ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 321 



There is a distinct diflfereiice between the larger branches of the portal vein and 

 those of the sub-intestinal vein. In the former case they have an obvious connective- 

 tissue sheath, and are surrounded by factors of the hepatic duct, whilst in the latter 

 case they are irregular spaces between the liver tubules, apparently unlined,* and 

 usually contain a lesser quantity of blood. Nevertheless, the two series of spaces 

 communicate freely within the liver. A forward injection of the portal vein at once 

 passes from the liver substance backwards along the sub-intestinal vein. 



The portal vein, on entering the liver, splits into a number of large vessels of 

 considerable blood capacity compared with that of the vein itself. The pressure of 

 blood in the liver, therefore, must be low. 



As that division of the portal vein which enters the posterior lobe of the liver is 

 coursing backwards over the surface of the liver preparatory to plunging into it, there 

 is given off first a large vessel which courses forwards in the interior of the anterior 

 section of the lobe, and which preserves its identity almost up to the anterior extremity 

 of the latter. The portal vein then detaches another large vessel, which supplies the 

 more central section of the gland. Finally, it enters the lobe at the point where the 

 hepatic duct leaves it, coursing at first alongside and external to the associated larger 

 factors of the hepatic duct near the surface of the lobe, and giving off large vessels to 

 the interior. It then penetrates to about the centre of the lobe, still accompanying the 

 larger factors of the hepatic duct. Finally, as the posterior extremity of the lobe is 

 approached, and the hepatic duct is represented by a large number of very small 

 tubules, the portal vein splits first into two and then into four, and so on until its 

 identity is no longer maintained. 



With regard to the anterior division of the portal vein to the anterior lobe of the 

 liver, the vessel, on entering the lobe, gives off two large branches which pass back- 

 wards, and supply that part of the gland behind the point of exit of the hepatic duct. 

 The remainder passes forwards, accompanied by the factors of the hepatic duct, soon 

 splits into two and then into four, and thereafter behaves in the same way as in the 

 case of the posterior division. 



Near the point of exit from the liver we encounter the usual portal canals, con- 

 sisting of one or more branches of the portal vein, hepatic artery, and hepatic duct, 

 enclosed in the connective-tissue sheath of Francis Glisson. This association, however, 

 has by no means the definite significance and regularity of the higher animal. The 

 artery is distinguished by a well-defined lining and its conspicuous closely set nuclei 

 projecting into the cavity of the vessel. 



The, posterior hepatic duct, after leaving its lobe of the liver, receives a large duct 

 from the anterior section of the lobe. If this itself be now traced forwards, it almost 

 at once, and before becoming actually buried among the liver tubules, splits into a 

 very large number of small ducts which are arranged round the portal vein and its 



The boundaries exhibit a few flattened nuclei at intervals which belong to the inconspicuous lining of the 

 vascular space and not to the basement membrane of the tubules. 



