The hepatic ducts are extremely vascular, and in a well-injected liver are always com pletely covered with the ramifications of the hepatic artery. The rugs: upon their internal surface are formed by large vessels, " arteries as well as veins," which are distributed beneath the mucous membrane. This membrane, seen beneath the microscope, appears plaited over every part of its surface by innumerable lami nated papilla' of a semilunar form. The vessels distributed upon these papilla consist of an artery which ascends upon each side of the lamina, and divides into a beautiful net-work of capillaries which are collected after their distribution into a small vein and returned to the portal vein. " It is," says Kiernan, " to the rupture of the delicate vessels forming these papilla' that is to be attributed the facility with which Soemmering and other anatomists in jected the ducts from the arteries and veins, and not to any direct communication between the vessels and the ducts." The mucous lining of the ducts is provided with a considerable number of muciparous follicles which mingle their secretion with the bile during its passage along the excretory tubes. These follicles have been described by all anatomists as existing in the larger ducts, but they were not known to be present in the smaller branches until they were discovered and figured by Kiernan. In the larger ducts they are irregularly dispersed, but in the smaller tubes are found arranged in two longitudinal rows upon opposite sides of the ducts. Hence the vascularity of the hepatic ducts is intended to perform a higher function than the mere nutrition of those tubes; it provides an im portant secretion as an auxiliary to the compo sition of the bile.
The hepatic artery arises from the coeliac axis and ascends through the right border of the lesser omentum to the transverse fissure of the liver, where it bifurcates into two branches for the right and left lobes. The right and left hepatic arteries ramify in the portal canals, and give off branches which accompany each twig of the portal vein and hepatic duct. Their branches, like those of the vein and duct, are the vaginal, the interlobular, and the lobular.
The vaginal arteries arise from the hepatic arteries in the portal canals, and assist in form ing the vaginal plexus in the capsule of Glisson, from which the interlobular branches are given off to accompany the interlobular portal veins and ducts. In the larger canals the plexus completely surrounds the portal vessels, but in the smaller canals the plexus is situated only on the side opposite to the cylinder of the artery, and in the tissue of Glisson's capsule. This vaginal plexus has the effect of supplying the lobules which are the most distant from the essel to which they belong, as certainly, as those hich are in immediate contact with its cylin er. The vaginal arteries anastomose so freely with each other, that if the hepatic artery of one side be injected, the injection will return by that of the opposite side.
The interlobular arteries enter the intervals of the lobules through the interlobular spaces and ramify upon the capsular surface of the lobules. They are distributed principally to the interlobular ducts, around which they form a vascular net-work. The question of the inosculation of these vessels is very difficult to decide by dissection on account of their ex treme minuteness; but analogy would lead us to infer that they must communicate.
The lobular arteries, " exceedinuly minute and few in number," so as to be demonstrable with much difficulty in the structure of the lo bules, enter the circumference of these bodies with the lobular ducts upon which they are distributed. They are the nutrient vessels of the lobules, and terminate in the lobular venous plexus formed by the portal vein.
