200 MICROSCOPIC ANATOMY OF THE ORGANS. 



work surrounding the bile ducts in the interlobular connective 

 tissue forms veins which enter the interlobular veins. 



It will be seen from the above descriptions that there are in 

 the liver two units, a secretory and a, blood vascular unit. The 

 former is quite definite, and has for its centre one of the small 

 interlobular connective-tissue spaces in which an interlobular 

 bile duct is present. In these spaces there is also usually an 

 artery and one or more veins. The periphery of the secretory 

 unit varies considerably in outline, but can always be marked 

 by lines drawn between all the nearest central veins. It thus 

 takes in parts of at least three and sometimes several liver 

 lobules. The bile capillaries of these lobules run in different 

 directions toward the ducts into which they empty, so that those 

 of one liver lobule may belong to many secretory units. The 

 blood vascular unit is less definite, for the organ is built up 

 around the venous system more than the arterial. Taking the 

 arterial system as a centre, the vascular unit would be much 

 like that described for the biliary system. With the veins, 

 however, a much more definite unit is formed, which can be 

 taken in two ways according as we consider the entry or the 

 exit of the blood. Taking the interlobular veins as a central 

 point, units can be mapped off which include parts of various 

 liver lobules, as in the secretory unit. If, however, the central 

 vein be considered as the centre, the unit would correspond 

 exactly with what is known generally as the liver lobule. This 

 is shown in Figs. 155 and 156. The liver lobule itself is a 

 unit formed by the division of the organ by connective-tissue 

 septa. The framework of the liver includes the parenchyma 

 in the form shown by these lobules. 



As shown in Fig. 153, the blood pigments contained by the 

 body and nucleus of the liver cell may, under certain condi- 

 tions, become crystallized. 



In connection with the walls of the intralobular blood capil- 

 laries may here be mentioned the stellate cells of v. Kupffer. 

 At first these were considered as perivascular connective-tissue 

 cells, but in later years it has been determined that they belong 

 to the endothelial coating of the intralobular blood capillaries. 



