724 THE REACTION OF THE URINE 



the account of his experiments, which we have seen, at what interval 

 after ligature the injections were made, but presumably following 

 Nussbaum they were made soon after the operation. He con- 

 trolled his ligature by injection of a pigment and a microscopical 

 examination of the kidneys, and found in every case glomeruli in 

 circulation, but he adds that their number was so small as to be 

 a physiologically negligible factor. The conclusion to be drawn 

 from these repetitions of Nussbaum 's experiments seems to be that, 

 while they do not afford a final proof that the tubules excrete, 

 they bring forward strong evidence in favour of it. 



Gurwitsch experimented on frogs by a method which is the 

 converse of Nussbaum's. Having introduced urea into the intes- 

 tine and tied a canula into each ureter, he ligatured the reno- 

 portal vein and its tributaries to the kidney on one side. In 

 this the kidney circulation through the glomeruli would be undis- 

 turbed, but that round the tubules must have been reduced. In 

 two experiments he found that the kidney on the ligatured side 

 secreted O'o c.c. urine, and on the unligatured side '8 or 1 c.c. 

 This result affords evidence that the function of the tubule is not 

 absorbent, unless we imagine that the tubule had been stimulated 

 to absorb more water owing to the reduction of its blood supply. 

 He experimented also with indigo-carmine and other dyes, and 

 found that the tubule cells on the ligatured side were nearly free 

 from pigment, but on the unligatured side were deeply pigmented. 

 He admitted that small quantities of pigment were passed out 

 through the glomeruli, but concluded that pigments were passed 

 out chiefly by the tubules. 



The Reaction of the Urine. No fact in the secretion of urine 

 is more striking than the power which the kidney has of producing 

 an acid urine from alkaline blood. Numerous experiments have 

 been devised to discover where this change in reaction takes place 

 and the means by which it is brought about. 



The site of this change in reaction has been settled by Dreser's- 

 experiments. He injected into frogs a large variety of indicators 

 and examined their kidneys microscopically. His most successful 

 results were obtained with acid fuchsin, which is nearly colourless 

 in alkaline solutions and red in acid ones. After a single injection 

 of the dye into the dorsal lymph sac, he found that the urine 

 was red in an hour or two, and that the red colour was confined 

 to the fluid in the collecting tubules. After repeated injections 

 the glomeruli still remained colourless, but in the convoluted 



