PHYSIOLOGICAL ANATOMY OF THE LIVER. 239 



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 ar- 

 rangement of the hepatic veins is accordingly very simple. 



Intralobular Veins. The innumerable capillaries of 

 the lobules converge into three or four venous radicles (rep- 

 resented in Fig. 9), which empty into a central vessel, from 

 ToVo- to TFF f an mcn m diameter. 1 This is the intralob- 

 ular vein. If a liver be carefully injected from the hepatic 

 veins, and 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. 



The hepatic veins differ somewhat in their structure 

 from other portions of the venous system. Their walls are 

 thinner than those of the portal veins ; they are not en- 

 closed in a sheath, and are very closely adherent to the he- 

 patic tissue. It is this provision which makes the force of 

 respiration from the thorax so efficient in the circulation in 

 the liver. 2 Here, indeed, a force added to the action of the 

 heart is especially necessary ; for the blood is passing in the 

 liver through a second capillary plexus, having already been 

 distributed in the capillaries of the alimentary canal and 

 other abdominal organs, before it is received into the portal 

 vein. It has also been noted that the hepatic veins possess 

 a well-marked muscular tunic, very thin in man, but well 

 developed in the pig, the ox, and the horse, and composed 



1 KOLLIKER, op. cit., 1867, S. 442. 2 See vol. i., Circulation, p. 322. 



