EXCRETION 205 



as independent "ions," is an example of the complications 

 which produce apparent departures from these laws. It must 

 further be added, and with emphasis, that, important though 

 it be that anyone who attempts to explain the interchanges 

 which occur between the various fluids of the body should be 

 conversant with the laws of osmosis, it is impracticable, and in 

 some cases misleading, to rigidly apply them. Living mem- 

 branes and dead membranes do not necessarily control diffusion 

 in the same manner. Still less do the laws which govern 

 diffusion through dead membranes hold good, without qualifica- 

 tion, to living cells. 



To return to the sanitary engineer whose opinion we asked 

 regarding the mode of working of the drainage system of the 

 kidney. Probably he would deny that the problems came 

 within his province. " They are not physical, but vital," he 

 would say. " I know nothing about the vital action of the 

 cells which line the tubule." Objection may be taken to the 

 form of expression, albeit he was fully justified in declining to 

 discuss the question any further. He does not know enough 

 about the internal structure of a cell to be able to predict the 

 phenomena of osmosis which will occur within it. No one can 

 say what capacity living cells may have of taking substances 

 from the blood, returning some of them, and excreting others. 

 This unknown capacity leads to results which, when they do 

 not appear to be in accordance with the laws of physics, 

 are commonly termed " vital." The term is a stumbling- 

 block which has tripped up generations of physiologists. The 

 expressions " vital action " and " physical phenomena " have 

 been used as if they were antithetical, whereas all vital 

 actions are physical phenomena. "Vital" in this sense connotes 

 " as yet unknown." Yet, in truth, there is abundant excuse 

 for the use of a term which covers ignorance, so long as its con- 

 notation is not extended until it assumes a positive, anti- 

 physical sense. " Physical " and " vital " are expressions 

 which point a contrast constantly present to a physiologist's 

 mind. He knows perfectly well that the passage of water and 

 salts through a membrane, and their passage into and out of a 

 living cell, are equally phenomena of osmosis. But the former 

 process he can test and measure in his laboratory ; the latter he 

 can but observe in much obscurity in the living body. He 



