HUMAN beings are singularly alike in their general anatomical construction ; yet when we come to investigate one particular region with more detail it is surprising how frequently we meet with variations of one sort or another. More especially does this apply to the vascular system, and in no region more than to the liver. This is, I think, generally appreciated by anatomists. There are, however, variations in the excretory ducts of the liver almost as frequently, and anatomists have not given the attention to this subject that a part. so important from the surgical point of view, deserves.I have made 200 dissections on post-mortem subjects of the vessels and ducts of the liver.There has been no selection of cases, and it is claimed, therefore, that the anatomical details given are a very fair representation of the state of the parts as found in the human subject, and that the results of anyone undertaking so large a number of dissections will be found to corroboratc mine. In addition, I have paid particular attention to these parts during operations, but I have not included the results in the statistics ; I shall refer to them later.If we exclude the appendix, the gall-bladder and its ducts more often call for operative treatment than any other intra-abdominal viscus, and a great deal of this surgery is very badly done, if one may judge by the great number of cases one sees which require a second operation, mostly for reasons that should not arise if proper care and knowledge had been applied a t the first attempt. Technically, gall-bladder surgery is much the most difficult of any abdominal surgery, and inadequate appreciation of the abnormalities of this region does not lessen the risks.The arrangement of the vessels and ducts given as normal in the text-books of anatomy is shown in Fig. 410. In my series of 200 cases, only 69 conform to this type. So frequent are variations that it is impossible to regard any one type as normal ; the arrangement found in the 69 cases can only be described as the most usual one.The dissections were made as far as possible consecutively. I will describe first the vessels as found in my series. The Right Hepatic Artery.-This arises from the main hepatic trunk in 158 cases, and to reach the liver passes behind the common hepatic duct in 136 (Fig. $lo), and in front of this duct in 25 (Fig. 411). In 42 the right hepatic artery arises from the superior mesenteric artery (Fig. 412), and always passes behind the common duct. In 7 cases there are two right hepatic arteries, one from the hepatic trunk and one from the superior mesenteric (Fig. 413). In 2 cases there are two right hepatics both from the main hepatic, one passing in front of, and the other behind, the common hepatic duct. In 4 cases, in addition to passing behind the ducts, the main hepatic or the right hepatic artery also passes behind the portal vein (Figs. 414 and 432).I am aware that the right hepatic artery very occasionally arises from the aorta, the right renal, the gastric, or the inferior mesenteric artery, but ...