en. xxxviu.] LUDWIG'S THEORY 577 



Ringer's fluid causes a copious flow of urine which is in its composi- 

 tion virtually Ringer's fluid. On physico-chemical grounds such a 

 flow would not necessarily demand work on the part of the kidney 

 cells, and as a matter of fact, estimation of the oxygen used during 

 such excretion reveals no measurable rise in the amount of work 

 performed. The general relationship shown in the table on p. 575, 

 between the pressure of blood in the kidney capillaries and the 

 volume of urine secreted, points also to the possibility that under 

 normal conditions filtration is a factor in urine formation. The cells 

 of Bowman's capsule, thin though they are, can, however, exercise 

 selective action. The simplest phase of this is that massive 

 molecules (protein) do not pass through them in health ; this, however, 

 is also true for films of gelatin. In addition to this, these living 

 cells may exercise a more active selection, as is seen in the type of 

 urine excreted by kidneys in which the tubules have undergone 

 necrosis as the result of injection of uranium salts. 



We may take next the point mentioned last, namely, that of 

 reabsorption. Carl Ludwig imagined that the urine filtered off at 

 the glomerulus becomes more concentrated as it descends the 

 tubule, and that the work of the cells in the tubule is to reabsorb 

 the water and pass it back into the blood. There is in our 

 opinion no satisfactory evidence that this occurs ; still it is not 

 possible to categorically deny its existence ; if it does occur it does 

 not take place on the massive scale * which Lud wig's views involve, 

 and is limited to the clear epithelium of the descending limb of 

 Henle's loop. The majority of physiologists do not admit even this, 

 and one piece of evidence which we owe to Brodie appears to me 

 to be quite conclusive ; it is this : if the pressure of urine in the 

 ureter is artificially raised by partially blocking it, absorption of 

 water back into the blood ought to be increased ; but as a matter 

 of fact this does not take place, but the exact contrary, for the flow 

 of urine provoked by the injection of sodium sulphate is more 

 abundant from the kidney with the partially blocked ureter, than 

 in the other kidney which serves as a control. 



This brings us then to our main conclusion concerning the 

 function of the epithelium of the tubular part of the apparatus. 

 Here undoubtedly the main function is secretion, and the obviously 

 secreting nature of the cells is revealed by microscopic examination. 

 The following experiments, among others, support this view : 

 (a) In frogs the glomeruli can be cut out of action by ligaturing the 

 renal artery ; the kidney is then supplied only by the renal portal 

 vein, a vessel which goes to the tubules only. If urea is then 

 injected under the skin, a secretion of urine occurs, which though 



* Ludwig's theory would involve the reabsorption of nearly 70 litres of water 

 in the day ! 



2 O 



