390 PROTOPLASMIC ACTION AND NERVOUS ACTION 



the current at its regions of entrance and exit also 

 traverses the plasma membrane. The current passing 

 lengthwise along the fibers of a tissue like a nerve is 

 undoubtedly subject to electrostatic retardation, as in 

 the analogous case of a cable; and Crehore and Williams^ 

 have calculated that the speed with which its variations 

 are transmitted would thus be reduced to a value similar 

 to that of the nerve impulse. It is evident that an upper 

 limit to the speed attainable by transmission of the kind 

 above is set by the speed with which variations of current- 

 intensity can be transmitted lengthwise along such a 

 conductor, since the general physical conditions present 

 in conductors with boundary surfaces having capacity 

 are undoubtedly present in protoplasmic structures like 

 nerves. Such conditions are common to all conducting 

 paths having a certain structure (core-conductors). 

 It would be erroneous, however, to infer that trans- 

 mission in nerve is identical with transmission along a 

 cable. The nerve impulse, or any other protoplasmic 

 excitation-wave, is an active process whose energy is 

 derived at each portion of its path from local chemical 

 reactions. Its electrical component furnishes the condi- 

 tions for transmission from each region to the next, 

 but does not constitute the whole process in the physi- 

 ological sense. Nevertheless the conclusion that certain 

 physical factors are common to both systems must on 

 general scientific grounds be regarded as correct. 



Since a portion ('' return path") of the bioelectric 

 current traverses the external medium, we should expect 

 that varying the electrical conductivity of the medium 



^ Crehore and Williams, Proceedings of the Society of Experimental 

 Biology and Medicine, XI (1913), 59. 



