60 PHYSIOLOGICAL CHEMISTRY. 



Sugar and starch, accordingly, in all their varieties, are closely allied, 

 both in their chemical and physiological relations. They are all carbo- 

 hydrates, and their mutual convertibility in the vegetative process has 

 been shown by abundant investigations. Starch and sugar, in the 

 living plant, represent the same nutritive material under two different 

 conditions ; starch having the form of a solid deposit, glucose that of 

 solution and activity. The organic substance passes from one to the 

 other of these two conditions by hydration or dehydration. It is at 

 last either decomposed in the immediate changes of nutrition, or is 

 stored up as a deposit for future consumption. 



Glycogen, C 6 H ]0 O 5 . 



Grlycogen, so called from its capacity for the production of glucose, is 

 an amylaceous substance of animal origin. It is isomeric with starch 

 and dextrine, and resembles the latter in its physical properties, except 

 that a watery solution of dextrine is clear, while that of glycogen is 

 opalescent, and that when treated with iodine, dextrine in solution 

 gives a rosy red, glycogen a deep brown-red color. It is insoluble in 

 alcohol and in ether, but soluble in water, either cold or hot. Its watery 

 solution deviates the plane of polarization strongly to the right, its 

 specific power of rotation for yellow light being about 130. 



This substance is constantly present in the tissue of the liver in all 

 vertebrate animals, in the healthy condition. It is found at an early 

 period of development in the integument and mucous membranes of 

 the embryo, in a portion of the placenta and amnion, in the muscles 

 during their formative condition, and in the pulmonary tissue. It does 

 not exist at this time in the liver, or in any other of the glandular 

 organs. But about the middle of foetal life it begins to be found in 

 the liver, where it increases in quantity, at the same time gradually 

 disappearing from the other organs ; and after birth it is a character- 

 istic and abundant ingredient of the liver alone. It has been found, 

 however, in moderate and varying amount in the muscles of some 

 adult quadrupeds and birds, and in considerable quantity in molluscous 

 animals, as the oyster and the cockle-shell. 



Glycogen is obtained from the liver of a well-fed animal in the fol- 

 lowing manner : The organ is taken out immediately after death and 

 cut into small pieces, which are then coagulated by a short immersion 

 in boiling water. This arrests the changes which would otherwise take 

 place under the influence of a ferment contained in the hepatic juices. 

 The coagulated tissue is then ground to a pulp and boiled for half an 

 hour with a small quantity of water, making a concentrated decoction, 

 which is afterward treated with animal charcoal, to remove the color- 

 ing matters, and filtered. The filtered decoction is opaline, but does 

 not hold in suspension any solid granular matters visible with the 

 microscope. It is allowed to fall by drops into strong alcohol, by which 

 the dissolved glycogen is precipitated, subsiding to the bottom as a white 

 deposit. It is still contaminated by a little glucose, a certain quantity 



