DIGESTION. 693 



can be derived from glycerin ; hence it can be derived from the fats 

 which contain glycerin, but whether it is normally derived from the 

 fats is not known. When the tissues are in need of sugar to supply 

 them with a source of energy, the glycogen of the liver is split and 

 is distributed by the blood-stream in the form of dextrose. This is 

 seen especially clearly in the case of the muscles, which require a 

 large amount of sugar, and have also the power of storing up a local 

 supply very much as the liver stores up a general supply for he 

 whole body. It is found that in starvation, and particularly in 

 starvation with extensive muscular work, that the store of glycogen 

 in both the liver and the muscles is rapidly exhausted. 



Indole and skatole, formed by putrefaction in the intestine, are 



brought to the liver by the portal vein. Indole, C 6 H / NH ">CH, 



is oxidized, forming indoxyl, C 8 H 7 NO, which combines with potas- 

 sium acid sulphate, with elimination of water, forming indoxyl 

 potassium sulphate, C 8 H 6 NKSO 4 , which is excreted in the urine. 

 Skatole, methyl-indole, C 6 H 4 (CCH 3 CH)NH, is similarly converted 

 into the oxidation product skatoxyl, C 9 H 9 NO, and skatoxyl potassium 

 sulphate, C 9 H 8 NKSO 4 . These substances appear in the urine as the 

 conjugate or ethereal sulphates. 



The formation of glycogen from sugar has been mentioned, and its physical 

 properties were considered in Chapter 48. 



Experiment 87. (Preparation of glycogen.} Digest 50 grammes of fresh liver 

 with 500 c.c. of boiling water containing about 5 c.c. of acetic acid. Strain 

 the liquid through muslin. The solution contains besides glycogen some pro- 

 tein, which remove by concentrating the liquid to a small volume and adding 

 alternately a few drops of hydrochloric acid and of potassium mercuric iodide 

 as long as a precipitate is formed. Filter and mix filtrate with 2 volumes of 

 alcohol, when glycogen is precipitated ; purify it by pouring off the super- 

 natant liquid and washing it with 65 per cent, alcohol by decantation. Then 

 cover with absolute alcohol, let stand for an hour, collect the glycogen on a 

 filter, and dry between filter-paper. 



Tests for glycogen. 



1. Dissolve some glycogen in warm water : an opalescent solution 

 resembling soluble starch solution is formed. 



2. To a portion of solution add iodine solution : a reddish-brown 

 color resembling the one produced by erythrodextrin is produced. 



3. Heat some of the solution with Fehling's solution : no change 

 occurs. 



4. Acidify solution with hydrochloric acid, boil a few minutes, 



