496 EXCRETION 



of the kidney had been cauterized with nitrate of silver before in- 

 jection of the pigment, the spinal cord being left intact, a wedge of 

 the renal substance, corresponding to this area, remained coloured 

 only in the cortex, although the rest was blue in the medulla 

 also. The ' rodded ' epithelium was filled with blue granules as 

 before (Fig. 191). 



(i) shows that the epithelium is capable of excreting some sub- 

 stances at least. (2) appears to show that when the blood-pressure 

 is normal water is poured out from some part of the tubule, and 

 washes the pigment separated by the ' rodded ' epithelium down 

 towards the papillae. (3) suggests that it is through the glomeruli 

 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. It must be carefully noted that 

 these experiments do not prove that urea is secreted by the tubular 

 epithelium. Indeed, after section of the cord no accumulation 

 of urea takes place in the kidney (Cushny). It would be equally 

 erroneous to conclude from this that the cells of the tubules do not 

 secrete urea. For the reduction of the blood-supply may have 

 rendered them incapable of doing so. 



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 



