EXCITATION AND TRANSMISSION 79 



degree of correlation and mutual dependence exists 

 among the essential conditions and reactions, change in 

 any essential factor cannot proceed very far without 

 involving changes in other factors. Undoubtedly, 

 therefore, oxidation is not independent of the condition 

 of membranes or phase boundaries and, on the other 

 hand, the condition of these is not independent of 

 oxidation. In other words the most primitive and the 

 most highly specialized forms of excitation and trans- 

 mission in protoplasms probably involve much the 

 same complex of factors, though the specialization of 

 different protoplasmic mechanisms may determine various 

 modifications of the general process with different factors 

 predominant in different mechanisms. It is possible, 

 for example, that in nervous excitation and conduction 

 oxidation may play a role of minor importance, and it is 

 even conceivable that in certain protoplasmic mechan- 

 isms some other reaction than oxidation may be involved. 

 Again, in some mechanisms the plasma membrane and 

 its changes may be of fundamental importance, while in 

 others this factor may perhaps be secondary and inci- 

 dental to others. Unquestionably the protoplasmic 

 system affords a basis for the specialization of various 

 mechanisms of excitation and transmission, but it does 

 not follow that the primitive process is exactly like 

 any one of the more highly specialized processes. The 

 general problem has apparently been somewhat obscured 

 by the fact that a large part of the work on excitation 

 and transmission has been concerned with the process as 

 it occurs in the muscle and nerve of the higher animals, 

 and this work has constituted the chief basis for gen- 

 eralization. It is commonly held, for example, that 



