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one organ, capillary new formation would occur when the pressure of 
the capillary blood exceeds the limit c, while, in the other organs, this 
limit might be higher at the blood-pressure C. 
“The number of capillaries, their lumen, and the rate of flow of the 
blood-stream passing through them, determine, as has been observed, the 
total quantity of the blood which flows through the entire organ. We 
thus arrive at the remarkable result that it ts the organ iself which de- 
termines the quantity, the rate of flow, and the pressure of the blood 
flowing through it; and that this is effected by means of fixed relations 
which are expressed generally in the three histo-mechanical principles. 
The conditions which produce the uniformity of pressure and rate of 
the blood-current in all capillary areas of the same organ are included 
in these principles. 
“ According to the generally accepted view of the problem of the cir- 
culation, which was formerly quite sufficient to serve as a basis for the 
account of its general disturbances, the pressure, the rate, and the amount 
of the blood-flow appeared to be directly dependent upon the action of 
the heart. According to the view given here, on the other hand, it is 
the metabolic processes in the organs, which determine first for the in- 
dividual organs, then for the whole of the organs—that is, for the circu- 
lation as a whole—the amount of blood propelled within a given time, 
its pressure and its rate of flow. In this case, the working-power of the 
heart appears as the equivalent of the sum of the histo-mechanical de- 
mands made by the organs.” 
It will be seen that Thoma concludes, and I think properly, that capil- 
laries of like component parts of an organ are of equal size and length, 
and that the rapidity of the circulation through them is also equal. This 
idea I have also tried to develop in various papers upon the structural 
unit of organs. It appears that each organ is broken up into units 
which are of equal value from anatomical and physiological standpoints. 
What takes place in one unit takes place in all of the rest. A good ex- 
ample is to be found in the intestine where the structural unit is a villus 
surrounded at its base with a circle of intestinal glands (crypts). In the 
center of the group is the main artery which passes directly to the apex 
of the villus and ending there divides abruptly into an umbrella of capil- 
laries which lie at the periphery of the villus. These capillaries are about 
of one diameter and length, and no matter what course is taken by the 
blood the distance and resistance in passing from the artery to the vein 
is always the same.” Ludwig pointed out that the capillaries of an organ 
™MVall, Abhandl. d. K. S. Gesell. d. Wiss:, XIV, 1887. 
