246 A Study of the Structural Unit of the Liver 
On account of numerous calculations of the lobules, terminal portal veins 
and terminal hepatic veins, I have fixed upon 480,000 as the average 
number. The terminal lobule or structural unit, either portal or hepatic, 
is about one-third of a cubic millimeter in volume, and with this size as 
a basis I have estimated the number of capillaries in the liver. How- 
ever, lobules are usually put up in clusters, as described by Kiernan, and 
the volume of such a cluster is about 2 cu. mm. But the clusters can 
easily be diminished or increased at will to two structural units or to a 
whole lobe. 
The average diameter of the portal vein is found to be 9 mm. The 
branches of the first order may be considered six in number and supply 
respectively the six lobes of the dog’s liver. They may be designated as 
right upper and right lower, left upper and left lower, cystic and omental 
branches to correspond with the terms given by Rex in his excellent paper 
on the morphology of the mammalian liver.” Each lobe usually receives 
two or more branches varying from 1 to 5 mm. in diameter which when 
bunched for each lobe will give an area represented by the single vessels 
from 4 to 8 mm. in diameter. So in order to round my figures without 
interfering with the total area of the veins of the first order I have indi- 
cated in the table that these are six in number averaging 5 mm. in diam- 
eter. This liberty is based upon measurements taken from nine corrosion 
specimens in celloidin and in wax, and may be controlled by the six 
excellent illustrations to scale given by Rex. It may be pointed out here 
that the left main branch ends quite abruptly in a dilatation at the point 
of communication with the umbilical vein; from this point veins radiate 
in all directions. This dilatation, the recessus umbilicalis of Rex, is even 
better marked in the human liver than in that of the dog (Fig. 5) where 
it makes its appearance during the fifth week of development (Fig. 25). 
From the branches of the first order to the capillaries it is relatively 
easy to compute the number of vessels of a given order. To be sure a 
few of the branches which were bunched with those of the first order are 
of the second order, but they are so insignificant in number that they 
need not be considered. The main subdivisions of the branches of the 
first order may be collected and counted. One of average size may be 
selected and dissected out with the liver tissue it supplies. The volume 
of the whole liver divided by the volume of this piece will give a second 
estimation. The count and the estimation should not be far apart if 
both are made accurately. Slight variations will be neutralized in esti- 
mating the number of vessels of succeeding orders. The sixth order of 
% Rex, Morph. Jahrbuch, XIV, 1888. 
