536 COMPARATIVE ELECTRO-PHYSIOLOGY 



stability, where the record made by the spot of light appears 

 as a horizontal line. Under normal conditions, then, the 

 response to excitation is a sudden movement, due to con- 

 traction, say upwards, followed by recovery down. In 

 the response-curve projected on the screen, the vertical 

 movement or ordinate represents the amplitude of the re- 

 sponse, and the horizontal abscissa the time. Under tetani- 

 sation a series of curves corresponding in frequency to the 

 frequency of the shocks is observed as serrations. 



Another very interesting observation often made in the 

 mechanical response of nerve is that of the after-effect on 

 the cessation of continuous stimulation by tetanic shocks. 

 It has been found, it will be remembered (p. 428), that the 

 response of the retina to the action of continuous light often 

 shows on its cessation a sudden transient increase. This 

 phenomenon has been regarded as peculiar to the retina. But 

 I have found exactly parallel effects to occur in the mechanical 

 response of nerve. Under continuous stimulation there is a 

 tendency to the attainment of a maximum contraction, which 

 suddenly, on the cessation of stimulation, overshoots, to be 

 followed by the usual recovery. I have already referred to 

 the two different effects of an opposite character caused by 

 incident stimulus, namely, the effect of negativity, and its 

 converse positivity. In the case of mechanical response, it is 

 the former which is effective in inducing contraction, while 

 the latter is associated with expansion, and is a factor in re- 

 covery. It will also be seen, in a general way, that by the 

 antagonistic action of these two elements, and by the differ- 

 ing relative intensities of their after-effects, many diverse 

 results may be exhibited. In the case of the after-effect in 

 question, which occurs by a sudden positive variation of the 

 contraction, the excitatory effect would appear to be pre- 

 dominant. Even when, after this brief positive variation, 

 recovery is taking place, the excitatory element, with its 

 contractile tendency, appears to persist ; for if a second 

 stimulus be applied, some time before the recovery is com- 

 plete, the consequent contractile response takes place almost 



