THE KIDNEY. 219 



shaken occasionally, but not used for a week or more. The affinity that water 

 has for anhydrous sulphate of copper is greater than that of the alcohol, and 

 the latter readily gives it up. As soon as the anhydrous sulphate regains its 

 water of crystallization it assumes a blue color again. 



Everywhere in the sections it will be seen that the glomeruli 

 or their capsules are entirely free from color, while all the 

 tubules possessing the rod-epithelium have a more or less blue 

 color, according to the quantity of indigo-carmine excreted. 

 The lumina of the convoluted and other tubules are generally 

 filled with the crystallized indigo-salt. In examining sections, 

 it soon becomes evident that the convoluted tubules and that 

 part of Henle's loop which possesses the rod-epithelium, alone 

 excrete the indigo-salt, while the other tubules merely contain 

 it in their lumen, the salt having been washed down, as it were, 

 from above by the water filtered through the capillaries of the 

 glomeruli. 



Instead of using the sulphindigate of soda, Heidenhain, in his second series 

 of experiments, substituted a solution of uric acid in caustic soda. The renal 

 artery was injected with alcohol containing acetic acid. The result showed 

 that urate of soda, like the indigo-salt, was excreted only by the tubules pos- 

 sessing the rod- epithelium. The capsules were entirely free. The addition of 

 acetic acid to the alcohol caused the uric acid to be precipitated in the shape 

 of rhomboid crystals within the tubules. In this condition Heidenhain found 

 them. The hypothesis set down by Bowman, years ago, that the tubules of 

 the kidney excrete the solid constituents of the urine merely, while the glome- 

 ruli serve as a filter for the fluid portion, is therefore correct. 



If the quantity of indigo solution injected into the jugular 

 be small, and the animal killed soon after, the kidney being 

 treated as above detailed, the microscopic sections exhibit 

 the following appearance : glomerulus and capsule are not 

 acted upon ; the narrower branch of the loop and the collect- 

 ing tubules are free from any crystallized salt, and their epi- 

 thelium clear. In the convoluted tubules and the broad part 

 of the loop, the following phenomena may also be observed : 

 their lumina are entirely free from any deposit of indigo-car- 

 mine, though here and there the rod-epithelium is not stained. 

 In the greater number it is colored of a light blue color. In some 

 the rods and nuclei are uniformly stained ; in others the rods 

 alone show the blue color, while the nuclei are not stained. 

 This constitutes the first stage of the excretion of indigo-car- 



