EXTERNAL SECRETIONS 491 



this amorphous powder is treated with iodin, it strikes a port-wine color. 

 When digested with saliva, pancreatic juice, or boiled with dilute acids, the 

 solution becomes clear, and testing with Fehling's solution reveals the pres- 

 ence of sugar. 



For the reason that this starch is capable of being transformed into or of 

 generating glycose or glucose it received the name of glycogen; and inasmuch 

 as the liver continually produces glycogen it may be said to have a starch- 

 forming or an amylo genetic or a glycogenic function. 



If the liver be allowed to remain in the body of an animal for a period 

 of twenty-four hours before the decoction is made as above described, it will 

 be found that the solution contains only a small amount of starch but a 

 relatively large amount of sugar. The inference drawn is that after death 

 the starch is transformed by some agent, possibly a ferment, into sugar 

 (glucose). From this fact as well as from the results of different lines of 

 investigation, it is the generally received opinion that the same change is 

 constantly taking place in the living condition and therefore the liver is said 

 to have a sugar-forming or a glycogenetic function. The liver cells are thus 

 characterized by two processes amylo genesis smdglyco genesis. To the trans- 

 formation of glycogen into sugar the term glycogenolysis has recently been 

 applied. 



The presence of glycogen in the liver cells can be shown microscopically 

 in the form of discrete hyaline and refractive masses, which show a blue or 

 violet color with iodin. As they are soluble in water they can be readily 

 dissolved out from the cells, leaving small vacuoles separated from one 

 another by strands of cell substance. The amount of glycogen in a well- 

 fed animal varies from 1.5 to 4 per cent, of the total weight of the liver. By 

 experimental methods it has been shown that the production of glycogen is 

 dependent very largely on the consumption of carbohydrates, the greater 

 the amount of sugar and starch in the food, the greater being the production 

 of glycogen. Nevertheless it is also certain that glycogen can be derived 

 from proteins; for if the carbohydrates are excluded from the food and the 

 animal fed on a pure protein diet plus fat, glycogen will continue to be 

 formed in the liver though in far less amounts. 



The facts connected with the formation of glycogen, as well as with its 

 transformation as at present generally accepted, may be stated as follows: 

 The dextrose or glucose into which the carbohydrates are mainly converted by 

 the action of the digestive fluids is absorbed into the blood of the portal vein 

 and carried direct to the liver, where a certain portion of it diffuses across 

 the capillary walls into the surrounding lymph-spaces; by the action of the 

 cells or by a special enzyme it is then dehydrated, and temporarily deposited 

 under the form of the non-diffusible body glycogen. At a subsequent period 

 and in proportion to the needs of the system the liver cells, through the agency 

 of a ferment, transform the glycogen into glucose or dextrose by the addition 

 -of a molecule of water, after which it is returned to the blood, by which it 

 is transported to the systemic capillaries, where it disappears again, diffusing 

 across the walls of the capillaries into the surrounding lymph-spaces to play 

 a part in the general nutritive process. Though the final disposition of the 

 sugar is uncertain it is highly probable that after its delivery to the muscles, 

 for example, it may be directly oxidized or temporarily stored as glycogen 

 or possibly be used in the formation of living material. Ultimately, how- 



