258 THE NERVOUS SYSTEM 



must it last in order to be effective. The following figures 

 by Keith Lucas show this : — 



Duration of cvirrent Strength of current 



in seconds. in volts. 



00 -086 



•00087 -179 



Here the smallest current which is effective, given unhmited 

 time, is '086 volt. When the strength of current is doubled 

 the minimum duration required is -00087 sec. This figure 

 was called by l/ucas the excitation-time. He found it to 

 be smaller in nerve-fibre than in muscle, and much smaller, 

 again, in the nerve-ending. 



Factors influencing the Activity of the Nerve-fibre 



The two properties possessed by the nerve-fibre — 

 excitability or the capacity to initiate a disturbance, and 

 conductivity or the capacity to propagate that disturbance — 

 are both profoundly modified by various circumstances. 



1. Temperature.— The rate of conduction increases con- 

 siderably with rise of temperature. The change in excit- 

 abihty depends upon the form of current used. With rise 

 of temperature nerve becomes more irritable to induction 

 shocks and less irritable to mechanical stimulation. 



2. Previous Activifi/.— 'Piomded that the nerve-fibre is 

 liberally supplied with oxygen it seems to be completely 

 immune to fatigue. When, after the motor nerve-endings 

 have been paralysed with curare, a nerve is subjected to 

 prolonged stimulation the muscle which it supplies con- 

 tracts when the effect of the drug has passed away. 



But the effect of a stimulus is influenced by an impulse 

 which has just occurred, the direction in which it is 

 influenced depending upon the interval between the 

 stimulus and the previous impulse. The most notable 

 change is a change in excitabihty. This is shown in 

 Fig. 38. For a period of about -002 sec. after an impulse 

 has passed along it a nerve is completely inexcitable. This 



