434 



EXCRETION. 



when they are fully distended by artificial injection, their diameter is greater than that 

 of the intervascular spaces. It must be remembered, however, that, in the study of the 

 liver by minute injections, as in other parts, the vessels probably are distended so that 

 they occupy more space than they ever do under the physiological conditions of the cir- 

 culation The blood, having been distributed in the lobules by this lobular plexus, is col- 

 lected by venous radicles of considerable size into a single central vessel situated m the 

 long axis of the lobule, called the intralobular vein. A single lobule, surrounded with 

 an interlobular vessel, showing the lobular capillary plexus, and the central vein (the 

 intralobular vein* cut across, is represented in Fig. 130. 



FIG. 130. Transverse section of a single hepatic lobule. (Sappey.) 



1, intralobular vein, cut across ; 2, 2, 2, 2, afferent branches of the intralobular vein ; 3, 3, 3, 3, 3, 3, 3, 3, 3, interlobular 

 branches of the portal vein, with its capillary branches, forming the lobular plexus, extending- to the radicles of 

 the intralobular vein. 



With regard to the mode of origin of the hepatic duct in the substance of the lobule, 

 recent researches have shown that it begins by a very fine, anastomosing plexus of ves- 

 sels, with amorphous walls, situated between the liver-cells ; but there are many differ- 

 ent opinions on this subject, and we shall defer its full consideration until we take up the 

 anatomy of the secreting structures in the lobules. 



Origin and Course of the Hepatic Veins. The blood distributed in the lobular capil- 

 lary plexus furnishes the materials for the formation of bile and undergoes those changes 

 produced by the action of the liver as a ductless gland ; in other words, it is in and 

 around this plexus that all the physiological functions of the liver are performed. It is 

 then only necessary that the blood should be carried from the liver to go to the right 

 side of the heart ; and the .arrangement of the hepatic veins is accordingly very simple. 



Intralobular Veins. The innumerable capillaries of the lobules converge into three, 

 or four venous radicles (represented in Fig. 130), which empty into a central vessel, from 

 Tirotf to 477 of an inch in diameter. This is the intralobular vein. If a liver be carefully 

 injected from the hepatic veins, and if sections be made in various directions, it will be 

 seen that the intralobular veins follow the long axis of the lobules, receiving vessels in 

 their course, until they empty into a larger vessel, situated at what may be termed the 

 base of the lobules. These vessels have been called, by Kiernan, the sublobular veins. 

 They collect the blood in the manner just described from all parts of the liver, unite 

 with others, becoming larger and larger, until finally they form the three hepatic veins, 

 which discharge the blood from the liver into the vena cava ascendens. 



