KIDNEY AND SKIN AS EXCRETORY ORGANS. 743 



posed that in these structures water filtered through from the 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 con- 

 voluted 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 tubules 

 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 Ludwig and the Bowman 

 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, but which 

 implies that the epithelial cells participate in the process and do 

 not act 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 between 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 the controversies between 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 physiologists adhere to the 

 more conservative view of Bowman-Heidenhain to the extent at 

 least of recognizing that the physical laws of filtration, diffusion, 

 and inhibition, so far as they are known, do not suffice for a satis- 



