THE FUNCTIONS OF THE LIVER.
The structure of the liver has been described on pages 200 and 201, and its function in diges tion has been referred to on p. 205. But the part the liver plays in the digestive process is only one of its duties, and perhaps not the most important. The consideration of its other busi ness is not properly included in a description of digestion. It is advisable, nevertheless, to have a complete view of all the functions of the liver in order to estimate the great importance of that organ in the bodily economy.
The Characters of Bile. — The functions of the liver are indeed not single, but several. As we have seen, it secretes the bile, and therefore ranks (1) as a secretory organ, an organ, that is, which elaborates a fluid for use in the body. But the bile is not wholly a digestive fluid. It aids in the digestive pro cess, but it also contains ingredients which are separated from the blood, for the purpose of being cast out of the body, because their re maining in the blood would impair its quality. In respect of this the liver ranks (2) as an ex cretory organ, an organ, that is, which separates material of no use to the body, and which is des tined to be expelled as waste matter. This will be more easily understood by noting, in detail, the constituents of bile. It contains roughly 86 per cent water, and 14 per cent solid matter. The solid matter consists (a) of the bile salts, the glycocholate and taurocholate of soda, (b) of colouring matters, (c) of fats, (d) of inorganic salts, chiefly chloride of sodium (common salt), with a smaller quantity of phosphates, and traces of iron and manganese, and (e) of a crystalline substance called cholesterin, a sub stance found in the brain,and seemingly brought to the liver by the blood. There is also in bile a considerable quantity of mucus, obtained from the bile-ducts and gall-bladder. These sub stances are in the following proportions:— Of these the chief are the bile salts, and the colouring matters—the bile pigments. They do not exist already formed in the blood, as do the salts and the cholesterin, and must be formed from materials in the blood by the activity of the liver cells. Now it is the bile salts that act on fats in the alimentary canal and aid in their emulsion and absorption. They appear to be themselves split up into other substances and absorbed, for they are not found in the faeces. The colouring matter of bile is derived from the colouring matter of the blood. The pigment of human and carnivorous animals is bilirubin, of a golden red colour. In herbivor ous animals it is biliverdin, a green pigment. The red pigment is readily converted by oxida tion processes into the green. These pigments are cast out in the faeces. Their presence ins, the blood gives rise to the yellowness in cases of jaundice.
The Glycogenic Function of the Liver.
—The third function of the liver is very different from those already considered. A French physiologist, Claude Bernard, was the first to point out that the liver formed a sub stance like starch, which was readily converted into sugar. He called it glycogen ; it is also called animal starch. If an infusion of pieces of the liver of any animal be made, it will be found to be rich in sugar (grape-sugar, or glu cose). But if the liver of an animal, just killed, be rapidly removed from the body and thrown into boiling water, an infusion does not contain sugar. It is opalescent or even milky. By add ing alcohol to it a white precipitate of glycogen falls. If to the opalescent infusion saliva be added (which converts starch into sugar, p. 202), the infusion clears up, and sugar may now be de tected in it—the glycogen has been transformed into sugar. Moreover, if water be injected into the portal vein (p. 200) of a liver, removed from an animal, and the injection continued till the water issues from the hepatic vein; sugar will be found in abundance in the water. If the injec tion be continued till the liver is well washed out, the washings will at last contain no sugar. If the liver be now left for a few hours, and then the injection repeated, sugar will again be found. It appears from such experiments, and many others, that the liver forms glycogen, which is stored up in the liver cells, and that it also contains a ferment capable of transform ing the glycogen into sugar. The liver forms its glycogen chiefly from starch and sugar, taken as food, and passing as sugar to the liver by the portal vein. So far as can be learned, the fate of glycogen is to be gradually retrans formed into sugar and sent to the tissues, as their needs demand, to supply them with material for their energy and heat. The liver thus has a great purpose to serve in the nutri tion of the body. Its glycogenic function, as it is called, throws light on the disease diabetes, in which sugar appears in the urine.
Fat Formation by the liver. Fats may be formed or arrested by the liver-cells. Liver-cells usually exhibit bright dots of oil globules (p. 200), which may so increase in number that the cell appears to contain no thing but fat. The liver of domestic animals, especially of those kept in confinement, tends to become very fatty. The luxury known as pite de grog is made of the fatty liver of Strasburg geese. These animals are kept in close confinement and stuffed with rich food, so that the fatty degeneration speedily occurs.
Thus, then, the liver aids in the process of digestion by secreting the bile ; it also separates certain waste substances from the blood, and it stores up in its cella substances (animal starch and fat) which are destined to take part in the general nourishment of the body.