96 EXPERIMENTAL PHYSIOLOGY 



separate twitches, each with its own time tracing, may he recorded, 

 and the difference between the two latent periods can then be 

 calculated. 



The velocity of a nervous impulse varies considerably in different 

 animals and under different conditions. For the sciatic of the frog it 

 is found to vary from 2 to 30 m. per second, the chief variation being 

 caused by differences in temperature, the rate being considerably 

 diminished by cooling the nerve. When a very rapidly moving re- 

 cording surface is not available for the preceding experiment, we may 

 make use of this last factor to delay the rapidity of the nervous im- 

 pulse, and so obtain curves which have a sufficient difference between 

 their latent periods to make the time measure well marked. 



