444 PHYSIOLOGY CHAP. 



sides of the membrane are isotonic. This membrane is accordingly 

 permeable to water and to electrolytes (at least to sodium 

 chloride) ; it behaves passively, and does not perform any 

 osmotic work. 



(d) In its passage through the tubules the solution which was 

 at first isotonic with the blood becomes concentrated. The 

 osmotic equilibrium which prevailed at first between the two 

 solutions (blood and urine), separated by a semi-permeable 

 membrane (walls of the tubules), therefore alters, inasmuch as the 

 osmotic pressure of one of the solutions (urine) increases. The 

 work involved is performed by the tubules, which must therefore 

 possess the property of developing energy; for no analogous 

 phenomenon takes place with a passive, semi -permeable mem- 

 brane. This is the reason why on the death of the protoplasm 

 of the epithelial cells the higher concentration of the urine ceases 

 also. 



(e) The fundamental question as to the mechanism by which 

 this increase in concentration is effected by the epithelial ceils of 

 the uriniferous tubules, i.e. the controversy between the two 

 theories of Bowman-Heidenhain and Ludwig-Sobieranski, is not 

 solved by Galeotti, although many physiological and physical data 

 seem to him arguments in favour of the former. 



Another method of cutting out the uriniferous tubules was 

 attempted by Bottazzi and Onorato (1904-5). By injecting a 

 solution of sodium fluoride into dogs' kidneys through the ureters 

 they proved microscopically that only the epithelia of the tubules 

 suffered, and not the glomeruli, since the poisonous solution did 

 not reach them. The following are the principal results of their 

 experiments : 



While in dogs operated on by unilateral nephrectomy the urine 

 secreted by the remaining kidney has a very high osmotic pressure, 

 sometimes twice as great as the normal, the urine secreted after 

 poisoning with sodium fluoride has a much lower osmotic pressure 

 than the normal. The fall in concentration seems to be approxi- 

 mately proportional to the intensity of the alteration of the 

 tubules ; it is certainly progressive ; the concentration is minimal 

 shortly before the death of the animal. 



The amount of urine eliminated from the poisoned kidney 

 depends on the degree of intoxication. If the epithelium of the 

 tubules is so much damaged that it becomes detached from the 

 walls, and blocks the lumen, the amount of urine secreted is 

 diminished : if, on the contrary, the epithelium is only functionally 

 injured and not detached, the amount of urine secreted is in excess 

 of the normal. This last fact can be interpreted in favour of 

 Bowman's theory. Bottazzi and Onorato assume that the 

 abnormal dilatation of the blood capillaries and excessive con- 

 gestion of the kidney poisoned with sodium fluoride cause a rise in 



