248 A Study of the Structural Unit of the Liver 
veins of the third order and then into the terminal veins, skipping en- 
tirely the veins of the first, second, fourth and fifth orders. In general, 
however, most of the blood passes through veins of all orders before it 
reaches the capillaries. The short cut some of the blood takes is neutral- 
ized by the increased resistance due to the increased angle of the small 
vessel to the main trunk; very small veins always arise at right angles 
from large trunks, while their direction when arising from a smaller 
trunk is at an acute angle. 
If there are 480,000 structural units in the form of small cylinders 
0.7 mm. in diameter and 0.7 mm. high, it is easy to determine the num- 
ber of capillaries which enter the unit in both transverse and longitudinal 
sections. My count in ten different injections gives 110 for the circum- 
ference of the hepatie unit and 35 for its height. These multiplied give 
3850 as the number of capillaries which enter the hepatic lobule at its 
periphery. If all of the anastomoses within the lobule are estimated also, 
as they should be, this number is at least doubled. In order to err on 
the safe side I have taken the smaller number and multiplied it by the 
number of structural units, giving 1850 million as the number of capil- 
laries which enter the periphery of the hepatic lobule of the liver. 
I have arranged certain data (marked with brackets in Tables I to VI) 
in Table VII. It is at once noticed that the vascular bed is about five 
times larger for vessels 0.05 mm. in diameter than for those a millimeter 
in diameter. While the lumina of the vessels diminish 20 times, their 
number increases 2000 times. In the pulmontary artery and the portal 
vein, in both of which the blood-pressure is low, the vascular bed increases 
much more than in the arteries from the coeliac axis, down to the 
arteries one millimeter in diameter. In the intestine, stomach, ad- 
renal and liver the increase in the vascular bed from vessels 0.05 mm. in 
diameter to the capillaries is much the same, averaging about 36 times; 
in the spleen it is much greater and in the lung it is much less. In the 
spleen with the numerous small slender vessels the resistance is enormous 
and in the lung it is insignificant, as is easily verified by making injec- 
tions; it is very difficult to inject from an artery into the vein in the 
spleen, while in the lung melted wax can be injected through. In the 
spleen the area of the terminal arteries is 800 times that of the splenic 
artery, while if the pulp spaces be compared, the ratio is fully 30,000. 
That this number lies within the hmit of probabilities is easily shown. 
The average volume of the spleen of the dogs used is 10 ec., which 
when distended increases to 80 cc. ; the spleen can add to itself seven times 
its own volume without bursting. The area of all of the pulp spaces of 
