THE SECRETION OF URINE 693 



Heidenhain revived Bowman's view, restating it with greater 

 exactitude and placing it on an experimental basis. He has 

 expressed his own views as follows (1) In the kidney, as in all 

 other glands, the secretion depends upon the activity of special 

 secreting cells ; (2) the first kind of these cells is represented by 

 the single layer of epithelium covering the glomerular capillaries. 

 The function of these cells is to secrete water and such salts of 

 the urine as are found in solution throughout the body, e.g. sodium 

 chloride ; (3) the second system of secreting cells is represented 

 by the epithelial lining of the convoluted tubules and ascending 

 loops of Henle. They secrete the specific constituents of the 

 urine, and under certain circumstances water at the same time ; 

 (4) the degree of activity of both kinds of secreting cells is deter- 

 mined by (a) the amount of water or urinary constituent con- 

 tained in the blood, and (6) by the velocity of blood-flow through 

 the renal capillaries, since on this depends the supply to the cells 

 both of substances to be secreted and of oxygen ; (5) the great 

 variations in the composition of urine are explained by differ- 

 ences in the secretory activity of both kinds of cell, either com- 

 bined or relative to each other. 



These two views represent the chief of those current at present, 

 and those most diametrically opposed to each other. For, they 

 differ greatly with regard to the function of the glomerulus, and in 

 the case of the function of the tubule they are exactly opposite. 

 It might be thought that it would have been easy to devise 

 experiments which would have proved or disproved one of them. 

 This has not proved to be the case. It is necessary to point out 

 that these two views do not exhaust all the possibilities. Neither 

 of them may be true in outline, much less in detail. In both 

 views it is assumed that the function of the tubule is the same 

 throughout, either excretory or absorptive. When we turn to the 

 histology of the kidney tubule and see great variations in the 

 structure of its different parts, we conclude that there must be 

 corresponding differences of function. It is possible that in 

 accordance with one or other of these two views the function of 

 the different parts of the kidney tubule may be broadly the same, 

 either absorptive or excretory ; but it is, a priori, equally possible 

 that one part of the tubule might absorb and another excrete. 



The problem to be discussed resolves into two fundamental 

 points (1) How does the glomerulus act ? Is it a mechanical 



