1922.] ‘* Renal Portal’’ System. 157 
is not secreted when the kidneys are perfused with Ringer’s 
or other solutions, I may say that no one acquainted with the 
facts would expect the fluid normally excreted from blood to 
excreted by the kidney from the fluid in the intertubular 
vessels and if by ‘‘urine’’ we mean fluid formed by the 
kidney, then the secretion of perfusion experiments is as much 
urine’? as that micturated by the living anima]. If, on the 
aaa hand, we restrict the term “urine” to that which is 
micturated, then neither can the term “ filtrate’’ be generally 
applied to the fluid secreted in perfusion experiments for the 
simple reason that only very rarely is the “filtrate ’’ isotonic 
with the perfusing fluid, and then it is due to the same condi- 
tions which cause the living animal to secrete what is practi- 
cally Ringer’s fluid after a copious injection of it into the blood. 
But I have quoted sufficiently to prove that the neo-Ludwig 
mode of interpretation is fully as elastic as the theory which it 
proposes to supplant. I will only add that if the renal func- 
tion could be fully explained on quasi-physical lines, it would, 
so far from assisting us to understand the mechanism of secre- 
tion in general and of the organism as a whole, only place one 
more difficulty in the way by adding the problem as to why 
such an unique organ as the kidney should exist. 
Summary of some of the Principal Conclusions contained in 
Parts I-IV. 
All true, i.e. functional, portal systems are known to be 
developed by activity of the vascular tissue; on the other hand, 
the ‘‘ renal portal” system is solely due to the encroachment on 
and mechanical subdivision of the posterior cardinal or other 
adjacent vein by the serene hy Mo tubules, and on this 
ground alone, cannot be regarded as a true portal system 
(Part I). O ther a priori stapes against the idea that the 
‘*renal portal ’’ system is functional are summarized in Part I. 
Frogs and toads can live as well without as with a venous 
supply to the kidneys, provided that other conditions remain 
the same (Part II), and the kidneys of frogs deprived of the ven- 
ous supply can, ~ provided that other conditions remain the 
same, excrete urine equal in quantity and strength (nitrogen 
and chloride) to that excreted by normal frog kidneys (Part a 
ese results are confirmed by perfusion experiments (Part IT 
Gurwitsch is mistaken en he asserts that in a frog ee 
kidney deprived of its venous supply excretes less urine than 
the “ normal ’’ comparison kidney ; as a soi of fact, si vein- 
deprived kidney excretes more urine because the ‘“ 4 
idney is hindered by diminished flow in its guten capillaries 
due to increased pressure in its renal afferent vein. 
