FUNCTIONAL INERTIA AS LATENT PERIODS, &c. 25 



Cell " does not hesitate to use the word " inertia " 

 in connection with refractory period of nerve-cells. 

 To this inertia he attributes the fact that the second 

 of two stimuli arriving within a " certain fraction of 

 a second " is disregarded, and that if two different 

 sense-organs are simultaneously stimulated, two 

 sensations are not perceived ; there must be an 

 interval of time greater than a certain minimum 

 between the two (differential time) in order that 

 both sensations be appreciated. These remarks I 

 had not seen when I wrote my first paper. 



Refractory period or period of neural insuscepti- 

 bility must underlie all such phenomena as insensitive- 

 ness to increments in strength of stimulus as is 

 involved in the Weber-Fechner Law, non-response to 

 maximal stimuli, insensitiveness to stimuli beyond 

 the limits in which the Law is applicable, and 

 several other phenomena involving consciousness 

 which will be taken up in the chapter on psychic 

 inertia. 



The central nervous system affords us many 

 examples of the inertial property : this ought not to 

 surprise us in a system in which functional differen- 

 tiation has been carried so far. In it we have a wide 

 range of graded responses towards environmental 

 stimuli, and also a not less interesting series of all kinds 

 of non-responses, insusceptibilities, and latencies. 

 Certain idiosyncratic insusceptibilities of the nervous 

 system towards drugs, come under this head ; for 

 instance, the considerable differences in suscepti- 

 bility to nicotine, alcohol, morphia, cocaine. The 



