432 EXCRETION. 



mammalia, the lobules are not so distinct, although their arrangement is essentially the 

 same. Although the lobules are intimately connected with each other from the fact that 

 branches going to a number of different lobules are given off from the same interlobular 

 vessels, they are sufficiently distinct to represent, each one, the general anatomy of the 

 secreting substance of the liver ; but, before we study the minute structure of the lobules, 

 it will be convenient to follow out the course of the vessels and the duct, after they have 

 penetrated at the transverse fissure. In this description we shall follow, in the main, 

 the observations of Kiernan, who has given, probably, the most accurate account of the 

 vascular arrangement in the liver. 



At the transverse fissure, the portal vein, collecting the blood from the abdominal 

 organs, and the hepatic artery, a branch of the coeliac axis, penetrate the substance of 

 the liver, with the hepatic duct, nerves, and lymphatics, all enveloped in the fibrous 

 vagina, or sheath, known as the capsule of Glisson. The portal vein is by far the larger 

 of the two blood-vessels, and its caliber may be roughly estimated at from eight to ten 

 times that of the artery. 



The vagina, or capsule of Glisson, is composed of fibrous tissue, in the form of a dense 

 membrane, closely adherent to the adjacent structure of the liver, and enveloping the 

 vessels and nerves, to which it is attached by a loose areolar tissue. The attachment of 

 the blood-vessels to the sheath is so loose, that the branches of the portal vein are col- 

 lapsed when not filled with blood ; thus presenting a striking contrast to the hepatic 

 veins, which are closely adherent to the substance of the liver and remain open when 

 they are cut across. This sheath is prolonged over the vessels as they branch and it fol- 

 lows them in their subdivisions. It varies considerably in thickness in different animals. 

 In man and in the mammalia generally, it is rather thin, becoming more and more delicate 

 as the vessels subdivide, and it is entirely lost before the vessels are distributed in the 

 interlobular spaces. 



The vessels distributed in and coming from the liver are the following : 



1. The portal vein, the hepatic artery, and the hepatic duct, passing in at the trans- 

 verse fissure, to be distributed in the lobules. The blood-vessels are continuous in the 

 lobules with the radicles of the hepatic veins. The duct is to be followed to its branches 

 of origin in the lobules. 



2. The hepatic veins ; vessels that originate in the lobules, and collect the blood dis- 

 tributed in their substance by branches of the portal vein and of the hepatic artery. 



Branches of the Portal Vein, the Hepatic Artery, and the Hepatic Duct. These 

 vessels follow out the branches of the capsule of Glisson, become smaller and smaller, and 

 they finally pass directly between the lobules. In their course, however, they send off 

 lateral branches to the sheath ; and those who follow exactly the description of Kiernan 

 call this the vaginal plexus. The arrangement of the vessels in the sheath is not in the 

 form of a true, anastomosing plexus, although branches pass from this so-called vaginal 

 plexus between the lobules. These vessels do not anastomose or communicate with each 

 other in the sheath. 



The portal vein does not present any important peculiarity in its course from the 

 transverse fissure to the interlobular spaces. It subdivides, enclosed in its sheath, until 

 its small branches go directly between the lobules, and, in its course, it sends branches 

 to the sheath (vaginal vessels), which afterward go between the lobules. The distri- 

 bution of the hepatic artery, however, is not so simple. This vessel has three sets of 

 branches. As soon as it enters the sheath with the other vessels, it sends off minute 

 branches (vasa vasorum), to the walls of the portal vein, to the larger branches of the artery 

 itself, to the walls of the hepatic veins, and a very rich net-work of branches to the hepatic 

 duct. When the hepatic artery is completely injected, the walls of the hepatic duct are 

 seen almost covered with vessels. In its course, the hepatic artery also sends branches 

 to the capsule of Glisson (capsular branches), which join with the branches of the portal 



