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 
