It was formerly believed that the liver served merely for the separation of the biliary secretion front the blood; but there is now abundant evidence that the blood itself is changed by its means, in such a way as to show that this gland possesses an assimilating as well as a depurating action. Thus, the albuminous matter contained during digestion iu the blood of the veins which pass from the intestine to the portal vein (tile mesenteric veins), is very different from the albuminous matter contained in the hepatic veins; the blood, before reaching the liver, containing a crude albuminous product, while the hepatic veins contain only true blood-albumen. That the liver possesses an assimilating power on albuminous substances is also shown by the experiments of Claude Bernard, who found that, if a solution of egg-albumen be injected into any part of the systemic circulation, albumen speedily appears (like other soluble substances which are foreign to the body) in the urine, and is eliminated as au extraneous matter; but if it be injected into the portal vein, it does not appear in the urine, but becomes a normal constituent of the blood (blood-albumen), through the agency of the liver. It is now also known that if the liver does not secrete a true sugar, as Bernard supposed, it at all events secretes a substance closely allied to, and readily convertible into sugar—viz., glycogen (q.v.)— which must be regarded as a, respiratory or heat-forming food. Further, It appears from. Bernard's researches that fatty matters are elaborated in the liver—the blood of the hepatic veins which leave the liver containing considerably more fat than that of the portal vein which enters it. Some of this fat is doubtless burned off in the lungs; but if a deficient supply should be introduced by the lacteals, some of it would doubtless be applied to the formative processes. Lastly, during the last three days of incubation of
the chick, the liver is made bright-yellow by the absorption of the yelk, which enters the branches of the portal vein, and is then converted partly into which enter the circulation, and partlyinto bile, which is discharged into the intestine. Hence, there is distinct evidence, from several points of view, that the liver is an assimilating organ. The depurating action of this organ is exhibited in the secretion of bile (q.v.), by which the hydro-carbonaceous portion of the effete matters of the blood is removed, just as tlte nitrogenous portion is eliminated by the kidneys. The use of the bile in the digestive process is sufficiently explained in the article DIGESTION.
Our limited space does not allow of our noticing at any length the comparative anat omy of this important gland, which first shows itself in the form of yellowish-brown cells in the polypes, and gradually becomes more concentrated and developed in the echinoderms, annelides, nudobranchiate gasteropods, insects, crustaceans, air-breathing mollusks, cephalopods, fishes, reptiles, birds, and mammals. Till we arrive at the verte brated classes, it consists of tubes or follicles containing cells, which stand to them in the relation of an epithelium, and its structure is easily made out; but when, as in the vertebrata, it is mainly composed of a solid parenchyma, made up of lobules, each of which is composed of aggregations of cells surrounded by the alternate ramifications of the ducts and other vessels, it presents an anatomical complexity which it is almost impossible to unravel.