460 A MANUAL OF PHYSIOLOGY 



ments 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 Bowman's capsule. The cells of the con- 

 voluted tubules were colourless, 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 oxidation 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, however, 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 substances as these foreign pigments which pass 

 through the Malpighian tufts unchallenged, renders it likely that 

 the tubules also exercise a special function in the secretion of the 

 normal constituents of urine. More direct evidence of this is 

 not wanting, for Bowman saw crystals of uric acid in the epi- 

 thelium 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 experiments, 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 circulation in the glomeruli, and 

 he found that after this was done there was no further spon- 

 taneous secretion of urine. But when urea was injected intra- 

 venously 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 

 the arteries going to the kidney are tied the glomeruli are com- 

 pletely and permanently deprived of blood. The spontaneous 



