578 THE URINARY APPARATUS [CH. 



scanty in. amount, is peculiarly rich in urea. Urea, therefore, in the 

 frog is secreted by the epithelium of the tubules. In order to 

 obtain this result, the kidney must receive also sufficient oxygen for 

 the maintenance of the functional activity of its cells; as the 

 arterial supply is cut off by ligature of the renal artery, this must 

 be accomplished in some other way, for instance, by keeping the frog 

 during the experiment in an atmosphere of pure oxygen. 



(&) The same result is reached in the frog in another way. 

 The renal portal system of the frog's kidney may be artificially 

 perfused with oxygenated Kinger's solution, the renal arteries as 

 before being ligatured ; if certain diuretics are added to the solution 

 (caffeine, urea, phloridzin, sodium sulphate, etc.), these induce 

 secretion which is accompanied by a marked increase in the oxygen 

 consumption of the kidney. 



(c) In mammals the arrangement of the blood-vessels is different, 

 and it is not possible to isolate the tubules as in the frog. 

 Nevertheless, the same diuretics cause a copious flow of urine, which 

 is accompanied in all cases by the evidence of increased work on the 

 part of the kidney cells, namely, a large rise in the amount of 

 oxygen used up. In all these cases the urine secreted is unlike the 

 plasma in composition, and varies somewhat with the diuretic 

 employed ; for instance, injection of sodium sulphate produces a urine 

 almost devoid of chlorides. 



(d) In mammals also it is possible to trace certain substances 

 with the eye. A research of great importance was performed along 

 this line by Heidenhain. By cutting the spinal cord he removed 

 the arterial tone from the whole visceral area, and consequently 

 produced a blood-pressure so low that no fluid came down the 

 tubules. Sulphindigotate of soda is a nitrogenous substance, and 

 it can easily be recognised by its blue colour. If it is injected into 

 the blood-stream it is excreted by the liver and kidney, and on 

 post-mortem examination of the kidney, it is found in those cells of 

 the tubules which bear the impress of secreting cells, whereas it is 

 absent from the cells of Bowman's capsule. 



Numerous hypotheses have been put forward to explain all 

 these results, and the two great historic theories are those of 

 Ludwig and of Bowman. The gist of Lud wig's views will have 

 been gathered from what has just been said ; he supposed that 

 the urine filtered through at the glomeruli is a fluid of the 

 same general composition as the blood-plasma, so far as its 

 crystalline constituents are concerned, and that this fluid is gradu- 

 ally turned into urine as it travels along the tubules by the 

 absorption from it into the blood of water and certain salts. 

 That fluid isotomic with blood-plasma can or does filter through 

 the flat epithelium cells of Bowman's capsule appears to be true, but 



