If there is great difficulty in following fats and proteins 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. Provision 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 substance which takes a port-wine colour with iodine. This is glycogen, or animal starch. It has the same empirical formula as starch (C₆H₁₀O₅)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 kingdom 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 unnecessary 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loaded with sugar (into which glycogen is rapidly converted after death), as well as with proteins and fats, which are withdrawn from it when the animal fasts.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 long period of starvation. The muscles lose during activity the glycogen which they contain when at rest.