THE SECRETION OF THE URINE 491 



that most of the water passes. For cauterization has not destroyed 

 the power of the epithelium to excrete pigment, and therefore, 

 presumably, would not have destroyed its power to excrete water 

 if it possessed this power in any great degree; and the glomeruli 

 and their capsules are the only other part of the renal mechanism 

 which can have been affected. The fact that in birds and serpents, 

 whose urine is solid or semi-solid, the glomeruli are smaller than in 

 mammals is corroborative evidence that the glomeruli have to do 

 with the excretion of water. 



When pigments are injected into the dorsal lymph- sac of a frog 

 without interference with the renal circulation, they are found 

 plentifully in the lumen of the convoluted tubules, and also in the 

 epithelial cells lining them. The suggestion has been made that 

 the pigments have been absorbed by the cells from the lumen, and 

 not excreted by them into it. And certainly pigments soluble in 

 the cytoplasm or in the substances that form the envelopes of cells, 

 and therefore capable, like methylene blue, of staining them during 

 life, might be taken up by the renal epithelium if excreted into the 

 tubules by the glomeruli, and might cause staining of them, par- 

 ticularly, of course, of the free ends of the cells next the lumen. 

 But this suggestion is inadmissible, since, on injection of the same 

 pigments after ligation of the renal portal vein, the convoluted 

 tubules contain little or no pigment in their lumen. And when the 

 urinary flow is stopped on one side in mammals by temporary com- 

 pression of the renal artery, the corresponding kidney takes up fully 

 as much carmine as its fellow (Carter). There is no doubt that not 

 only pigments capable of ' vital staining/ like methylene blue, but 

 also pigments which do not stain living cells, are taken up from 

 the blood (or lymph) by the epithelial cells, and, lying in vacuoles 

 in their cytoplasm, are transported towards the lumen, and there 

 extruded. It is not the solubility of the pigments in lipoids, and 

 therefore their solubility in the supposed lipoid envelope of the cells, 

 which determines whether they shall be excreted. The degree in 

 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 



