The liver is enclosed in a connective-tissue capsule, the peritoneum, which also gives off secondary folds, or duplicatures, called ligaments, by which the organ is held in proper connection with the adjacent parts. The thickness of the capsule is about 0.03 mm., and its free surface is covered with the flattened corpuscles that belong to 'serous membranes generally. This connective-tissue covering is furthermore composed of thin laminae, which contain a large number of elastic fibres.
At the transverse fissure, where it is continued into the interior of the organ, the same character is maintained. Here it encircles vessels, ducts, and nerves, forming the so-called Glisson's capsule, which, indeed, with its minute ramifications, traverses the whole interior of the gland. The liver contains, in addition to the glandular substance, blood-vessels, lymphatics, nerves, and gland-ducts, the whole held together by the framework of connective tissue just mentioned, which in the human species is but imperfectly developed.
The hepatic lobules. The glandular parenchyma consists of the so-called hepatic lobules, which in the human liver are not completely separated from one another, for the reason just named. In some of the lower animals, however, this separation is more perfect. In the hog's liver, for example, the septa are so well developed that the lobules are plainly recognizable by the naked eye.
To isolate the human lobules is a matter of some difficulty ; but it can be accomplished by macerating the organ in water from twelve to twenty -four hours.
These lobules are also known as the hepatic acini, or in Sula of the liver. Their form is irregularly polyhedral, and they usually measure about 4x1 mm. At their bases they are attached to short twigs of the hepatic vein, which have a thickness of from 0.080.06 mm., and traverse the lobules in the axes of their long diameters. As the hepatic vein ascends through the lobule, it gives off innumerable capillary branches, almost at right angles to its course. The latter pursue their way to the periphery of the lobule, and hence have a radial direction. These capillaries further subdivide within the lobules, and are united to each other by transverse branches, forming a network with small meshes. At the periphery the capillaries join the ramifications of the portal vein. The latter divides within the liver into numerous branches, which again subdivide at the surfaces of the lobules.
Their ultimate ramifications form the boundary lines between adjoining hepatic lobules, and it is for this reason that they have been called interlobitlar veins. For a similar reason the branches of the hepatic vein, which traverse the centres of the lobules, have been called intralobular or central veins.
The interlobular veins are contained within interlobular or intermediate (Hering) canals ; these are easily demonstrable in the hog's liver. In this animal the adjoining edges of three or four hepatic lobules combine to form a canal which con tains the interlobular vessels. The latter are surrounded by connective tissue, which is continuous with that of the septa between the lobules. In the human liver there is a similar arrangement, but in it the septa of connective tissue do not completely separate the lobules, and, excepting at the interlobular canals, the parenchyma of contiguous lobules appears to coalesce.
Nevertheless, the substance of the human liver can be di vided into distinct lobules, and the terminal branches of the portal veins may be regarded as their natural boundaries. Starting with the portal veins, therefore, the course of the blood is as follows : portal veins, interlobular veins, capillaries, intralobular veins, hepatic veins, and inferior vena cava.
Sublobular veins, according to Kiernan, are such branches of the hepatic vein as are placed under the bases of several lobules, and collect the blood from their central veins.
The liver may be injected either through the portal or hepatic veins, or through both. Good specimens may be obtained by injecting the fresh liver of a dog or rabbit with carmine gelatine through the portal vein, then injecting fluid Berlin blue into the hepatic vein, and afterward hardening the organ in alcohol. The central vein and adjacent capillaries will thus be filled with a blue mass, while the interlobular and portal veins, with the peripheral capillaries, will con tain the transparent red mass.
The color of the cut sur face of the liver in its natural condition is of a uniform reddish brown tint, and its lobular structure is not readily made out. Usually, however, we find two shades or gradations in color ; one, corresponding to the central veins of the lobules, is of a dark red ; the other, corresponding to the periphery of the lobules, is a lighter and yel lowish red.
Occasionally these conditions are found to be reversed, and the difference of color is due to the fact that after death the central and other hepatic veins are filled with blood, while the portal and its branches are empty ; and also because the deposit of bile-pigment takes place at the centres of the lobules about the intralobular veins ; whereas a fatty infiltration, such as may occur in normal livers, takes place at the periphery. Not uncommonly the yellowish red color at the boundary of the lobules exists under the form of delicate markings, which are nothing more than the empty interlobular branches of the portal vein.
Kiernan occasionally observed in young subjects that the portal vein was distended with blood, while the hepatic vein was empty. In such cases the periphery of the lobules was of a darker color than their centres.