THE KIDNEY 47 



to adduce the first evidence that the 'corpuscle' functions 

 in the wholly passive manner of a minute filter, and to 

 propose that the filtrate thus formed is sufficient in quan- 

 tity to carry into the 'uriniferous tubes' all the waste 

 products, which are finally concentrated by the reabsorp- 

 tion of water during the passage of the filtrate down 

 these tubes. 



Early in the present century Ludwig's filtration theory 

 received strong support from E. H. Starling, to whom 

 physiology is indebted for many basic principles pertain- 

 ing to the circulation, and a httle later from A. R. 

 Cushny, who in 1917 published the first definitive work 

 on urine formation, which (though not so entitled) is 

 familiarly known to all students as the 'Modem Theory 

 of Urine Formation'— the word 'modem' here meaning 

 that, with Ludwig and Starling, Cushny visualized this 

 process in modem terms of physical chemistry, free from 

 the ancient vitalism that still lingered in renal physiology. 

 Between 1924 and 1938, the Ludwig-Cushny theory of 

 glomerular filtration was removed from the realm of 

 speculation to the category of demonstrated fact by 

 A. N. Richards, A. M. Walker, Jean Oliver, and their 

 collaborators, who in a brilliant series of studies on a 

 variety of animals collected the minute quantity of fluid 

 available in Bowman's capsule and at various points in 

 the tubule by means of a micro-pipette, subjected it to 

 precise, qualitative analysis, and showed that this fluid 

 did in fact have just such a composition as was required 

 by Ludwig's filtration theory. 



The definitive demonstration of tubular excretion was 

 afforded in the years between 1923 and 1934 by E. K. 

 Marshall, Jr., and his collaborators, first in frogs, rabbits, 

 and dogs, and then in a theory-shaking series of observa- 

 tions, made simultaneously with those of J. G. Edwards, 

 on the aglomemlar marine fishes in which urine forma- 

 tion is entirely dependent on tubular excretion. 



From 1930 on, renal physiology has been chiefly con- 



