82 BILE. 



branches has become venous before it comes in contact with the 

 finest ramifications of the biliary ducts ; for, as Kiernan was the 

 first to show, the vasa vasorum of the hepatic artery enter into a 

 venous plexus which, instead of opening into the hepatic veins, 

 discharges itself into the smaller (but not the smallest) branches 

 of the portal vein, and in this manner forms the hepatic origin of the 

 portal system. Hence the secretion of the materials of the bile takes 

 place solely from pure venous blood. The secretion in the kidney, 

 for instance, is altogether different, to which arterial blood, and 

 with it the substances (such as urea, uric and hippuric acids, &c.,) 

 which are first rendered excrementitious by the respired oxygen, 

 are carried, and where, without having to pass through a dense layer 

 of cells, these substances are transmitted in a manner very similar 

 to simple transudation from the blood-vessels into the urinary 

 canals. From the extreme slowness with which the blood passes 

 through the liver, it follows that the conversion of the constituents 

 of the blood into bile in the hepatic lobules only takes place very 

 gradually, thus allowing of a more thorough and complete meta- 

 morphosis. (At these lobules, we find that the finest capillary 

 network of the portal vein is separated by the plexus of the hepatic 

 cells from the smallest biliary canals, which, according to E. H. 

 Weber, are far more minute than the finest capillary vessels.) 

 If we consider that the blood of the portal vein has been already 

 collected from a capillary network, and that now, without 

 further mechanical assistance, it has again to overcome the 

 resistance of friction in a second capillary system, and further, 

 that the veins into which the portal branches discharge them- 

 selves, are even deficient in the valves which usually aid 

 the circulation within the veins, we can comprehend why it 

 is that the blood passes very slowly through the liver. Muller 

 and E. H. Weber have convinced themselves of the correct- 

 ness of this assumption by direct microscopic observations on 

 frogs and on the larvae of salamanders. With these facts in our 

 possession, we need be as little surprised that Bidder and Schmidt 

 perceived that two hours elapsed after the administration of food, 

 before there was an augmentation of the biliary secretion, and that 

 it was not till the end of ten hours that the maximum flow took 

 place, as at the great frequency of hypersemic affections of the 

 liver and of the associated congestion of the haemorrhoidal veins. 



If, however, the great slowness of the circulation within the 

 liver forces us to the assumption that there is a peculiar elabora- 

 tion of the materials in question within the hepatic cells, so 



