carbo-hydrate, 

 depots of glycogen are formed, both in 

 adult and fcetal life, in other situations 

 where the strain of function or of growth 

 is exceptionally heavy in the muscles of 

 the adult (o'3 to o'5 per cent, of the moist 

 skeletal muscle, or on a carbo-hydrate regi- 

 FIG. 183. CELLS OF m en o p 7 to s'7 per cent.), in the placenta, 



PLACENTA CONTAINING 11- xi_ 



GLYCOGEN. m ma ny developing organs in the embryo 



(muscles, lungs, epithelium of the trachea, 



cesophagus, intestine, ureter, pelvis of kidney, and renal 

 tubules). The foetus, however, is not, compared with the adult, 

 especially rich in glycogen. In the adult under favourable 

 circumstances the absolute amount of glycogen in the muscles may 

 be several times greater than that in the liver, and usually the 

 hepatic glycogen makes up considerably less than half the total 

 glycogen of the body. That the muscles do not derive their 

 glycogen by the migration of hepatic glycogen, but can them- 

 selves form it from dextrose, has been shown by injecting that 

 sugar subcutaneously into frogs after excision of the liver. The 

 muscle glycogen was found to be increased. 



The glycogen store of the liver fulfils a different function from 

 that of the muscles. This is indicated by the fact that when dogs, 



* Such results, however, need confirmation in view of Pfliiger's recent 

 analysis of possible errors in work with the tortoise liver. 