The mode of distribution of the hepatic ar tery is a subject upon which some difference of opinion subsists between Miifter and Kier nan. .Kiernan states that the hepatic artery is distributed chiefly upon the coats of the ducts and gall-bladder, upon the coats of the other vessels to which it forms the vasa vasorum, and to the substance of the lobules. The ducts are highly vascular, and are abundantly supplied, the lobules sparingly, but " few" vessels, and those " exceedingly minute," being traceable into them. From the capillaries of the ducts and vessels, the blood having become venous during its circulation is returned into the portal vein, and thence conveyed onwards to the lo bules, where it is distributed through the lobular venous plexus. The blood of the terminal lobular arteries also becomes venous in the substance of the lobules, and is likewise poured into the lobular venous plexus. So that, ac cording to this author, the whole of the blood distributed through the hepatic artery is re ceived by the portal vein, either in the course of that vessel, or at its termination in the lo bular venous plexus, and therefore, that all the blood circulating through the plexus must ne cessarily be venous. He likewise affirms that no part of the blood of the artery is poured directly into the hepatic vein. "The intra lobular veins," he says, " convey the blood from the lobular venous plexus, and not from the arteries." These views are the results of the evidence of numerous experimental injec tions. With regard to the vascularity of the lobules, he observes, " These bodies cannot be coloured with injection from the artery, even in the young subject ; the adult, after the most successful injection, when the arteries of the cellular capsule, those of the excreting ducts and gall-bladder, and the vasa vasorum are mi nutely injected, a few injected vessels only are detected entering the lobules. I have fre quently tied the thoracic aorta in living animals, thereby cutting off all supply of blood from the abdominal viscera ; and in these animals, when injected from the aorta below the ligature forty-eight hours after death, the integuments, the secreting portions of the kidneys, the spleen, pancreas, intestines, and pelvic viscera were co loured in a remarkable degree by the injection; on the surface of the liver a few vessels only could be discovered, this organ presenting a curious contrast with the surrounding coloured viscera. The gall-bladder and ducts were, how ever, equally well injected with the intestines ; the vasa vasorum were also well injected." Perceiving in the progress of his experiments that the injection thrown into the artery passed freely into the portal vein by means of the ca pillary communication existing between these two vessels on the coats of the ducts, and through the vasa vasorum of the vessels, he imagined that the injected fluid might in this way be diverted from the lobules, and that this must be the cause of his want of success in filling the lobular arteries. To ascertain if such were the case, he injected the portal vein in the first instance with blue, and then the arteries with red. " On dissection, branches of the two sets of vessels were found in the coats of the vessels, and in those of the excreting ducts and gall-bladder ; the lobules were coloured with the blue injection ; the red was confined to their circumference, and appeared in points only. This experiment was varied by inject ing the portal vein and its branches as far only as the entrance of the latter into the lobules, the lobules thus remaining uninjected. The
injection propelled through the arteries had now free access to the uninjected lobules, and no exit by the injected portal vein ; and the artery having no communication with the he patic veins, the injection had no exit by these vessels: the lobules however were not better injected in this than in the preceding experi ments. From these experiments I conclude, that the secreting part of the liver" "is supplied with arterial blood for nutrition only. As all the branches of the artery of which we can ascertain the termination, end in branches of the portal vein, it is probable that the lobular arteries terminate in the lobular venous plex uses formed by that vein, and not in the intra lobular branches of the bepatic veins, which cannot be injected from the artery." Muller, who published upon this subject previously to the discoveries of Kiernan, and was therefore not aware of the exact distribution of the ves sels, was deceived by this free communication between the hepatic artery and portal vein. He conceived, with the older anatomists, that the arterial blood was mixed with the venous blood of the vena portre, in a capillary network, " vascula ultima reticulata," common to the three bloodvessels of the liver, the hepatic ar tery, portal vein and hepatic veins. Observing, moreover, in the injected preparations of Lie berkiihn,* that the "vascula ultima reticulata," the lobular venous plexus of Kiernan, appeared as well filled when the injected fluid was forced into the hepatic artery, as when intro duced through the portal or hepatic vein, he at once decided that the artery must pour its blood directly into this plexus. hence he writes, " Vascula ultima reticulate sanguinem tam ab arteriis quam a vena portarum accipere, ve nisque hepatitis reddere, ex hisce argumentis concludo : Post injectionem in arteriam hepa ticam non minus quam in