the manufacture of the bile pigments, bilirubin and biliverdin, which are made from the hemoglobin of the blood, seems to require some special action on the part of the liver cells.
The glycogen, which is manufactured and stored in the liver cells, is a clear hyaline substance, akin to starch and capable of being converted into sugar by the starch ferment. Probably there is some such ferment in the blood which converts the glycogen into sugar as soon as it passes from the liver into the blood, though what it is, is not known. Neither is it known just how glycogen is formed, but it is manufactured chiefly after a mixed meal in which carbohydrates predominate, proteins having little and fats no effect upon its formation. It is undoubtedly formed from the sugar in the portal blood and the process requires some work on the part of the liver cell itself. Probably there is always some sugar in the circulating blood which, as it is used up, must be made good. If there it not enough in the diet, the liver supplies the deficiency from its store of glycogen.
Glycogen is found also in the muscles, in the placenta as food for the fetus, in leucocytes, and to a slight extent in cartilage. In fact, it is the form in which carbohydrate material is supplied to the tissues as needed. Normally, much of the sugar is used up by the blood and its cells in metabolism, giving rise to heat and energy. In muscles glycogen is probably digested as lactic acid, as before action muscle is neutral or slightly alkaline and after action acid.
When the liver is deranged and allows the glycogen to pass out into the blood too freely, or when the glycogen is not held as such but turned to sugar and passed out in large quantities, sugar in the urine or diabetes mellitus results.
Besides its secreting function the liver has an eliminative function and plays an important part in purifying the blood, removing from it many poisonous and narcotic substances. It is thought by some, though it has not
