VELOCITY OF TRANSMITTED IMPULSE 147 



Effect of Fatigue 



It should be stated here that very strong stimulation has 

 a tendency to induce fatigue, the result of which is seen 

 in the reduction of the rate of transmission in subsequent 

 experiments. The effect of fatigue can also be shown 

 under moderate stimulation by reducing the period allowed 

 for rest between two stimulations. In summer the period 

 of complete recovery of conductivity is about 15 minutes. 

 In winter the same process requires from 20 to 25 minutes. 

 In fig. 87 a pair of records is given showing the reduction 

 of the velocity under fatigue. The upper of the two records 



Fig. 87. — Effect of fatigue. Upper record when plant fresh ; 

 lower record when fatigued, 



was taken when the plant was fresh, the distance of the 

 point of stimulation was 10 mm., and the intensity of 

 stimulus was 2. The next and lower of the two records 

 was taken after allowing the incomplete resting-interval 

 of only 10 minutes. It will be seen that fatigue has here 

 prolonged the time taken for transmission of excitation. 

 This prolongation is due chiefly to fatigue of conductivity 

 and partly to the prolongation of the latent period. The 

 variation of the latter factor, however, is relatively insigni- 

 ficant, being only, as already stated, of the order of 

 hundredths of a second. Taking the approximate value of 

 the latent period to be 'i second, the period required for 



l2 



