Starch is quite indigestible in the uncooked state; when cooked it is insoluble, and incapable of passing through the membranes of the blood vessels of the stomach and intestine. To be rendered soluble and capable of assimilation it must be converted into glucose or grape sugar. This conversion of starch into glucose takes place partly in the mouth, by the action of the diastase of the ptyaline of the saliva, but much more rapidly in the duodenum by the action of the pancreatine of the pancreas. Cane sugar is dissolved by the fluids of the stomach, or is swallowed in a soluble condition. The soluble glucose and sugar are absorbed by the veinules of the stomach and intestine, and carried at once by the portal vein to the liver. In the liver the sugar is lost sight of; though after the ingestion of food the portal vein may contain an abundance of saccharine matter in solution, there is not a trace of sugar in the blood of the hepatic vein of a healthy person.

What, then, becomes of the sugar in the liver? - This is a question which has been partially solved for us by the researches of Claude Bernard. He discovered the presence of an amyloid or starchy substance, which he called glycogen, stored in the liver cells; and the result of his researches tends to show that the soluble glucose which is brought to the liver by the portal vein is converted by means of a strange and not well-understood action of the liver cells into this insoluble substance or glycogen, which is then stored up in the cells. It is there available, as Claude Bernard insisted, for conversion again into sugar to be carried off by the capillaries and burnt up in the tissues in the course of tissue change and the production of energy. Claude Bernard contended that the liver was a magazine for the storage of sugar in the form of glycogen, and the regulation of its supply to the economy. Without the interposition of the liver, sugar would obviously be introduced into the circulation in irregular quantities at the moment of digestion, which would have a very disturbing influence on the system. It has since, however, been strenuously denied by Pavy that glycogen is ever reconverted into sugar, or that sugar is burnt up in the tissues. Recent researches tend to show that the formation of glycogen in the liver is the first step in the metamorphosis of starch and sugar into fat, and that it is fat and not sugar which is the hydro-carbon burnt up in the tissues.