304 PROTOPLASMIC ACTION AND NERVOUS ACTION 



solution in the same manner as the potential difference 

 between a metallic electrode and its adjoining solution 

 (e.g., Zn in ZnS04 solution). When the demarcation 

 potential between the cut surface of the sciatic nerve 

 and an uninjured area one centimeter distant was 

 measured (by the usual compensation arrangement), 

 after leaving the nerve for five minutes in differently 

 concentrated solutions of a given salt, results of the 

 following kind were obtained. (The potential in the 

 original physiological salt solution is represented in the 

 bracketed expression by E.) 



«;^i„f,v„ Demarcation 



S°^"^°° Potential 



m KCL o. lo (=£ log I. 2) 



m/2 o . 34 ( = £ log 2 . 2) 



m/4 0.6 {=E log 4) 



m/8 0.9 (=£log 8) 



It will be noted that the potential difference increases 

 with increasing dilution, and very nearly in direct 

 proportion to the logarithm of the dilution. This is the 

 characteristic relation found in electrode potentials, and 



expressed by Nernst in the formula E = RT log — where 



C2 



Ci represents the concentration corresponding to zero 

 potential (equivalent to that required to compensate the 

 ionic solution-pressure of the metal) , and C2 the concentra- 

 tion of the ions in solution; thus with zinc in contact 

 with ZnS04 solution, the potential decreases with increase 

 in the concentration of the zinc ions in a logarithmic 

 curve. The fact that a similar relation is found with 

 the living tissue, and that the results obtained are 

 reversible, as Macdonald found, shows that the living 

 tissue behaves as if its surface were an electrode reversible 



