THE SECRETION OF URINE 1195 



All these appearances are susceptible however to another explanation. 

 If indigo carmine is turned out by the glomerulus it will be so dilute that 

 unless very large doses are injected the glomerulus will not be stained. As 

 the glomerular transudate descends the tubules it undergoes concentration. 

 The precipitation of the dyestuff in the tubules may be simply a result of this 

 concentration, and the granular deposit in the cells may be evidence not 

 of secretion but of absorption of the dyestuff by the cells. In fact we 

 must acknowledge that the evidence for secretion by the cells of the con- 

 voluted tubules is very defective. Since all the microscopic appearances 

 observed after the injection of dyestuff are susceptible of two explanations, 

 there remains only the experiment of Nussbaum, as repeated by Bainbridge 

 and Beddard, as evidence of secretion on the part of the tubular epithelium ; 

 and this evidence would lose its weight if one or two glomeruli in the operated 

 kidney still received some blood supply, even though they failed to be 

 injected with the injection mass used at the end of the experiment for con- 

 trolling the completeness of occlusion of the renal arteries. 



ABSORPTION BY THE RENAL TUBULES. The experiments of Ribbert, 

 mentioned above, in which removal of the medullary portion of the 

 kidney led to the formation of an increased quantity of a more watery urine, 

 points to the possession by the tubules of a power of absorbing water. We 

 have other evidence that this power of resorption is not confined to water, 

 but may affect also the dissolved constituents of the glomerular transudate. 

 It was pointed out by Meyer that, if two salts such as sodium sulphate and 

 sodium chloride" were present at the same time in the glomerular transudate, 

 any process of resorption should affect chiefly the more diffusible salt, namely, 

 sodium chloride. Such a differential resorption would account for the much 

 greater-diuretic power of sodium sulphate as compared with sodium chloride. 

 In certain experiments Cushny produced a diuresis by the injection of 

 equal parts of equivalent NaCl and Na 2 S0 4 solutions into the veins of a 

 rabbit. An increased flow of urine was produced which lasted two hours 

 and a half. The chlorides of the urine rose with the diuresis and reached 

 their maximum at the height of the urinary flow. They then sank, and in 

 some experiments had practically disappeared from the urine towards the 

 end of the observation. The concentration of the sulphates however con- 

 tinued to rise in the urine to the end of the experiment. Thus in the first of 

 two identical experiments, when the rabbit was killed at the height of the 

 diuresis, the serum contained 0-547 per cent, chlorine and 0-259 per cent, 

 sulphate, while the urine contained 0-372 per cent, chlorine and 0-546 per 

 cent, sulphate. In the second, in which the rabbit was killed when the rate 

 of the urinary flow had considerably diminished, the serum contained 0-493 

 per cent, chlorine and 0-191 per cent, sulphate, while the urine contained 

 094 per cent, chlorine and 2-0 per cent, sulphate. These results are illus- 

 trated in Fig. 547. 



The difference between the two salts can be made still more striking if 

 the process of secretion be slowed by increasing the pressure within the 

 tubules by partial obstruction of one ureter. Thus in one experiment, 



