This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Vena Portae in the Foetus divides into two branches; one of which sinks into the right lobe, the other runs towards the left lobe of the liver and terminates in communicating with the umbilical vein.
On examining the distribution of these two large veins in the foetal liver, we will perceive that in reality the right lobe receives blood from the vena portae only, but the left from both the portal and umbilical veins. This explains why the left lobe is so well developed at this period of life. After birth, however, the ductus venosus becomes entirely obliterated, but of the umbilical vein its left branches and a part of its trunk remain pervious, viz., that part of it in the immediate neighborhood of the transverse fissure; for otherwise there would be no way for the blood of the porta to arrive at the left lobe of the liver. All the rest of the umbilical vein is obliterated.
The use generally ascribed to the vena portae is to convey to the liver the materials for the secretion of the bile. Some, however, insist that the bile is secreted from arterial blood; and others, as Mr. Phillips, that it may be indifferently secreted from either. In favor of the secretion from the portal blood, it has been said that the bile is of an oily character, and that the venous blood, being highly charged with carbon and hydrogen, is the best suited for its production. To this it has been replied, that fat, though compounded of carbon and hydrogen, is nevertheless separated by exhalation from the arterial blood. Again, the size of the vena portae is said by those who suppose the bile derived from it, to be suited to the size of the liver; while the other party reply, that the vessel should be compared with the duct and not with the gland, they maintain that there is a proper proportion between the size of the hepatic artery and hepatic duct. There are at least four* cases on record, in which the vena portae did not go at all to the liver, but terminated in the inferior cava. It would appear, however, that in one of these cases the portal system of the liver was not absent, but the peculiarity was, that it commenced in a cul de sac. The same would probably be found in the other cases if they had been accurately examined. M. Simon found, that if the hepatic artery be tied in pigeons, the secretion of the bile continues; if the portal vein and ducts be tied, it ceases; and if the ducts alone be tied, the liver is gorged with bile. Mr. Phillips infers from his experiments, that the blood may be secreted from the blood of either artery or vein, as, whichever vessel was tied, the secretion continued.
We shall now present the student with an abstract of Mr. Kiernan's valuable observations concerning the circulation and structure of the liver. Previous to his researches, it was supposed that the liver consisted of two different kinds of substance, termed the red tissue and yellow tissue. Mr. Kier-nan, however, by a series of well-conducted experiments, has shown that the red color depends on congestion of the bloodvessels, and the yellow color on the absence of it. In order to make this more clear, let us attend to his exposition of the structure and arrangement of the vessels in the liver. According to Mr. Kiernan, each lobule of the liver has a conical form, and when divided longitudinally presents a foliated appearance; and through its axis passes a small vein, termed the intralobular vein. This vein terminates at a right angle in a larger vein, which is applied to the base of the lobule; this is accordingly called a sub-lobular vein. The sub-lobular veins terminate in the venae cavae hepaticse, and these again in the vena cava inferior. Now, all that portion of the exterior of a lobule, which does not constitute its base, is termed its capsular surface, because it is in contact with, and separated from, the surrounding lobules by a process of the capsule of Glisson, which serves as a capsule for the lobule. Mr. Kier-nan considers that the capsule of Glisson "is to the liver what the pia mater is to the brain; it is a cellulo-vascular membrane, in which the vessels divide, and subdivide to an extreme degree of minuteness; it lines the portal canals, forming sheaths for the larger vessels contained in them, and a web in which the smaller vessels ramify; it enters the interlobular fissures, and with the vessels forms the capsules of the lobules; and it finally enters the lobules, and with the blood-vessels, expands itself over the secreting biliary ducts. Hence arises a natural division of the capsule into three portions, a vaginal, an interlobular, and a lobular portion ; and as the vessels ramify in the capsule, their branches admit of a similar division." Thus, according to Mr. Kiernan, the capsule of Glisson enters the transverse fissure of the liver, and forms an internal lining for those canals called the portal canals, which convey the larger divisions of the vena portae into the interior of the organ; this, which is called the vaginal portion of the capsule of Glisson, invests the primary divisions of the vena portae, hepatic artery, and the larger portions of the hepatic ducts: the term vaginal branches has, therefore, been applied to these divisions of the vessels surrounded by the vaginal portion of the capsule. The minute divisions of these three sets of vessels, together with the lobular portion of the capsule of Glisson, constitute the principal part of the lobules, so that, according to Mr. Kiernan, each lobule is composed, "on the outer surface of a capsule formed by a process of Glisson's capsule, of a plexus of biliary ducts, of a venous plexus formed by branches of the portal vein, of a branch of an hepatic vein, and of minute arteries: nerves and absorbents, it is to be presumed, also enter into their formation, but cannot be traced to them." By taking the duct, artery, and vena portae separately, we shall find how they are disposed of in the liver. First; the ducts penetrate the capsular surfaces of the lobules, and form in the interior of each, an extensive lobular biliary plexus; from this plexus the bile passes into the interlobular branches, and then into the vaginal biliary plexus which goes to form the hepatic ducts. Secondly; when the branches of the hepatic artery pass into the portal canals, they give off vaginal branches; these form the vaginal plexus of arteries which gives off the interlobular branches; these pass through the interlobular fissures and give off the lobular branches which also penetrate the capsular surface of the lobules: they supply the parenchyma of the lobules, and the coats of all the vessels; and the surplus quantity of blood not required for the nutrition of these parts, is conveyed into the minute branches of the vena portae. Mr. Kiernan remarks concerning the function of the hepatic arteries, "I conclude that the secreting portion of the liver, like the excreting portion of the kidney, 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 plexus formed by that vein, and not in the intra-lobular branches of the hepatic veins, which cannot be injected from the artery, the blood of these arteries, after having nourished the lobules, becoming venous, and thus contributing to the secretion of bile." Thirdly ; after the vena portae has reached the transverse fissure of the liver, it divides into branches which enter into the portal canals: here they give off the vaginal branches which constitute the vaginal plexus; these then give off the interlobular branches, which in their turn give off the lobular branches. Finally; these last, after piercing the capsular surface of the lobules, form, in their interior, a portal plexus or the lobular venous plexus of the porta, which, having received, as above mentioned, part of the blood conveyed by the arteries, furnishes the material for the secretion of the bile. Hence it appears that the bile is not secreted from arterial blood, but from venous derived from two different sources, one from the returned blood of the hepatic arteries which flows into the portal veins, the other from the venous blood of the porta itself. Thus, the branches of the vena portae act as veins to the hepatic artery, and as arteries to the hepatic veins, which receive the surplus quantity of blood not employed in secretion. Now the appearances of the yellow and red tissues can be readily explained. The red tissue is owing to venous congestion : when this occurs in the hepatic system of veins, the centre of each lobule will be dark from engorgement of the intra-lobular veins; and, vice versa, when the congestion occurs in the portal system, the centre of the lobule will be light (constituting the appearance of yellow tissue), but the circumference red, by engorgement of the portal veins investing it; so that in one case we shall have dark spots on a pale ground; and in the other, pale spots on a dark ground. Mr. Kiernan could produce these appearances at pleasure, in experiments on animals, both in the liver, and also in kidneys of those animals that have a portal renal circulation.
* Lieutaud, Hist. Anat. Med., tom. i. p. 190. Huber, Observ. Anat., p. 34. Abernethy, Phil. Trans., 1793. Lawrence, Med. Chirurg. Trans., vol. v. p. 174.
According to Todd and Bowman, in the human subject the lobules are not isolated by a distinct capsule, but are only imperfectly marked out by the several points of their exterior, to which the ultimate twigs of the portal vein and duct arrive.