FUNCTION OF TUBULE 163 



arguments against the supposition may be found. For instance, 

 the thin layer of epithelial cells is not strengthened in any way 

 to stand a large filtration pressure. Again it is doubtful whether 

 any such pressure exists in the glomeruli. Measurements are 

 given of general arterial pressure, say in the carotid artery. The 

 capillary pressure may be under r,th of this. 



Simple diffusion through a membrane impermeable to colloids 

 will answer as well. Increase in blood flow instead of pressure 

 regulates the amount of dialysate. Furthermore, it is generally 

 stated that capsular fluid has the same composition as blood, less 

 the colloids. No direct evidence of this has been produced. 

 Theoretically, it is not probable. Colloids have not only the 

 power of holding salts by adsorption, but globulin especially holds 

 water and sodium chloride in solution. This was proved by 

 Milroy and Donegan, who showed that even when 250 c.c. of water 

 per hour passed the outside of the collodion membrane, a solution 

 of globulin (containing 0-15 per cent, nitrogen) in 0-15 per cent, 

 sodium chloride, lost practically no salt in six hours. Any salt 

 over the amount mentioned rapidly passes through the mem- 

 brane. 



5. The bone of contention between the two modern schools is 

 the function of the tubule. One group holds that the tubular 

 epithelium absorbs water and threshold salts (i.e. salts of use to 

 the organism) from the fluid passing down the lumen. The other 

 group holds that salts are excreted into the lumen by its lining 

 epithelium. Much of the evidence produced is of equal value to 

 both sets of thinkers. Macallum'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 Nad-tension must be equal to the blood Nad-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 



