THE SECRETION OF THE URINE 495 



tioned in Chapter XL suggest a different interpretation of these ob- 

 servations. And Boyd, who repeated Ribbert's work, obtained quite 

 different results after partial removal of the medulla. He found 

 it impossible to remove the whole. So that hitherto the direct 

 method of eliminating the tubules has left the matter where it was. 

 Some light has been thrown on this question, by taking advantage 

 of the anatomical fact that the kidney of batrachians, and, indeed, 

 that of fishes and ophidia as well, has a double blood-supply. The 

 renal artery gives off afferent vessels to the glomeruli; the vena 

 advehens, or renal portal vein, breaks up, like the portal vein in the 

 liver, into a plexus of capillaries surrounding the tubules, and there 

 seems to be no communication between the two vascular systems. 

 By tying all the arteries going to the kidneys in frogs the circula- 

 tion through the glomeruli can be completely cut off, while ligation 

 of the renal portal vein does not affect the blood-supply of the 

 glomeruli, though markedly interfering with that of the tubules. 

 Gurwitsch has found that, after ligation of the renal portal vein of 

 one kidney in (male) frogs, the flow of urine from that kidney is 

 much diminished as compared with the other. He argues that if 

 reabsorption of dilute urine filtered through the glomeruli takes 

 place in the tubules, the opposite result ought to be obtained, since 

 the glomeruli are not affected, while any absorptive power of the 

 tubules must be crippled or abolished. 



Experiments on the Excretion of Pigments by the Kidney. In 

 connection with the second question, and also incidentally with the 

 first, the results of experiments on the distribution of pigments in 



the kidney after their injection into 

 the blood have often been appealed to. 

 Heidenhain injected indigo - carmine 

 into the blood of rabbits, and after a 

 variable time killed them, cut out the 

 kidneys, and flushed them with alcohol, 

 in which the pigment is insoluble. His 

 results were as follows: (i) When the 



Fi f- I 9 I -- Dia s r f n * Distri ] )U - spinal cord was cut before the injec- 

 tion of Pigment in Kidney after ,f ., ,, , 

 injection into Blood. The cor- tlon m order to reduce the blood- 

 tex between a and 6 and be- pressure, the blue granules were found 



tween c and d was cauterized ifl the < roddec l epithelium of the 



before the injection. In the 1.1.1.1 j 



blank wedge-shaped portions, i, convoluted tubules and the ascending 



there was no pigment, in the limb of Henle's loop, and in the lumen 



zones shaded like 2 there was of the tu b u l eS , but nowhere else. ' 

 some pigment, but no* so much -.-. i , 



as in the areas shaded like 3 . Bowman s capsules contained no pig- 

 ment. The renal cortex was coloured 



blue. (2) When the spinal cord was not cut, the pigment was found 

 in the medulla and pelvis of the kidney, as well as in the cortex, 

 but always in the lumen of the tubules, and not in the epithelium, 

 except in the situations mentioned. (3) If a portion of the cortex 



