LIVER (ante). The physiological anatomy of the liver may be briefly stated as fol lows: The lobules mentioned in the preceding article are about -14 of an in. in diame ter and of an ovoid shape. They are surrounded by a plexus of blood-vessels, nerves, and ramifications of the hepatic duct, comprising what are called the interlobular ves sels. These are all inclosed by a sheath which is a prolongation of the proper coat of the liver (capsule of Glisson), but attached loosely by areolar tissue. This sheath follows the vessels to the subdivisions within the interlobular spaces (spaces betw-een the lobules), but does not extend to the capillary vessels within the lobules. In a few animals, as the pig and polar bear, the lobular structure can be seen with the naked eye, but in man and most mammals it cannot. The lobules are intimately connected with each other, branches of the interlobular vessels being each distributed to several of the lobules. Any one lobule, however, may be considered as representing the physiological anatomy of the whole liver, and the study of its anatomy aud functions will answer for the study of the whole gland. The lobules receive blood at their surfaces from the capillary ter minations of the portal vein, these vessels haying received the terminations of the hepatic artery before passing into the lobules. It is very important to bear in mind this peculi arity of distribution, which is often overlooked. The branches of the hepatic vein, the vessel which carries the blood from the liver to the ascending great vein (ascending vena cava), by which it is returned to the heart and lungs. have their oriain within the lobules. Their capillary extremities arise from the capillary ramifications of the portal vein, and, passing toward the center of the lobule, converge into three or four radicles, which, uniting at the center, fortn the intralobular veins, which is the com mencement of the hepatic vein. These intralobular veins, which are in the center of each lobule, are from "lb° to Aff of an in. in diameter, and they follow the long axis of the lobule, receiving vessels in their course till they empty into larger vessels situated at the base of the lobules. These latter vessels have been called by Kiernan sub-lobu lar veins. They(collect the blood from all parts of the liver, and, increasing iu size by union with one another, they at last form the three hepatic veins' which discharge the blood from the liver into the ascending vena cava. Now, these hepatic veins are a long way from the influence of the heart's action, lying as they do between the portal circulation aud the veins going to the heart; but a provision has been made to assist in the propulsion of their contents, and they are supplied with a muscular coat, composed of unstriated muscular fibers, The minute anatomy of the liver has only recently been satisfactorily investigated, and it is to the labors of Beale, E. Wagner, Garlack Budge, Andrejevic, Koelliker, MacGillavry, Frey, Eberth, Hering, and others that we owe nearly ail the knowledge we have upon the subject. The most essential elements of the lobule, or of the liver, remain to be described. They are the hepatic cells, which are the true secreting elements of the gland. They are minute, polygonal-shaped bodies about 1-61-rs of an iu. in their longest and /Tiro- iu their shortest diameter, having one nucleus, or sometimes two nuclei, with some granular inatter. See CELLS. It has geu erally been supposed that these hepatic cells WM held within a net-work of the capilla ries of the portal and hepatic veins, but, according to the investigations of the above. named microscopists, this is not the case. They are surrounded by an independent net work of extremely minute vessels -1-dr,m of an in. in diameter, of uniform size through out, called the biliary capillaries, and in which the bile first makes its appearance.
We must pause here to refer to the fact that the liver is an organ which has no ana logue in any of the other organs of the body. It has two distinct functions, and a cellu
lar arrangement entirely unlike that seen in any other gland. It is excretory on one hand and secreting on another, and it is its secreting function which has been so long overlooked, and the knowledge of which has also thrown so much light on the physiology of what are called ductless glands, like the spleen (q.v.) and the lymphatic glands. The liver, in otie of its functions, is a ductless gland. It secretes (that is, not merely separates, but forms) a substance which is not carried away by any excretory vessel, but which is immediately returned to the blood, when it is washed away as soon as formed. The other function of the liver is the production of bile, which, although a true excretion, answers. a salutary purpose in the econotny. Let us now returu to the cousideration of the hepatic cells aud the lately discovered net-work of vessels which surrounds them, called the bil iary capillaries. It is with the utmost difficulty that they have been made out, and it is owing to this that so many hypotheses have been formed in regard to the histology aud physiology of the liver, only to be successively abandoned. The meshes which are formed by the passing round the hepatic cells of these minute capillaries are arranged in a. cubical tnanner, very much as if they had been woven around them. The question has. been whether these binary capillaries possessed independent walls or whether they were simply lacunar passages; but the manner in which they have been found to interlace with. the blood capillaries decides the question in favor of considering them as vessels having, walls, although their caliber is only -fa" of au in., which would require the membrane which forms the tube to be inconceivably thin, and perhaps destitute of any cellular structure, as is generally found in lining membranes of most organs. The precise rela tions of the ltepatic cells and the biliary ducts have been more particularly determined by the investigations of Eherth and Hering; and they find that they vary in different classes of vertebrata, being simpler the farther we descend in the scale of being. In. amphibia, for instance, the lobular form is altered, and the bile duct passes through a. tubular arrangement of hepatic cells. In reptiles the arrangement approaches more• towards that of mammals, but is still far behind in development; and it is only when. ascending to birds that a structure is reached capable of performing the excrementitious. functions of active, warm-blooded animals. The binary and blood capillaries never come into actual contact, but are always separated from each other by a distance some what less than the diameter of an hepatic cell, or about rat" of an inch. The biliary capillaries are undoubtedly the commencement of the finer hepatic ducts. In some dis eases they become so distended with bile as to become easily discernible with a good microscope. The livers of animals dying of Texan-cattle disease were examined by the. late Dr. R., C, Stiles a few years ago, and the observations of the German anatomists were. completely verified. The finest bile ducts and capillaries in the livers of these anitnals were found filled with bright yellow bile, and their relations to the liver cells were easily distinguishable. Favoring tlte view that they are lined by an excessively thin mem brane, Dr. Stiles found in his exatninations what appeared to be detached fragments of these capillaries. Between the lobules the bile ducts are still very minute, the smallest being only 12-gur t .5151" of an in. in diameter, and composed of a very delicate mem brane lined with pavement epithelium. When they reach a size of 457F of an in. in: diameter, they are supplied with a fibrous coat, composed chiefly of inelastic, with a few elastic fibers; but the larger ducts, as afore-mentioned, are supplied with non-striated. muscular fibers.