388 THE EXCRETIONS OF THE BODY 



For these and other reasons those authors who regard the process in the 

 glomeruli as a pure filtration are themselves inclined to explain the reabsorp- 

 tion from the urinary tubules postulated by this theory, as an active process 

 carried on in virtue of the vital properties of the parenchyma cells. By so 

 doing, however, the fundamental position of Ludwig's theory is surrendered, 

 for that theory set out to give a purely physico-chemical explanation of the 

 secretion of urine, without reference to vital processes. 



But can it be regarded as proved that a reabsorption of fluid passed through 

 the capsule actually takes place in the urinary tubules? Is it not possible 

 that we have here not an absorptive but a secretory process? 



In order to answer this question Heidenhain carried out some experiments 

 on the elimination of sodium indigosulphate, which is easily recognized in 

 microscopical preparations, and reached the conclusion that this salt is thrown 

 out by the epithelium of the urinary tubules. From analogy he concluded 

 that the same is true of urea and other specific constituents of the urine, and 

 that therefore the tubules have the function of enriching the fluid coming 

 from the capsule with solid constituents. 



Direct observations on the elimination of urea are not feasible because 

 there are no micro-chemical reactions by which urea can be recognized. But 

 in birds and reptiles it is not a difficult matter to demonstrate uric acid 

 microscopically. And yet investigators of the subject have not succeeded in 

 satisfactorily demonstrating uric acid within the epithelial cells. Hence, the 

 mode of separation of sodium indigosulphate cannot be regarded as deter- 

 minative for the elimination of the normal constituents of the urine. More- 

 over, the microscopical findings after injection of dyes are not harmonious, 

 for the pictures obtained have been regarded by other authors, like v. Sobier- 

 anski, as the indication of an absorptive process going on in the urinary 

 tubules. 



The following experimental fact speaks strongly for secretion by the epi- 

 thelium of the urinary tubules. The frog's kidney receives its blood vessels 

 partly from the renal artery, partly from the renal-portal vein. The former 

 provides the glomerulus, the latter the tubules. As was remarked by Nuss- 

 baum, and later verified by Beddard, the glomeruli or the urinary tubules can 

 be thrown out by tying off the one or the other of these vessels. After tying 

 the renal artery the flow of urine ceases entirely. If the fluid coming through 

 the capsule of Bowman during its passage along the tubule were to become 

 thicker by absorption of water, then tying the renal portal ought to produce 

 an increased flow of urine. But according to Gurewitsch this is not true; 

 instead, the quantity of urine is reduced by the operation. 



If this observation proves to be absolutely correct, it constitutes a conclu- 

 sive argument against the doctrine of absorption in the renal tubules. Accord- 

 ingly, the epithelium would have the function of taking up the specific con- 

 stituents of the urine from the blood and of delivering them to the urinary 

 tubules. 



We come therefore for the present to the following view, first expressed 

 by Bowman and further elaborated by Heidenhain, concerning the activity 

 of the kidneys. The cells covering the glomerulus give out water and salts 

 by a true process of secretion, those of the convoluted tubules and of 



