constituents of the bile, which has already been mentioned as necessary for the purification of the blood, and useful in aiding absorption, is probably but a secondary function of this great gland. The production of a special material-animal starch

Diagram of the Portal Vein (v) arising in the alimentary tract and spleen (s), and carrying the blood supply of blood which from these organs to the liver. flows through the intestinal tract and the spleen.

The blood in the vena porta during digestion can hardly be called venous blood, for much more passes through the intestinal capillaries when digestion is going on than is necessary for the nutrition of the tissue of the intestinal wall. The portal blood is also to be distinguished from ordinary venous blood from the fact that it has just been enriched with a quantity of the soluble materials taken from the intestinal canal, namely, pro

teids, sugar, salts, and possibly some fats; and it has been further modified by the changes taking place in the spleen.

It is from this blood that the liver cells manufacture the starchlike substance above mentioned. Animal starch was discovered by Claude Bernard, and called by him Glycogen, on account of the great facility with which it is converted into sugar in the presence of certain ferments which exist in the liver itself and in most tissues after death. Shortly after the death of an animal the tissue of the liver, and also the blood contained in the hepatic veins, are extremely rich in sugar, which has been formed by the fermentation of the hepatic glycogen. The quantity of sugar increases with the length of time that has elapsed since the death of the animal, and is minimal, if not nil, if the liver or hepatic blood be taken for examination while the tissue elements are still alive.

The peculiar blood of the great portal vein coming from the stomach, intestines, and the spleen has then to pass through a second set of capillaries in the liver, and undergoes such important changes that this organ must be regarded as occupying a foremost position among the blood glands. Differences of the utmost importance have long been thought to exist between the blood going to and that coming from the liver, and to it has even been attributed paramount utility as a blood elaborator; but the scientific knowledge of its power in this respect must date from the discovery of its glycogenic function.

GLYCOGEN.

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 winered 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.

CHAPTER XXI.

SECRETIONS.

The secretions which are poured into the alimentary tract have been already described in the chapter on digestion. There are other glands which can now be conveniently considered, since they more or less alter the blood flowing through them, and thus may be said to aid slightly in the perfect elaboration of that fluid. They are, however, subservient to very different functions, some having merely local offices to perform, and others having duties allotted to them of the greatest general importance to the economy. This becomes obvious from a glance at the following enumeration of the remaining glandular organs. Secreting glands (other than those forming special digestive juices) :

Most vertebrate animals that live in air have a gland in connection with the surface of their eyes, which secretes a thin fluid, to moisten the conjunctiva. This fluid commonly passes from the eye into the nasal cavity, and supplies the inspired air with moisture.

The lachrymal fluid is clear and colorless, with a distinctly salty taste and alkaline reaction. It contains only about 1 per cent. of solids, in which can be detected some albumin, mucus, and fat (1 per cent.), epithelium (1 per cent.), as well as sodium chloride and other salts (.8 per cent.).

The secretion is produced continuously in small amount, but is subject to such considerable and sudden increase, that at times