4 MODE OF PRODUCTION 



the production of this peculiarity, I will next pro- 

 ceed to sketch the train of reasoning by which I was 

 led to arrive at the discovery of a very simple rule 

 for its explanation. 



The first step to be taken is to have as clear a 

 view as possible of the nature of the mechanism by 

 which healthy secretion is performed. There are 

 certain difficulties and mysteries attending this vital 

 process which have not yet, and perhaps never will 

 be, fully understood ; but there are some facts now 

 pretty universally admitted with respect to secretion 

 in general, and that of the kidneys in particular, 

 which I may be allowed briefly to enumerate. 



1. All secreted fluids escape through the porous 

 coats of the capillaries into the excretory ducts, or 

 recipient cavities, where a free surface exists, and 

 not from the open mouths of exhalent vessels as was 

 formerly imagined. 



2. The principles of which some, if not all, secre- 

 tions are composed, exist previously in the mass of 

 the circulating fluid ; this is especially the case with 

 the secretion of the kidneys, and hence their function 

 would seem to be limited to the task of eliminating 

 or selecting from the blood, during its pase^e 

 through their capillaries, those substances, by the 

 union of which healthy urine is constituted. 



It is also probable that the structure of glands 

 exerts some influence on the nature of the secreted 

 products, otherwise the differences observed with 

 regard to the length, situation, and minute arrange- 

 ment of their constituent vessels and ducts would 

 not have existed, for we never find nature to employ 



