KIDNEY AND SKIN AS EXCRETORY ORGANS. 835 



blood, carrying with it not only the inorganic salts, but also the 

 specific elements (urea, etc.) of the secretion. There was thus 

 formed at the beginning of the uriniferous tubules a complete but 

 diluted urine, and in the subsequent passage of this liquid along the 

 convoluted tubes it became concentrated by diffusion with the more 

 concentrated lymph surrounding the outside of the tubules. 



Bowman's theory of urinary secretion, which has since been 

 vigorously supported and extended by Heidenhain, was based orig- 

 inally mainly on histological grounds. It assumes that in the 

 glomeruli water and inorganic salts are produced, while the urea 

 and related bodies are eliminated through the activity of the epi- 

 thelial cells in the convoluted tubes. 



The first of these theories (Ludwig) is sometimes spoken of as 

 the mechanical theory, since as originally proposed it attempted 

 to explain the formation and composition of the urine by reference 

 only to the physical forces of filtration and diffusion.* Adherents 

 of this view in recent years have modified it, however, to the extent 

 that the absorption supposed to take place in the convoluted tub- 

 ules is designated as a selective absorption, or selective diffusion, the 

 characteristics of which depend upon unknown peculiarities of struc- 

 ture in the epithelial cell, so that it is no longer a purely mechanical 

 theory. The difference between the mechanical and the secretory 

 theories may be stated briefly in this way. The former assumes that 

 in the glomerulus all of the constituents of the urine are produced 

 from the blood, probably by filtration, and that the function of the 

 epithelium lining the convoluted tubules is absorptive, like the 

 epithelium of the intestines, and not secretory. The Bowman view 

 as formulated by Heidenhain teaches that the glomerular epithe- 

 lium forms the water and salts of the urine by an act of secretion, 

 the ultimate chemistry or physics of which is not known. The 

 theory asserts that the epithelial cells participate actively in the 

 process of secretion and do not serve simply as a passive membrane. 

 The cells of the convoluted tubules are also secretory, their special 

 activity being limited mainly to the organic constituents, urea, etc., 

 although, in this respect, namely, in the precise distinction be- 

 tween the secretory products of the glomerular epithelium and those 

 of the convoluted tubules, the theory is not very explicit. Much 

 interest and a large literature have been stimulated by controver- 

 sies based on these theories, and to-day the facts accumulated are 

 not such as to demonstrate conclusively one view or the other, 

 although, on the whole, perhaps, it may be said that the majority of 



* Sir Lauder Brunton calls my attention to the fact that Ludwig, in some 

 of his earlier investigations (Ustimowitsch, Ludwig's "Arbeiten," 1870), 

 recognized the fact that the flow of urine through the glomeruli is influenced 

 by factors other than the mechanical pressure. He called attention especially 

 to the influence of the diuretic substances present in the blood, such as urea 

 and sodium chlorid. 



