682 COMPARATIVE ELECTRO-PHYSIOLOGY 



it. This difference between B and the point actually reached 

 may be so small as to be undetectable by any ordinary 

 mode of inspection. We therefore term the impression 

 latent. But the properties of this B area, formerly acted 

 upon, have been profoundly changed, being rendered more 

 excitable by the impressed effect of previous stimulus. In 

 this sensitive impression-surface will be certain areas in the 

 A and certain others in the B condition, the former sluggish 

 and the latter characterised by enhanced excitability. By 

 the shock of an internal diffuse stimulus, a differential excita- 

 tion may now be induced, exactly similar to that caused by 

 primary stimulus. This is the revival of the memory-image. 

 We may carry out a physical experiment exemplifying 

 this process of the rise of a latent impression into vividness 

 under the action of diffuse stimulus. We may take a sensi- 

 tive surface, in which different areas, in consequence of pre- 

 vious excitation, have impressed on them latent variations of 

 excitability. Thus indifferent portions of the surface A, A tl 

 may have their excitability represented by zero, another 

 portion B, whose excitability has been exalted as the after- 

 effect of stimulatory agents previously applied, will have its 

 normal excitability enhanced. In still a third portion, C, the 

 excitability is artificially depressed or abolished. The 

 responding substance was a tin wire ; dilute solution of 

 sodium carbonate, which is an exciting agent, was applied 

 on the area B. The depressing or poisonous reagent, oxalic 

 acid, was applied at C. After a short period of this applica- 

 tion the wire was washed, and there was no outward indica- 

 tion of any difference between the areas A, B and C. Elec- 

 trically also there was little or no permanent difference 

 between them. One non-polarisable electrode' connected 

 with a galvanometer was kept permanently applied on the 

 indifferent surface A y/ . The second exploring electrode was 

 now moved along the wire, and while it rested on any point 

 the wire was excited as a whole by vibration. The galvano- 

 meter, under this arrangement, would detect differential 

 excitability. As long as the exploring wire moved over 



