SEC. 3. THE SECRETION OF URINE. 



409. The facts which we have learnt in a preceding section 

 concerning 1 the structure of the kidney have shewn us that that 

 organ, unlike the other secreting organs which we have hitherto 

 studied, consists of two parts, so distinct in structure that it seems 

 impossible 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 what may be called in a general 

 way a filtering than as a truly secreting mechanism. Hence has 

 arisen the view, which frequently bears the name of Bowman since 

 he was the first to put it forward, 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 adequate quantity in the blood, are as 

 it were filtered off by the glomeruli of the Malpighian capsules. 

 We shall see later on reason to doubt whether we are justified 

 in applying the term 'filtration,' which has a definite physical 

 meaning, to the process by which water and other substances pass 

 from the blood vessels of the glomerulus into the lumen of the 

 tubule ; for that process is as we shall find peculiar and complex. 

 But such a doubt need not prevent us from recognizing that the 

 whole act of secretion of urine consists of two parts, one of which 

 is much more closely dependent on the flow of blood through the 

 kidney than is the ordinary process of secretion such as has 

 hitherto come before us, and another part which seems to bear the 

 same relation to the flow of blood as does ordinary secretion. 



That the work of the kidney is to an unusual degree dependent 

 on the flow of blood through it seems suggested by the vascular 



