ELECTROPHYSIOLOGY 343 



and the latter not. By a perfectly reversible chemical reaction 

 is meant one that requires the application of the same amount of 

 energy to reverse it as that yielded by the original reaction. 

 Oxidation-reduction systems may be completely reversible, 

 partially reversible, or irreversible, with the fact kept in mind 

 that a system that under one set of conditions is completely 

 reversible may under another be partially reversible or irreversi- 

 ble. There is no conclusive evidence that any system is irre- 

 versible under all conditions. 



The reductant Fe++ has one more (negative) electron than the 

 oxidant Fe+++; an electron was transferred from the oxidant to 

 the reductant. This fact has led to careless reference to potential 

 and flow of current in living systems. In the first place, there 

 is no convincing evidence that electrons can occur free in aqueous 

 solution for any appreciable length of time. Metal conductors 

 carry a flow of electrons, but aqueous solutions require discrete 

 carriers of electrons (ions) for the conduction of electricity. It 

 appears, moreover, that there does not exist in living systems 

 any mechanism that is analogous to the metallic electron con- 

 ductor. (We shall question this statement in a moment.) This 

 being true, then any theory of electric potentials in tissues must 

 be based upon a different viewpoint from that which holds for 

 the outer circuit of a galvanic cell. We know that living systems 

 are demonstrably chemical systems in which material of high 

 chemical potential is degraded to material of low chemical 

 potential. This means that one may calculate a theoretical 

 electron pressure difference corresponding to the difference in 

 chemical potentials. An electron pressure difference is an electric 

 potential difference. But we have made one assumption, viz., 

 that the chemical energy, concerning the reality of which there 

 is no doubt, is converted into electrical energy in the living 

 system. Some of it is so converted when we pick it up and 

 measure it by means of metal electrodes, but this is not evidence 

 that the energy liberated in the living system is in the form of 

 electric energy. We may restate this thought as follows: In 

 spontaneous chemical reactions, there is a decrease in the level 

 of free energy. The energy thus liberated is available for work, 

 which may be mechanical, chemical, electrical, thermal, etc., 

 depending on the paths that happen to be available. The paths 

 represent mechanisms, and we are at liberty to postulate any 



