V 



once accepted as adequate. To Bowman these evidences of design 

 afforded complete satisfaction, but to Ludwig they suggested 

 rather a bundle of unsolved problems than an explanation. To him 

 the first step seemed to be to compare the discharge of the renal 

 secretion from the blood, with the filtration of liquid through a non- 

 living membrane of a similar character, in other words, to investigate 

 the then little-known process of liquid diffusion. It was by applying 

 the results of this preliminary physical inquiry to the kidney that he 

 was led to make the experiments which will ever remain the founda- 

 tion of knowledge on the subject to which they relate, as proving 

 that the two processes — the physical and the organic — are affected 

 in the same way by difference of pressure between the two sides of 

 the membrane. It was not asserted by him that these were the 

 only conditions, or even the most important. Consequently, when 

 Heidenhain, many years after, proved experimentally that the separa- 

 tion of the organic constituents of the urine from the blood was the 

 work of living cells, actuated rather by chemical than physical condi- 

 tions, Ludwig was able to regard the evidence not as a contradiction 

 of his own results, but as an addition to them. Having promulgated 

 no theory, but merely summed up the relations which his own experi- 

 ments had proved to exist, he had nothing to withdraw from, 

 nothing to object to. The strongest evidence of all against what was 

 supposed to be his theory of renal secretion was, indeed, obtained by 

 the aid of an experimental method already referred to, which was 

 par excellence his own — that of observing the function of an organ 

 in which the circulation is maintained artificially after it has been 

 separated from the body of the animal to which it belongs. For by 

 the application of this method it was more conclusively shown than 

 by any other, not merely that the secreting cells are the indispen- 

 sable agents in the separation of the organic constituents of the urine 

 from the blood, but that a sufficient supply of oxygen, as well as the 

 presence of urogenetic substances in the circulating blood, are con- 

 ditions essential to their activity. 



Before I attempt to give a general account of Ludwig's character 

 as a teacher and as a man of science, I must say a word or two about 

 his academical lectures, and about the only book, so far as I know, 

 that he ever published — the famous ' Lehrbuch.' The interest of the 

 lectures — often of a couple of hours' duration — consisted (as 1 have 

 learned from a distinguished pupil of Ludwig) in the fact that, what- 

 ever the subject, they were throughout illustrated by the results of 

 work originated by Ludwig himself, and done, often recently, in the 

 laboratory, so that everyone felt, " this is not hearsay ; the evidence 

 is here on the spot." It was this that made them of such value to 

 the mature student. 



As regards the 'Lehrbuch,' it is necessary to carry oneself back to 



