SEC. 2. THE SECRETION OF URINE. 



We have already called attention to the fact that the kidney, 

 unlike the other secreting organs which we have hitherto studied, 

 consists of two parts so distinct in structure that it seems im- 

 possible to resist the conclusion that their functions are different, 

 and that the mechanism by which the urine is secreted is of 

 a double kind. On the one hand the tubuli uriniferi with their 

 characteristic epithelium seem obviously to be actively secreting 

 structures comparable to the secreting alveoli of the salivary and 

 other glands. On the other hand the Malpighian capsules with 

 their glomeruli are organs of a peculiar nature with an almost 

 insignificant epithelium, and their structure irresistibly suggests 

 that they act rather as a filtering than as a truly secreting 

 mechanism. Hence the view put forward by Bowman long ago, 

 that certain constituents only of the urine are secreted after the 

 fashion of other secreting glands by the tubuli uriniferi, and that 

 the rest of the constituents, including a great * deal of the water 

 with such highly soluble and diffusible salts as preexist in con- 

 siderable quantity in the blood, are as it were filtered by the glome- 

 ruli of the Malpighian capsules. This view is moreover, as we 

 shall presently see, supported by direct experimental evidence. 

 Assuming for the present the truth of it, we may remark that 

 the passage of fluids and dissolved substances through membranes 

 being in large part directly dependent on pressure, the extent and 

 rapidity of that part of the whole process of the secretion of urine, 