venam portarum aut venal hepaticas factam, eadem communia vas culorum minimorum retia replentur, quod ex injectionibus exsiccatis Lieberkiihnianis, Bero lini asservatis, facile quisquis sibi persuadebit." Having recourse himself to an extremely im perfect experiment, the injection of water into the hepatic artery, and finding that this fluid returned by the portal vein, and possibly by the hepatic vein, he became convinced of the com munications of all the vessels in the "vascula ultima reticulata," and added another argument to the injections of Lieberkiihn in favour of his opinion; for he says, " Injecti liquores co lorati ex alio vasorum ordine facile in ilium transeunt, qualis frequens Halleri veterumque, Walter', denique et ltudolphi eel. extat cape rientia. Ise equidem transitum aqua' lira pidm et coloratx swpius observari." Now with regard to the injections of Lieberkiihn, I can only repeat with Kiernan, that if the lobular venous plexus or "vascula ultima reticulata" were filled, actually, from the artery, the only route which the injection could have taken must have been through the capillaries of the excretory ducts and vasa vasorum, and then through the portal vein. But with regard to the water experiment, I am quite satisfied of its utter inadequacy to elucidate so delicate a point as that under discussion. In my own experiments, made with a view of assuring my self of the nature of these plexuses, I have not been content with my injection unless I could distinctly trace with the aid of the microscope each capillary vessel from the interlobular vein to the intralobular vein, and this I have never failed to do in a successful injection from the portal vein; or in the opposite course when the hepatic veins have been filled. But in the most successful injection from the artery, when the capsular arteries have been beautifully filled, I have never observed more than a few red points in the circumference of the lobules. There is, however, in the consideration of this question, one circumstance which appears to have been altogether overlooked by Muller, but which seems to me to be fatal to the opinion which he entertains with regard to the distribu tion of the arterial blood. The ducts areabun dantly supplied with blood from the arteries; indeed to so great an extent, that in a well injected liver their coats appear to consist almost wholly of the ramifications of minute vessels. Now if the aggregate of the surface formed by the ducts, which is thus covered with vessels supplied from the artery, be considered, it must be evident that very little can be left for the " vascula ultima reticulata." And if con jointly with this fact, the difficulty of injecting the lobules from the artery be considered, it must be admitted that Muller =ries his dogma somewhat too far, in asserting without limita tion "that the arterial blood of the hepatic artery and the venous blood of the porta be come mixed in the minute vessels of the liver." The hepatic veins return the whole of the ve nous blood from the liver to the general venous circulation. They commence in the centre of each lobule by means of a small vein, the intralobular, which collects the blood after its circulation through the lobular venous plexus. The intralobular veins pour their current into the sublobular veins, and these latter unite to form the hepatic trunks, which terminate in the inferior vena cava. The hepatic differ from the portal veins in being more immediately in con tact, and more closely connected with the sub stance of the lobules. Thus the intralobular veins are embedded in the substance of each lobule, and the sublobular inclosed in canals formed by the bases of the lobules, and there fore by that part which is uninvested by the lobular capsule. The hepatic trunks differ from the preceding in being lodged in canals formed by the capsular surface of the lobules, the hepatic venous canals, which are analogous to the portal canals excepting in the absence of a proper investment of Glisson's capsule. It follows from this circumstance, that there are no vessels in connection with the hepatic veins at all resembling the vaginal branches and plexuses of the portal vein. The general course of the hepatic veins is from the two surfaces and free margin of the liver towards the vena cava in the posterior border ; that of the portal vein radiates from the transverse fis sure in the centre of the under surface to all parts of the circumference ; hence the two veins cross each other in their course, the former proceeding from before backwards, and the latter from the centre towards the circum ference. In examining either of these sets of vessels, we should, therefore, be guided in the direction of our section by this peculiar ar rangement. There is another mode by which we arrive at a knowledge of the means of dis criminating between the two veins in a section. The hepatic vein being closely adherent to the lobules forming the canal in which it is lodged, remains open, and retains the form of its cy linder upon the face of a section ; it may also be recognised by being solitary. The portal vein, on the contrary, being surrounded by the loose, vasculo-cellular web of Glisson's cap sule, is permitted to collapse; it is also charac terised by being associated with a branch of the hepatic artery and duct. In the consideration of the hepatic veins I shall describe, first, the intralobular, next the sublobular, and then the hepatic trunks.