vin CONDUCTIVITY AND EXCITABILITY OF NERVE 107 



single shocks, to perceive a marked dilatation of the pupil, and 

 Piotrowsky (62) also found this kind of excitation effective as 

 regards constriction of the vessels of the ear. Still, the action 



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of single shocks is extremely small, while tetanising excitation in 

 both cases produces marked effects. 



If the stimulation-frequency is altered, with constant strength 

 of current, it is easy to show that the excitatory action (dilatation 

 of pupil) is augmented, within a wide range, with increasing- 

 frequency. At an excitation interval of about two seconds Muhlert 

 could not find any summation of stimulation - frequency, at a 

 strength of 85*19 E, with even sixty-two consecutive stimuli. 

 Where number and interval of stimuli are so arranged that an 

 effect may be anticipated, the influence of strength of current may 

 easily be determined, in the sense that niydriasis first begins above 

 a certain range, and then with growing intensity rises to its 

 maximum, at first rapidly, and afterwards more slowly. In this 

 case the smooth muscle-fibres in which the summation occurs 

 give a precisely similar reaction to that of the reflex centres of 

 the spinal cord with excitation of the centripetal nerves. The 

 more sluggish of the striated muscles seem to give a similar 

 response. Thus Piotrowsky (56), on exciting the claw-nerves of 

 the crayfish with single and intrinsically ineffective induction 

 shocks, sent in at an interval of half a second, observed a weak 

 contraction after every seven stimuli. 



The striking insusceptibility of centripetal (sensory) nerves, 

 or more correctly of their central end-organs, to single induction 

 shocks has long been known. 



Munk (63) pointed out that no reflex twitches were elicited 

 from the frog by single induction shocks, impinging on a sensory 

 nerve, unless it had previously been weakly strychninised. 



Setschenow (64) also showed that induction currents with 

 the vibrating hammer that were quite perceptible to the tongue 

 discharged no reflex from the central end of the sciatic. In 



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determining the upper limit of current intensity at which the 

 animal remained undisturbed by single shocks, and the lowest 

 intensity at which it was first excited with the vibrating hammer, 

 he invariably found a great difference between the two, " because 

 the sensory nerves (especially the central apparatus of transmission), 

 which are so unsusceptible to single induction shocks, exhibit 

 almost the same activity to a succession of shocks as the motor 



