Glycogen is a substance nearly allied to starch in its chemical composition, and is converted with great readiness into grape sugar by the action of certain ferments and acids. Many of the animal textures contain these ferments, among others the liver itself, at least when its tissue is dying; and consequently the liver with the blood coming from it (if examined in an animal some time dead) does not contain glycogen, but sugar which has been formed from it. If a piece of liver taken from an animal immediately after it is killed be plunged into boiling water, so as to check the action of the ferment, no trace of sugar is found in it, but only glycogen. After the lapse of a little time another piece of the same liver, which has lain at the ordinary room temperature, will give abundance of sugar.

The mode of preparation of glycogen depends upon the foregoing facts. The perfectly fresh liver taken from an animal killed during digestion is rapidly subdivided in boiling water. When the ferment has been destroyed by heat the pieces of liver are rubbed up to a pulp in a mortar, and then reboiled in the same fluid. The liquor is then filtered, and from the filtrate the albuminous substances are precipitated with potassio-mercuric iodide and hydrochloric acid, and removed on a filter. From this filtrate the glycogen may be precipitated by alcohol, caught on a filter, washed with ether to remove fat, and dried.

Glycogen thus prepared has the following properties. It is a white powder, forming an opalescent solution in water, which becomes clear on the addition of caustic alkalies. It is insoluble in alcohol and ether. With a solution of iodine it gives a wine-red color, and not blue, like starch, which it otherwise much resembles in chemical relationship.

Glycogen is widely distributed among many other parts besides the liver, namely, in all the tissues of the embryo, and in the muscles, testicles, inflamed organs, and pus of adults; in short, where any very active tissue change or growth is going on, some traces of glycogen can be found.

Some light is thrown upon its origin by the fact that the amount of glycogen in the liver depends in a great measure on the kind and quantity of food used. It rapidly increases with a full, and decreases with a spare diet, though it never disappears even in prolonged starvation. The formation of glycogen is much more dependent on the carbohydrate food than on the proteid, for it rapidly rises with increase in the quantity of sugar taken, and falls, as in starvation, when pure proteid (fibrin) without any carbohydrate is used either with or without fat. Although the large supply of glycogen normally manufactured in the liver is probably derived from the sugar of the food, we must not conclude from this that the liver cells cannot make glycogen from other materials. Possibly anything that suffices for the nutrition of their own protoplasm enables the cells to produce glycogen. The slowness with which glycogen disappears in starvation would seem to point to this.

The ultimate destiny and physiological application of glycogen have been for some time vexed questions. Whether it is converted into sugar, and as such carried off by the blood of the tissues, or whether it is simply distributed as glycogen, is disputed by different observers, while others say glycogen is a step in the formation of fat out of carbohydrate.

The want of clear evidence on the subject, together with the obvious chemical difficulties, force us to abandon the theory that fat can be an immediate outcome of liver glycogen, though we must admit that carbohydrates, or any form of nutriment, may indirectly aid in the ultimate formation of fat by protoplasm.

The difficulty of determining the exact amount of sugar or glycogen in the blood makes this a very unsatisfactory means of determining the physiological application of liver glycogen. It seems probable that glycogen forms the general carbohydrate nutriment of the textures - the diffusible sugar being transformed in the liver, into indiffusible glycogen, in order that it may be distributed throughout the various tissues without being lost in the excretions.