ELECT ROPHYSIOLOGY 339 



protoplasm of the starfish egg, a value equivalent to a 0.25 M 

 potassium chloride solution. 



Potentials in General. — There are a number of types of 

 electric potential which are definite in character, in that we know 

 exactly to what they are due and under what conditions they 

 may be expected to arise. This is true of the potentials ordinarily 

 studied by the physicist, such as that between a metal and its 

 surrounding solution, which is evaluated by determining the 

 difference in potential between two such electrodes. Equally 

 definite is the potential existing between the outlet and intake 

 of a dynamo. In colloidal and biological systems, the situation 

 is not so simple. Most potentials there are due to a combination 

 of electric forces. 



Eucken and his translators Jette and La Mer have given a 

 satisfactory treatment of potentials from the physicochemical 

 point of view, but it is incomplete from the colloidal and biological 

 viewpoints. The following is a modified and augmented 

 classification: 



Electrode potential, metal/solution. 



Concentration potential, solution /solution. 



Diffusion potential, liquid/liquid (miscible). 



Liquid junction potential, liquid /liquid (immiscible). 



Oxidation-reduction potential, oxidizing system /reducing system. 



Membrane potential, solution/membrane/solution. 



Injury potential, injured tissue/normal tissue. 



Electrode Potentials. — The potential difference at the interface 

 of a metal in contact wih a solution was first mathematically 

 expressed by Nernst (1889). The 

 assumption is made that the metal goes 

 very slightly into solution. It gives off 

 ions which are held at its surface (in 

 the interface) so that a double layer 

 of ions is formed, with a resulting charge 

 on the electrode (Fig. 151). - ++ 



Such an electrode potential as the ; — ; — , ,— — 



, 1 r Fig. 151. — The double layer 



preceding exists on the two poles Ot an ^f i^ns formed at the surface 



electric cell, e.g., the copper and zinc of a metal electrode within an 

 , . , . 11 -r. , electrolytic solution. 



electrodes of a galvanic cell. Between 



these electrodes there is a potential difference which is designated 



as the electromotive force (e.m.f.) of the cell. Anions migrate in 



+ 1 I + 



- + 1 I + 



— + 11+ — 



+1 1+ - 



+1 1+ 



+ 1 I + 



— — +1 1+ ~" 



-+i 1+ — — 



+ 1 1+ 



