TIIRKSUOLD VALVES JOI 



oi' ccjLuil value to botli sets of thinkers. Maeallum's work, already 

 mentioned, showing that a constant low surface tension was 

 maintained at the cell-lumen interface rather weights the scales in 

 favour of the second view. There is also no doubt that experiments 

 where dyes, etc., are injected show that matter does pass, under 

 these conditions, from tubular capillary through the tubular cells to 

 the lumen of the convoluted tubule. On the other hand, there is 

 nothing to hinder the reverse process from taking place if need 

 arises. Consider the cell as middleman between blood and 

 secretion. Any abnormality in the blood would produce an 

 alteration in the cell, which, if it could, would pass on the change 

 to the secretion. Let us take a concrete example. Say there is 

 a deficit in NaCl in the tubular capillary. As a result, because 

 the cell NaCl-ten*?'o/? must be equal to the blood ^a£\-tension, 

 salt will pass from the cell to the blood. Similarly, if the dialysate 

 or filtrate in the lumen has any NaCl at all, some of it will pass 

 into the cell to make up the deficit. The experiments of Milroy 

 and Donnegan, already referred to, contain a series demonstrating 

 the passing of NaCl from an aqueous solution to one containing a 

 globulin. 



Cushny's obstructed ureter experiment referred to abo\'e may 

 be interpreted in this light. Assuming, as he does, that the fluids 

 coming from both capsules are identical in both kidneys, then on 

 the obstructed side there is a fluid in prolonged contact with the 

 tubular epithelium, while on the other, this fluid passes away more 

 or less rapidly. Take for granted, for the sake of a standard, 

 that the freely passed urine remains unaltered, that is, it is equal 

 to the glomerular filtrate on the obstructed side — e.g., 24 grms. 

 water, 0-08 grm. NaCl and 0-11 grms. NaaSO^. Sodium chloride 

 is more diffusible than sulphate and readily penetrates cells, there- 

 fore the positive tension of NaCl in the tubule wdll cause some 

 NaCl and water to enter the lining cells and so into the blood 

 stream. The sulphate, not being so diffusible, will not so enter 

 the cell. Thus the result would be a concentration of the urine 

 with a decrease in chloride, i.e., with sulphate steady ; chloride 

 would drop to a quarter = 0-02 and water to a half = 12 grams. 

 But no great energy need be expended here, only sufficient to 

 evaporate urine to half its bulk. The water and salt so secreted 

 into the blood stream would cause a further diuresis and so on. 

 On the other hypothesis, viz. that the sulphate is to a great extent 

 secreted by the tubular epithelium into the lumen, this difficulty 

 does not to the same extent arise. 



In short, two factors may come into play in the secretion of 

 urine, {a) the adjustment of the cell to alterations in its environ- 



