THE SECRETION OF THE URINE 499 



mechanism, and urea through the convoluted tubules (Underbill, 

 Wells, and Goldschmidt) . These results constitute a direct and 

 striking confirmation of the Bowman hypothesis. 



As regards our first two questions, we may conclude that there is 

 no good evidence that reabsorption of water or other constituents of the 

 urine in the renal tubules plays an important part in the preparation 

 of that secretion. Many facts favour the conclusion that the glomeruli 

 and the renal epithelium act as distinct, although, of course, mutually 

 supplementary mechanisms, the glomeruli separating the larger portion 

 of the water and salts, the epithelium the larger portion, if not the 

 whole, of the characteristic organic constituents. 



As regards the third question, it is now generally admitted, even 

 by those who uphold a modified ' mechanical ' theory, that if 

 the urine is originally separated from the blood by filtration at the 

 expense of the energy of the heart-beat represented by the pressure 

 of the blood in the glomeruli, the reabsorption in the tubules cannot 

 be attributed to simple diffusion, but must be a selective process 

 analogous to absorption in the intestine and entailing the expendi- 

 ture of a large amount of work at the expense of the food materials 

 or the protoplasm of the epithelial cells. Every attempt at a 

 strictly mechanical explanation breaks down for the kidney, as for 

 other glands. 



The practical absence from urine of the proteins and sugar of the 

 blood under normal circumstances, and the elimination by the 

 kidney of egg-albumin, peptone, and other bodies when injected 

 into the veins, show a selective permeability inexplicable by refer- 

 ence to any known structural or physico-chemical property of the 

 renal epithelium or the glomeruli, but precisely the kind of thing 

 which the physiologist has, without being hitherto able to explain 

 it, learnt' to associate with the activity of living cells. Urea and 

 dextrose, both highly diffusible substances, circulate side by side 

 in the bloodvessels of the kidney. The one is taken and the other 

 left. The urea is a waste-product of no further use in the economy. 

 The sugar is a valuable food-substance. The kidney selects with 

 unerring certainty the urea, of which only 4 parts in 10,000 are 

 present in the blood, but rejects the sugar, of which there is three 

 times as much. The theory that the dextrose of the blood or a 

 part of it is combined with substances in the colloid state, and not 

 in ordinary solution, has been advanced from time to time as an 

 explanation of the practical impermeability of the kidney for this" 

 sugar under normal conditions. But no proof of the truth of this 

 hypothesis has ever been given. On the contrary, there is good 

 evidence that all the dextrose which is estimated in blood by 

 analytical methods is in the free condition. For instance, dextrose 

 easily escapes from blood circulating in the vivi-diffusion apparatus 

 previously described (p. 48). And when the plasma of shed blood is 

 placed in a dialyser tube of animal membrane surrounded by a liquid 



