542 THE URINARY APPARATUS. [CH. xxxvi. 



that the cells, if they excrete the pigment, excrete urea and other 

 substances also. 



But the proof is not absolute, for the pigment is a foreign 

 substance. Urea is a very difficult substance to trace in this 

 way because it does not leave any coloured trail behind it. In 

 birds the place of urea is taken by uric acid, and the urates can 

 be actually traced, because they are deposited as crystals, and 

 can be seen in the cells and convoluted tubes much in the same 

 way as Heidenhain's blue pigment. 



Other experiments, however, have been undertaken to prove 

 the point for the case of urea. 



If the part of the cortex of the kidney which contains the 

 glomeruli is removed, urea still continues to be formed. This is 

 an additional proof that the excretion is performed by the portions 

 of the convoluted tubules that remain. 



By using the kidney of the frog or newt, which has two distinct 

 vascular supplies, one from the renal artery to the glomeruli, and 

 the other from the renal-portal vein to the convoluted tubes, 

 Nussbaum has shown that certain foreign substances, e.g. peptones 

 and sugar, when injected into the blood, are eliminated by the 

 glomeruli, and so are not got rid of when the renal arteries are 

 tied; whereas certain other substances, e.g. urea, when injected 

 into the blood, are eliminated by the convoluted tubes, even 

 when the renal arteries have been tied. These experiments 

 have, however, been subjected to considerable criticism, and 

 some observers have failed to obtain the same result. It is 

 a subject of so great importance that it demands complete 

 re-investigation. 



The Work done by the Kidney. 



Recent work by Starling, Hamburger, Dreser, and others has 

 shown the great importance a proper study of osmosis in the 

 body has in the understanding of many physiological facts. 



The subject is by no means a simple one, but the following 

 account of its bearing on urinary secretion (abstracted from 

 Starling) will not lead us into anything very abstruse. 



We have already seen that the urine is separated from the 

 blood by a process which is not the simple one called filtration. 

 This is further supported by the fact that it is possible to 

 measure the work of the kidney, and it is found to be vastly 

 greater than could be carried out by the intracapillary blood- 

 pressure. The following facts will also teach us that reabsorption 

 of water cannot, as Ludwig held, take place in the tubules. 



