364 Experiments on the Kidneys of the Frog. 



perfusing fluid, but this effect gradually passes off and, unless more caffein is 

 injected, the urine once more becomes hypotonic to the perfusing fluid. "We 

 found that if 0*1 per cent, caffein were continuously perfused through the 

 renal portal veins the urine remained isotonic with the arterial perfusing 

 fluid throughout the experiment. Unfortunately it is impossible to trace 

 the caffein histologically into tbe tubule cells, and we do not know whether 

 it attacks the whole length of the tubules or not. We regard these results, 

 therefore, as merely subsidiary to and confirmatory of those in which the 

 tubules were poisoned with mercury. 



The formation of a urine which is hypertonic to the perfusing fluid has 

 ■occasionally been observed after poisoning the tubules with corrosive 

 sublimate and even in the dead kidney. It occurs only when the formation 

 of urine is extremely slow ; we are not yet satisfied as to its significance. 



Summary and Conclusions. 



"When the frog's kidneys are perfused through the aorta and the renal 

 portal veins with oxygenated normal or hypotonic Ringer's solution, the 

 urine formed is hypotonic to the perfusing fluid and is derived entirely from 

 the glomeruli, since the tubules secrete no urine under these circumstances. 

 When the tubules are poisoned with corrosive sublimate or (temporarily) 

 with caffein, the urine becomes isotonic with the perfusing fluid. On the 

 •contrary, if the glomeruli are killed by the arterial perfusion of boiled 

 Ringer's solution, while the tubules still receive an adequate supply of 

 •oxygen through the renal portal veins, the urine formed continues to be 

 more dilute than the perfusing fluid. These results suggest, first, that the 

 glomeruli form by filtration a urine isotonic with the perfusing fluid, and, 

 secondly, that during the passage of the glomerular filtrate down the tubules 

 sodium chloride is absorbed by them. Whether any water is also absorbed 

 we do not know. 



The expenses of this research have been defrayed by a grant from the 

 Government Grant Committee of the Eoyal Society. 



REFERENCES. 



(1) Barcroft and Straub, ' Journ. Physiol.,' 1910, vol. 41, p. 145. 



(2) Beddard, 'Journ. Physiol.,' 1902, vol. 28, p. 20. 



(3) Bainbridge and Beddard, ' Bio-chem. Journ.,' 1906, vol. 1, p. 255. 



<4) Walter, ' Ann. Phys. Chem.,' vol. 38, p. 107 ; ' Journ. Chem. Soc.,' 1890, A, p. 202. 



