vm THE EXCEETION OF URINE 437 



opinion, have been too dilute to produce such lesions in any part 

 of the urinary tubules; while if it had given rise to them, not 

 only the cells of the convoluted tubules, but also Bowman's 

 capsules, the straight tubules, etc., would have been implicated. 

 According to these authors the fact that the epithelium of the 

 convoluted tubules alone is attacked, indicates some special attrac- 

 tion of these cells to the above substance, which contradicts the 

 mechanical theory. 



A last fact which shows the inadequacy of the mechanical 

 theory and the necessity for active intervention of the secretory 

 epithelial cells of the kidney, is the formation of hippuric acid, 

 which does not pre-exist in the blood, and which (in dogs at any 

 rate) is exclusively formed by a synthetic process in the kidneys, 

 initiated by the vitality of the epithelium of the urinary tubules. 

 (See Chapter VII. p. 393.) 



V. These objections to the mechanical theory led to a revival of 

 Bowman's physiological theory, according to which the elimina- 

 tion of the specific constituents of urine (urea, uric acid, etc.) is 

 the effect of a specific vital activity of the epithelium of the con- 

 voluted tubules. But before this theory could be unconditionally 

 accepted it was necessary to obtain direct experimental evidence 

 that the vital activity of the epithelial cells of the convoluted 

 tubules and the ascending limb of Henle's loop (which, as we have 

 seen, possess all the morphological characters of specific secretory 

 cells) is expressed in an external secretion, i.e. by picking out the 

 urea, uric acid, etc., from the lymph, and excreting them into the 

 lumen of the tubules, and not in an internal secretion, i.e. re- 

 absorption of the water and part of the salts of the glomerular 

 filtrate, and return of them to the lymph, so as to condense the 

 filtrate, and bring about the concentration and osmotic pressure 

 proper to urine. 



Unfortunately it is not' possible, owing to the great diffusi- 

 bility and solubility of urea (the chief constituent of urine), to 

 follow its course through the kidney by micro-chemical reactions. 

 As regards uric acid, first Bowman and then v. Wittich described 

 the presence of crystals of this acid, in the epithelia of the con- 

 voluted tubules. If these observations had been confirmed, they 

 would certainly have afforded a strong presumption in favour of 

 Bowman's theory; but the later researches of Adolf Schmidt 

 (1890) showed that uric acid in a crystalline form is never seen 

 in the epithelial cells, even when, owing to ligation of the ureters, 

 the uriniferous tubules are charged with urates. On the other 

 hand, he saw that the urates in this case were never present in 

 the cavity of Bowman's capsule, but always along the tubule. 

 This fact is not, however, sufficient to demonstrate that the uric 

 acid is secreted by the cells of the tubules, because it is also 

 legitimate to assume that it is excreted by the glomeruli, and is 



