Of all the organs that modify the composition of the blood flowing through them, the liver plays the most important part in elaborating the circulating fluid. The elimination of the various 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 essential to the nutrition and growth of the textures is probably the most important duty of the liver cells, and possibly the constituents of the bile are but the by-products, which must be got rid of, resulting from this and other unknown chemical processes.
In the chapter on the digestive secretions the structure of the liver was mentioned, and attention was directed to the peculiarities of its double blood supply. A relatively small arterial twig carries blood to it from the aorta, while the great portal veins distribute to it all that large supply of blood which 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, proteids, sugar, salts, and possibly some fats; and it has been further modified by the changes taking place in the spleen.
Fig. 164. Diagram of the Portal Vein (p v) arising in the alimentary tract and spleen (s), and carrying the blood from these organs to the liver.
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.