MEMORY 68 1 



properties ; and, second, the effect of an internal stimulus, 

 delivered as a blow from within, by an impulse of the will, 

 upon the sensitive surface in which the image is latent. 



We shall now first observe in some detail those changes 

 which remain as an after-effect of previous stimulation. The 

 differential effect caused by primary stimulus fixes the latent 

 image, and it is only by the reproduction of the same differen- 

 tial excitation, that the memory-image can subsequently be 

 revived. 



We must here recall briefly the results which were estab- 

 lished in Chapter XLII., on the modification of response 

 under cyclic molecular variation. It was there shown that, 

 under the action of stimulus, the molecular condition of a 

 substance undergoes a progressive variation, exhibited in its 

 characteristic curve ; that the forward and return curves do 

 not exactly coincide, because the history behind the two 

 half-cycles has not been the same ; and, finally, that on the 

 cessation of stimulus, the original molecular condition is not 

 exactly restored, a certain effect being residual. Owing to 

 this residual effect, the properties of the responding sub- 

 stance are changed. We also saw that, in consequence of 

 this impressed change, the conductivity and excitability of 

 the substance might be enhanced. A frequent repetition 

 of a stimulus was thus shown to create a habit or disposition 

 by which the mass of a substance, formerly almost non-con- 

 ducting, might be made a conductor of excitation. 



These impressed molecular changes may not leave any 

 visible impression behind. But let us look at the responding 

 properties of a given substance at different points on the 

 characteristic curve. In a sluggish A condition, that is to 

 say, before it has even been excited, the power of response of 

 the substance to a given stimulus will be slight or negligible. 

 Let us suppose next that by the action of stimulus the sub- 

 stance is raised above B. On the cessation of stimulus a slow 

 recovery will then take place, whose completion may be 

 indefinitely prolonged. The substance will thus approach 

 very near the point B in the curve, without actually reaching 



