686 COMPARATIVE ELECTRO-PHYSIOLOGY 



Diffuse stimulus, moreover, whether internal or external, 

 acting on the differentially excitable tissue, gives rise to a 

 marked indication, which may be either motile, electrical, or 

 psychic. A stimulus is applied to the stem of Mimosa. 

 This is transmitted as an excitatory impulse, and reaches the 

 differentially excitable organ, the pulvinus. As far as this 

 organ is concerned, the transmitted stimulus may be re- 

 garded as internal. This internal stimulus, then, gives rise 

 to a conspicuous differential effect, shown in the fall of the 

 leaf. In electrical fishes, similarly, the internal stimulus, 

 delivered by the will of the animal upon the differentially 

 excitable organ, becomes evident as an excitatory discharge 

 In man, again, the revival of memory constitutes a psychic 

 response, due to the play of the diffuse internal stimulus of 

 will upon a sensitive surface rendered differentially excitable 

 by the presence of a latent image. 



It will thus be seen that various after-effects of stimulus 

 find expression as the phenomena of memory. The effect 

 of primary stimulus does not disappear at once, but fades 

 gradually, with a concomitant fading of the sensory impres- 

 sion. From the fact that the after-effect of feeble, is less 

 persistent than that of strong, stimulus, we understand that 

 the sensory or memory impression also lasts longer in the 

 latter case than in the former. Very intense stimulation, 

 again, is apt to give multiple responses as its after-effect, and 

 the corresponding psycho-physiological phenomenon is seen 

 in the recurrent after-images in the retina. 



When a considerable interval has elapsed after the 

 primary stimulus, there is apparently no trace left of the 

 latent image. But the properties of the impressed portions 

 of the sensitive surface have undergone a more or less 

 permanent change in consequence of stimulation. Certain 

 channels have been rendered more conducting, and certain 

 areas more excitable. By an internal diffuse impulse it 

 is now possible to cause differential excitation, and thus to 

 revivify the latent image. 



