TRANSMISSION AND CONDUCTION 207 



able nervous structure and to consider certain aspects of 

 the problem of conduction in nervous tissue. 



THE DEVELOPMENT OF EXCITATION AND CONDUCTION 



It was pointed out in chapter iv that protoplasmic 

 transmission in its most primitive form is probably 

 little or nothing more than the gradient in physio- 

 logical condition associated with the electrical gradient 

 which results from a local excitation. With the develop- 

 ment of protoplasmic excitability and conductivity, the 

 transmission of what we call impulses, which consist 

 of more or less definite waves of excitation, becomes 

 increasingly possible. And, finally, the specialization of 

 more or less definite paths affords the basis for a more 

 definite conduction, as distinguished from the general 

 protoplasmic transmission. 



It is a familiar fact that the excitation of living proto- 

 plasm depends not merely upon the amount of change in 

 action of an external factor, but also upon the rate of 

 change. In fact the rate of change is, in most cases, if 

 not always, more important than the amount. Similarly, 

 the rate of change at which the excitatory process takes 

 place at a given point is a factor of fundamental impor- 

 tance in determining its effectiveness as a means of 

 excitation of ot^ier points or regions, and so of trans- 

 mission. A slow excitatory process is likely to be rela- 

 tively ineffective, a rapid one much more effective. 

 The development of excitability and conductivity in 

 protoplasm very evidently consists in large measure of 

 an increase in rate of the excitatory process and of the 

 consequent increase in its effectiveness in exciting other 

 points. Such development is a process not only of 



