1922. ] ** Renal Portal’’ System. 127 
assumption is untenable. We have shown repeatedly in 
Part I and in the present Part III that when the fluid in the 
intertubular plexus does become changed as the result of 
excess of fluid pressure in the renal venous meshwork, causing 
actual admixture of the venous blood or fluid with the arterial 
blood or fluid in the intertubular plexus proper, then the urine 
immediately indicates the admixture by an alteration in 
co 
result of such admixture between the blood in the renal venous 
meshwork and the lexus, because it is impossible, with such a 
Pentel high pressure in the glomoruli, for the venous fluid 
affect the blood or fluid contained in the glomeruli (Part I). 
Ohne of we of the blood or fluid in the intertubular 
plexus does then affect the composition of the urine secret 
and this i is proof that the tubules secrete the urine, because no 
in the venous blood but absent from the arterial blood (dye 
and ferrocyanide present only in the renal afferent vein supply 
e. 
” That the tubules can and do secrete urine, has already 
injecting strong diuretics into e sacs) urin 
measurable quantity (ie. at least 0°5 ¢.c.) in 11 out of 22 
experiments. This urine was found to contain “‘ most of the 
normal urinary constituents ’’—all that care tested for. Since 
in these experiments an arterial Sieh was absent, it was 
niles from the point of view of deciding whether the 
urme excreted was tubule or agecleely ived, to determine 
whether or not the blood supplied by the two renal afferent 
veins ever intruded so far into the intertubular capillary 
plexus as to reach any glomeruli. This necessity was 
apparently complied with when, at the end of each 
ut (from the aorta) with saline “‘ and then thoroughly injected 
with a saturated solution of soluble Prussian blue ” (presumably 
at a pressure not less than normal arterial pressure). This 
injection was made ty the authors apparently solely in order 
to be certain that all the branches of the renal arteries 
been severed by cautery and they did pier recognize the 
souaiiey of the dye reaching the glomeruli from the 
intertubular capillary plexus; nevertheless the injection 
served both purposes, provided that the dye was injected at 
a pressure not less than the normal arterial pressure. In 6 out 
