REFRACTORY PERIOD 227 



to cooling. The firing of a train of gunpowder is a series of local 

 acts. Moreover, an increased resistance in a straight electrical 

 circuit would produce a decrement of current which would not 

 recover its pre-resistancc value no matter how good a conductor 

 it passed into. 



9. Refractory period. Tlic passage of a nervous impulse pro- 

 duces some change in the physico-chemical state of the nerve, 

 so that it is followed by a state during which its function is de- 

 pressed. A certain time must elapse between each nervous 

 impulse. This spare time is called the refractory period, during 

 which a stimulus will not receive normal treatment. The length 

 of the period varies inversely as the temperature. The refractory 

 period may be divided into three stages : («) The ahsolutely 

 refractory period when no stimulus, however strong, is effective. 

 (6) During the relative refractory period the nerve is recovering 

 and will respond to stimuli of supernormal strength, (c) The 

 supernormal stage follows during which subnormal stimuli are 

 effective. Two factors at least come into play to cause the 

 refractory period, viz. alterations in excitability and alterations 

 in conductivity. These two factors go hand-in-hand, i.e. the 

 nerve is non-irritable and offers a resistance to the passage of the 

 impulse sufficient to swamp it during the absolute refractory 

 period ; during the relative refractory period the nerve steadily 

 recovers its irritability and its conducting power ; while the last 

 period is one of supernormal irritability and conductivity. Take 

 a very simple analogy. In a game common to Boy Scouts, and, 

 I understand, borrowed from the NaAv, a row of players are so 

 arranged in unstable equilibrium that on the word or sign of 

 command (stimulus) number one falls towards number two, and 

 so on, till all the players are horizontal. The disturbance has been 

 propagated from one end of the line to the other. Note in the 

 first instance that the stimulus has not supplied any of the energy. 

 Secondly, that the disturbance alone has been propagated, and that 

 each unit of the team has supplied its own energy (+ gravity). 

 Thirdly, before a second impulse can be propagated all the players 

 must be restored to the vertical. Until this is done, no amount of 

 stinuilation is of any use. This is the absolutely refractory 

 period. We cannot push the analogy further. 



10. Summation. If a second stimulus be applied to the nerve 

 during the third phase of the refractory period, it will give rise to 

 an impulse which will meet with less resistance in its passage along 

 the nerve. Now, if the first impulse be subminimal, i.e., insufficient 

 to cause a manifestation of energy in the motor mechanism which 

 the nerve supplies, then the second impulse if it be projiagated along 



15 — 2 



