Fig. 7.—Diagram of a Lobule of the Liver
divided vertically through its Axis.
In its centre is a space, the intralobular vein, through which the blood falls into a branch of the hepatic vein, on its way to the heart. An interlobular branch of the portal vein, which brings the blood from the digestive organs, pours it by many smaller vessels over the surface of the lobule. It filters into the lobule through innumerable pseudo-capillary vessels, or spaces, between the radiating columns of liver-cells. Arterial blood is brought to the lobule by a twig of the hepatic artery. Bile is drained away from it by an affluent of the hepatic duct. In the lower part of the diagram seven liver-cells are shown, forming a divided column, magnified about 300 diameters. The cells are loaded with glycogen, and contain minute globules of fat. Red blood-corpuscles and two leucocytes are seen between the columns of liver-cells. One of the leucocytes has ingested two blood-corpuscles.
The liver is supplied with blood by the hepatic artery. This vessel is small for so large an organ. Although responsible for the nutrition of the liver, it does not bring it the materials which are stored in its cells. A much larger supply of blood is derived from the portal vein, which breaks into capillaries, or, to speak more accurately, into sinuses, or pseudo-capillaries, in the liver. The blood, whether conveyed to the organ by the hepatic artery or by the portal vein, is drained away by the hepatic veins. The plan of structure of the liver is best understood when viewed with reference to the hepatic veins. These, if traced backwards, are found to break up into fairly straight vessels, each of which has a large number of lateral branches. Each of the lateral branches is in the centre of a mass of cells, which are packed round it in radiating columns. These masses, which have a diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual pressure the lobules are squeezed into a pentagonal or hexagonal form. The vein in the centre of the lobule is the intralobular vein. Turn now to the portal vein; this is seen to break up into branches which run between the lobules, and are therefore termed “interlobular veins.” The branches of the hepatic artery also run between the lobules, as do the radicles of the bile-duct. Each lobule is a liver in miniature. The blood of the portal vein, which has come from the spleen, in which red blood-corpuscles are destroyed, and from the stomach and intestines, from which it has absorbed the products of digestion, is poured over the surface of the lobule, to be filtered through into its central intralobular vein. In its passage from the interlobular veins (and branches of the hepatic artery) to the intralobular vein the blood is confined to radiating capillary channels; but since these merely prevent the escape of red blood-corpuscles without imposing any restrictions upon the exudation of blood-plasma, the portal blood is to all intents and purposes filtered through the columns of liver-cells. The body-substance of the liver-cells is soft, destitute of envelope, and capable, when free on the (warmed) stage of a microscope, of changing in form, somewhat after the manner of a leucocyte. Such cells have a great capacity for taking up the products of digestion. Possibly they take up and store fats and proteins, but undoubtedly it is their chief business to absorb sugar which accumulates as glycogen in their substance. The glycogen is handed out to the hepatic blood as required. The pigment which results from the disintegration of red blood-corpuscles in the spleen is secreted, along with the bile-salts, into minute channels, or canaliculi, which groove the flat surfaces of adjacent liver-cells. These canaliculi converge to the bile-ducts. The liver is therefore at the same time the storehouse of sugar which it takes up from the blood when it is in excess, and passes out to the blood when it is deficient, and an excretory organ which eliminates the refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and returns to the blood.
Another function of the liver has been referred to already. It is the organ, and, as far as we know, the only organ, in which urea is made in mammals, and uric acid in birds. If the liver of a freshly killed animal be excised and a stream of blood passed through it, the blood which leaves the organ contains urea. If a salt of ammonia, even the carbonate, be added to the blood, it is converted by the liver into urea. When a bird’s liver is made the subject of the same experiment, uric acid appears instead of urea. The liver can convert many nitrogenous substances into urea, but it seems probable that, normally, the salt with which it has chiefly to deal is lactate of ammonia ([cf. p. 146]).
A few words must be added with regard to the functions of the liver during prenatal life, obscure though these functions are. The liver develops very early, and attains a relatively enormous size. At the third month it weighs as much as the whole of the rest of the body ([cf. p. 34]). Yet it cannot, one must suppose, have to do much of the work which falls to its share in postnatal life. Food is reaching the embryo in a constant stream, and not as the result of intermittent meals. The embryo has no need to store glycogen; nor does its liver, on analysis, yield much of this substance. In the embryo glycogen is widely distributed throughout the tissues, not specially accumulated in the liver. No digestion is occurring in the alimentary canal. Bile is not needed to aid the hydrolysis and absorption of fats. A small quantity of cholesterin and less lecithin is being eliminated, but not much bile is needed to facilitate this process.
A process which is proceeding at a great rate in the embryo, in various situations, is the formation of red blood-corpuscles. In this the liver takes part. But its duty in regard to blood-formation is not sufficiently onerous to account for its size. The formation of blood-corpuscles in the liver is observed with difficulty in microscopic sections. It is therefore impossible to speak with certainty as to the extent to which it is going on, but it may be safely asserted that this function by itself cannot be held to account for the great size of the organ in embryonic life. What other office it fills at this period is a question which still awaits an answer.
There is no more curious chapter in medical history than the story of the views held at various periods with regard to the functions of the liver. From being a mere mass of “parenchyma” serving as packing for the abdominal viscera, it was elevated to the rank of Grand Purifier of the “humours” of the body. Next, its excessive activity became the cause of that form of dyspepsia known as “biliousness.” Still later its want of activity was its chief vice. A “sluggish” liver was held responsible for mental perversity and moral dulness. Calomel, podophyllin, and other drugs were used as whips to stir it up; and the increased secretions of the alimentary canal were mistaken for bile. Poor patient organ! It is the still-room of the body, in which the day’s supplies are stored, and from which they are served out, without haste and without delay. And it makes urea. What else it does we have yet to find out; and it is not impossible that when physiologists have quite shaken themselves free from the explanations based upon conjecture, which their predecessors have handed down, they may discover that it has other duties which are not obvious, but of great importance.