1922. ] ‘“* Renal Portal”’ System. 135 
(though the arterial “ head of fluid ”’ pressure remains con- 
stant) and cause increased output of urine, provide still fur- 
ther proof of the statement that fluid pressure in the glome- 
rulus is not concerned with the production of quantity of 
urine 
Si ince the results of experiments dealing with filtration 
through animal membranes prove that the quantity of filtrate 
‘rises with the pressure (though in lower ratio in most cases) 
and that with a fluid devoid of unfiltera ble constituents, rate 
It may however be objected that Gltration by the midney, ; 
i.e. the production of urine more or less identical in composi 
tion with the perfusing fluid, or at least the portion of it cathe. 
ed, undoubtedly does occur under certain conditions, as e.g. 
Ww 
quantities as to render mae insignificant the colloid content 
of the blood, or when Ringer’s fluid or simple saline is perfused 
through the kidneys at a certain pressure, but to admit this 
is a 
that this filtration, like all other forms of kidney pBke ea 
occurs through the walls of the tubules, which, un 
peculiar conditions, fail to modify the composition of the per- 
fusing fluid. Filtration in the dead kidney certainly occurs 
through the tubule walls, as may be seen when a kidney is first 
perfused via the artery with say chromic or picric acid fixative 
(the filtrate exuding from the ureter being small in quantity 
and soon ceasing) and afterwards perfused in the reverse direc- 
tion via the renal vein (when the filtrate is large in quantity 
and continues for along. time), and if this be the case in the 
dead kidney. what reason is there for Tia daa that it is not 
the tubules which filter when the same results are ica in 
& similar experiment with saline on a living kidney ? 




! It has been argued that the ee that the dead kidne: ney can 
pass fluid from the vessels into the tubules is ee thatthe *‘ urine ”’ of 
ordinary saline — usion wi penn one oa mere filtrate, and nota true 
secretion. But, a cording to o Cus me eo t 1 ict nas d urine i- itself only 
a filtrate (modified by ption) and rt ithe neo-Ludwig 
the result f perfus ion experiments on that 
score, What sie have. failed to ecognize is that the — comes from 
the tubules and not the sna The chief point to notice however is 
that what is sole a aap (i.e. passage of substances without con- 
centration) does occur under abnormal conditions even in the living 
organism (as when the blood con raged: be great excess of water) ‘and. af = 
dead kidney is proved to filter 
the living kidney almost certainly does likewise. To argue that the anid 
y the ureter in saline perfusion secretion 
n the sense that ast a urine is, is to suppose that the kidney is 


