EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 385 



was applied once more. It is here seen that this stimulus 

 induces a further acceleration. The latent period, however, 

 of this second response is reduced from forty-five seconds, 

 which was its value in the first case, to thirty seconds. 



This variation of latent period is brought out still more 

 clearly by the application of a stimulus of shorter duration, 

 in which case the latent period is more prolonged, and its 

 variations, therefore, more easily observed. In order to show 

 this, I took a fresh specimen of Croton, and, after the initial 

 balance, applied stimulus of five seconds' duration. It will 

 be seen from the photographic record (fig. 229) that the 



P'IG. 229. Photographic Record showing Variation of Latent Period 

 as After-effect of Stimulus 



Stimulus applied was for 5 seconds. Moment of application represented 

 by vertical line. Lower record shows latent period to be 25 minutes. 

 After re-balance, stimulus of 5 seconds was again applied. Upper 

 record now shows reduction of latent period to 20 minutes, 



latent period was here very long, being as much as twenty- 

 five minutes. After re-balance stimulus was once more 

 applied, lasting, as before, for five seconds. The latent 

 period in the second case is seen to be reduced to twenty 

 minutes. 



As an interesting and independent verification of the 

 enhancement of suction by stimulus, I now took a number 

 of response-curves, using the Method of Over-balance. Here, 

 it will be remembered, the normal over-balance is indicated 

 by a down-curve, and acceleration of suction by diminution 

 of the slope, or even by reversal, of this curve. In fig. 230 

 is seen a record obtained in this manner the first down part 



C C 



