SEC. 2. THE SECRETION OF URINE. 



329. The kidney, 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 

 the functions of the two parts 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 in- 

 significant 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 con- 

 stituents, including a great deal of the water with such highly 

 soluble and diffusible salts as pre-exist 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 glome- 

 rulus 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 

 arrangements ; for these are extremely favourable to a full and 



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