DIGESTION 147 



after their absorption, it is quite otherwise when we come 

 to deal with sugar. Carbohydrates are the great sources of 

 energy. Muscular work may be generated by the oxidation 

 of either of the three classes of foods, but undoubtedly the 

 carbohydrate glycogen is its most constant source. Pro- 

 vision is therefore made for the storing of glycogen in the 

 liver, and the distribution to the muscles of a regular supply. 

 After a meal the portal blood, on its way from the intestines 

 to the liver, contains a higher percentage of sugar than the 

 blood in the hepatic vein or in any other vessel. If sections of 

 liver be examined after feeding, and compared with those 

 obtained after a period of starvation, it is found that the 

 cells of the well-fed liver contain glancing masses of a sub- 

 stance which takes a port- wine colour with iodine. This is 

 glycogen, or animal starch. It has the same empirical formula 

 as starch (C 6 H 10 5 ) n . In the dry state it is a greyish powder, 

 which, unlike starch, forms an opalescent solution in cold 

 water. Like starch, it is non-diffusible. In the animal king- 

 dom it stands to sugar in the same relation as starch to sugar 

 in plants. If a sheep be killed while it is feeding in the 

 paddock, and its liver removed and weighed, it will be found 

 that it is from one-third to one-half heavier than the liver of 

 a sheep of the same weight obtained from a butcher ; for 

 butchers have the stupid practice of starving animals before 

 they kill them. It was long ago discovered that it is unneces- 

 sary to feed an animal for a day or two before it is killed, 

 and this option has been elevated into a prohibition. A 

 tradition has grown up that it is undesirable to give food for 

 some time before killing. Not only will the liver of a sheep 

 killed during active digestion be found to be heavier than 

 that of a starved sheep, but it will also prove more succulent ; 

 for it is loaded with sugar (into which glycogen is rapidly con- 

 verted after death), as well as with proteins and fats, which 

 are withdrawn from it when the animal fasts. It appears 

 that the liver cannot secure the whole of the sugar which is 

 absorbed after a full meal. Some of it passes into the general 

 circulation, and is stored in the muscles ; but the liver 

 always maintains a considerable reserve. Even after prolonged 

 deprivation of food, it holds on to a certain quantity, especially 

 in carnivora. Glycogen is found in the liver of a dog after a 



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