436 PHYSIOLOGY CHAP. 



(on Dreser's calculation) have to admit the absurd conclusion that 

 they are capable of developing an energy six times greater than 

 that exerted by the human muscles, i.e. 8000 grins, per sq. cm. 

 So that if we hold with Ludwig that the glomerular filtrate is 

 concentrated and converted into urine along the renal tubules, we 

 are still bound to admit that this concentration is the effect not 

 of simple osmosis, but of a specific physiological activity of the 

 epithelial cells that line the tubes. 



Another obvious objection to the mechanical theory is the fact 

 that while in all animals the blood serum is alkaline, the urine on 

 the contrary (except that of herbivora) is acid. Is it possible to 

 explain this difference of reaction by a simple process of filtration 

 and osmosis ? Some authors maintain that it is, since it has been 

 proved that when an alkaline fluid which (like urine) contains 

 mono- and dibasic phosphates filters through an inert animal 

 membrane, the filtrate is acid because the monobasic phosphates 

 filter through it by preference. Thus the filtrate collected from 

 Bowman's capsules may be acid although it conies from alkaline 

 blood. This explanation, however, does not hold in face of 

 Dreser's experiments (1885). 



In order to decide the question he examined the micro- 

 chemical reaction of the various parts of the kidney in the frog, 

 using fuchsin (rubin-S) as an indicator, which loses its bright red 

 colour in alkaline solution. One to two hours after injection of a 

 concentrated solution of fuchsin into the lymph-sac of the frog the 

 acid urine secreted by the animal became red ; the kidneys were 

 colourless in the cortical region which contains the glomeruli, 

 while the tubules of the central part were stained red. From this 

 Dreser concluded that the filtrate of the glomeruli is alkaline, and 

 that the acidity of the urine depends on the secretory activity of 

 the epithelium of the convoluted tubules. 



This result is confirmed by the use of diuretics. If the filtrate 

 of the glomeruli is alkaline, and becomes acid along the tubules, 

 then the faster the urine passes through the tubules the more 

 rapidly will the reaction of the urine approximate to that of the 

 glomerular filtrate. This can be seen after the injection of a 

 diuretic, e.g. caffeine ; the urine first becomes neutral, and eventu- 

 ally is little less alkaline than the blood. We shall presently see 

 whether the urine becomes acid in the tubules owing to the 

 secretion in them of acid salts, or the reabsorption of alkaline 

 salts. 



Albertoni and Pisenti, after administering a few c.c. of acetone 

 in aqueous solution (1 : 3) to rabbits, for several days in succession, 

 found serious alterations in the epithelium of the convoluted 

 tubules only, while that of Bowman's capsule and of the straight 

 tubules was uninjured. If the acetone had been secreted along 

 with the water by the Malpighian glomerulus, it would, in their 



