VITALISM 19 



see that in many instances simple mechanical 

 theories of various physiological processes 

 were put forward without sufficient experi 

 mental evidence of their correctness. Sub 

 sequent investigation has often shown that 

 these theories were incorrect, and this result 

 has seemed to be a justification for returning 

 to the prevalent vitalistic ideas of earlier 

 times. 



As an instance of this we may take 

 Ludwig's early theory as to the process by 

 which urine is secreted. He supposed that 

 urine is separated from the blood in the 

 glomeruli of the kidney by a process of 

 simple filtration, and that the dilute liquid 

 thus formed is concentrated during its 

 passage down the tubules by a process of 

 osmosis, in which water passes from the 

 tubules back into the blood, leaving the fully 

 formed urine as a relatively concentrated 

 liquid which passes out towards the ureter. 

 This theory turned out, when experimentally 

 tested, to be far from correct. It is the cells 

 lining the tubules of the kidney which are 

 the active agents in the secretion of urine ; 

 and they act against osmotic pressure, picking 



