252 A Study of the Structural Unit of the Liver 
lation. Ultimately the arrangement is such that all capillaries of an organ 
are equally favored by the circulation. This means that the capillaries 
have about the same diameter and length with about the same amount 
of blood passing through them during a given period of time. If too 
little blood passes through them, in a lobule of the liver for instance, 
some of the capillaries disappear; if the circulation comes to a standstill 
all of the capillaries are obliterated. So their very life is dependent upon 
a proper or normal circulation. If a capillary is too long, the resistance 
within it is increased, and the circulation is slowed with a subsequent 
reduction of length. So in order that a capillary may remain, it must 
have a definite lumen, a definite length and a definite amount of blood 
passing through it in a given time. The diameter varies from .005 to 
.01 mm., according to the organ in which it is located. The average is 
.0O8 mm., the diameter of the mammalian red blood corpuscles. In some 
of my tables the diameter is given as .003 mm., but these measurements 
are from hardened and mounted tissues. The length of the capillary is 
less than half of a millimeter, through which the blood flows in less than 
a second. So, morphologically, a capillary is a blood-vessel .008 mm. in 
diameter, .5 mm. long with a renewal of blood every half-second. If this 
renewal of blood is permanently diminished enough of the capillaries are 
obliterated to reestablish the normal circulation in those that remain. 
If the quantity is permanently increased, according to Thoma’s first law, 
some of them are converted into arteries and some into veins. If the 
increased circulation is continued without a corresponding increase in 
the number of capillaries, the artery will extend into the vein just as is 
the case in the liver when the ductus venosus is formed. 
The anlage, then, of the vascular system is the capillary; artery and 
vein are secondary and are differentiated out of them by the flow of blood 
set in motion by the beat of the heart. As the capillary bed increases the 
flow through the arteries increase, and the heart hypertrophies and the 
vascular proportion is maintained. In round numbers, in the dog, the 
arteries continue to grow until the rapidity of flow is 30 mm. a second in 
an artery .05 mm. in diameter, 150 mm. in an artery, 1 mm. and 300 mm. 
in the aorta. Thoma fixes the rapidity of the circulation in the aorta of 
man at 228 mm. a second, and over 34 mm. a second in an artery .04 mm. 
in diameter. 
The unequal growth of different portions of an organ accounts for the 
unequal size of the arteries which supply them. The whole thing works 
from the periphery to the center. In this way a succession of organ 
units is formed all the way from the first divisions of the artery which 
supply the lobes, to its final twigs, which supply the lobules. 
