THE SECRETION OF THE URINE 497 



which they are capable of being presented to the cells in non-colloid 

 solution appears to some extent to be a determining factor. The 

 pigments not taken up are highly colloidal (Gurwitsch, Hober). 

 Shafer has recently confirmed Heidenhain's statements as to the 

 place of excretion of indigo-carmine. When leuco-indigo-carmine 

 (a colourless reduction-product of indigo-carmine) was injected, the 

 blue oxidized substance was found in the lumen of the convoluted 

 tubules and in the collecting tubules, but not at all in the Bow- 

 man's capsule. The cells of the convoluted tubules were colour- 

 less, because they kept the pigment in its reduced condition, and it 

 only became oxidized in the lumina of those parts of the tubules 

 whose contents, according to Dreser, show an acid reaction. On oxi- 

 dation by peroxide of hydrogen the cells of the convoluted tubules 

 became faintly green, but the Bowman's capsule remained colourless. 

 This can only be explained on the assumption that the leuco-product 

 of the pigment was excreted by the cells of the convoluted tubules. 



But these cells are far from taking up all pigments indifferently. 

 Some pigments are extruded mainly by one part, others mainly by 

 another part, of the renal tubule, and some even by the glomeruli, 

 as shown long ago for ammonium carminate. The glomeruli, how- 

 ever, are in general far less active in this regard than the epithelial 

 cells, and the fact that the latter pick out from the blood such sub- 

 stances as these foreign pigments, which pass through the Mal- 

 pighian tufts unchallenged, renders it likely that the tubules also 

 exercise a special function in the secretion of the normal con- 

 stituents of urine. More direct evidence of this is not wanting, 

 for Bowman saw crystals of uric acid in the epithelium of the 

 convoluted tubules of birds. Heidenhain found that urate of soda 

 injected into the blood of a rabbit is excreted by the epithelium of 

 the convoluted tubules and the ascending part of Henle's loop, 

 just as is the case with indigo-carmine. And Nussbaum's experi- 

 ments, although not quite conclusive, have made it probable that 

 in the frog urea is actually separated by the epithelium of the 

 tubules. They were founded on the anatomical peculiarity in the 

 renal circulation of the frog already mentioned. By tying the renal 

 arteries in that animal, he thought he could at will stop the circula- 

 tion in the glomeruli, and he found that after this was done there 

 was no further spontaneous secretion of urine. But when urea was 

 injected intravenously the secretion of urine again began, urea 

 being eliminated by the kidneys, and water along with it. Sugar; 

 peptone, and egg-albumin, injected into the blood, no longer passed 

 into the urine, even when the secretion was excited by simultaneous 

 injection of urea, although they readily did so when the arteries 

 were not tied. He concluded that the Malpighian corpuscles have 

 the power of excreting water, sugar, peptone, and albumin, while 

 the epithelium of the tubules excretes urea as well as water. 



Beddard has confirmed Nussbaum's statement that when all th<? 



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