NUTRITION. 467 



glycogen is to be obtained from the organ. Pure glycogen 

 is a white amorphous inodorous powder, readily soluble in 

 water, forming an .opalescent milky solution; insoluble in 

 alcohol, and giving with iodine a red coloration which dis- 

 appears on heating and reappears on cooling again. 



About four per cent of glycogen can be obtained from 

 the liver of a well- nourished animal (dog or rabbit). This 

 for the human liver, which weighs about 1500 grams (53 

 oz.), would give about 60 grams (2.1 oz.) of glycogen at any 

 one moment. The quantity actually formed daily is, how- 

 ever, much in excess of that, since glycogen is constantly 

 being removed from the liver and carried elsewhere, while a 

 fresh supply is formed in the organ. Its quantity is subject, 

 also, to considerable fluctuations; being greatest about two 

 hours after a good meal, and falling from that time until the 

 next digestion period commences, when it begins to rise 

 until it again attains its maximum. If a warm-blooded 

 animal be starved glycogen disappears from its liver in the 

 course of four or five days. We are, thus, led to believe that 

 glycogen is being constantly used up, and that its mainte- 

 nance in normal quantity depends on food supply. 



The accumulation and disappearance of glycogen can be 

 demonstrated histologically. The liver is essentially a com- 

 pound tubular gland, but its structure is obscured by the fact 

 that the hepatic cells are very large in proportion to the 

 tubules wnich they surround, so that these are reduced to 

 in^re ductules, formed by the apposition of grooves on the 

 adjacent sides of two cells; and by the fact that cells and 

 ductules form an irregular network interlaced with the capil- 

 laries of the lobule (Chap. XXII), which capillaries are far 

 larger than the interlobular bile-ducts. When properly pre- 

 pared hepatic cells, taken from a healthy well-fed animal, 

 are examined, the side of the cell nearest the bile-ductule is 

 seen to be granular, and it also picks up readily most of the 

 ordinary protoplasmic stains. * The rest of the cell contains 

 few granules and does not stain with carmine, but it does 

 stain red with iodine. It is in fact mainly filled with glyco- 

 gen, and if this be dissolved out by digestion with saliva there 

 is left a loose protoplasmic network. If sections from the 

 liver of a starved animal be compared with those from a 

 well-fed, the liver-cells are seen to be considerably smaller, to 

 be granular throughout, and to stain everywhere with carmine 



