308 A Study of the Structural Unit of the Liver 
of bile ducts is of great importance and helps to clinch much that has 
been said above about the development of the liver. In an embryo 
10 em. long this plexus encircles portal branches of the second order 
(Fig. 73). In an embryo twice as long the plexus has passed the veins 
of the third order and now encircle completely those of the fourth or 
fifth orders. So as the liver tissue is shifting towards the periphery 
is) Qs Das, - EOE Re as at ee ni . 23 ~ 
Fic. 74. Injected bile ducts of the liver of a pig, 20 cm. long. x 50. B, bile 
duct; p, portal vein; h, hepatic vein; II, III, IV, respective order of the 
branches. From one of Mr. Hill’s specimens. 
PNG Se 
branches which were once central in every respect, are reduced entirely 
to main trunks, and throughout this process of growth the structural 
units remain practically of one size. Thus from one vein encircled by 
one structural unit a million are formed in the dog. Throughout this 
growth the vascular proportion is constant. Within the center of the 
unit the duct expands into a plexus from which regeneration takes place. 
The periphery of the unit is marked by nodal points which in one sense 
are embryonic units. 
