186 THE INTELLIGENCE SERVICE 



decreased ; that is, nerve-conduction has a positive temperature 

 coefficient. It was pointed out by Van't Hoff that the velocity 

 of chemical reactions is increased twofold or more for each ten 

 degrees in temperature, i.e. the temperature coefficient for chemical 

 reactions is greater than 2. On the other hand, the temperature 

 coefficient for physical processes is less than 2. The temperature 

 coefficient (i.e. ratio of velocity of propagation of nervous impulse 



at (T+10) to its rate at T=^ '} has been estimated 



v at 1 J 



by Lucas as approximately 1-8. This value has been proved to 

 be right by later workers. Therefore, physical factors are involved 

 in the propagation of a nervous impulse. This does not exclude 

 chemical reactions but tends to show that the process is not 

 purely chemical. 



G. Decrement of the nervous impulse. If a length of nerve is 

 cooled, not only does the velocity of the propagation of the 

 impulse suffer diminution but there seems to be a diminution in 

 the intensity of the impulse as well. If the degree of cold is 

 sufficient, or if the length cooled is extensive, the impulse may be 

 stopped entirely. If, however, any of the impulse is propagated 

 through the cooled region into a normal piece of nerve it seems to 

 recover its full intensity and velocity. Lucas compares this 

 phenomenon to the transmission of fire along a fuse of gunpowder. 

 If a section of the fuse is slightly damp, the rate of burning as 

 well as the heat evolved will be decreased but will recover as 

 soon as combustion starts on a dry section. Narcotisation of a 

 nerve by ether, alcohol, cocaine or other drug has a similar effect 

 to cooling. 



7. Refractory period. The passage of a nervous impulse 

 produces 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 : (a) The absolutely 

 refractory period when no strength of stimulus is effective. 

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

 and will respond to stimulus stronger than usual, (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 



