XXIX.] THE PHYSIOLOGY OF MITn^D. 29 



follow that the memory of a sensation is a continuation or 

 reproduction not of the sensation itself, but of the con- 

 sciousness of it. The simplest and most rudimentary form its rudi- 

 of memory is when the conscious ness of a sensation ^^^^^ 

 continues for some little time after the sensation itself conscious- 

 has disappeared. This is usually the case, and I shall sensation 

 have to show in a future chapter, how it is necessary outlasting 



. the sensa- 



to some of the most elementary and important mental tion. 

 operations, that the consciousness of a sensation should 

 thus outlast the sensation itself. There is no difficulty 

 whatever in understanding it as a fact. Sensations, when 

 intense, outlast the impressions that produce them ; thus, 

 to take the most decided instance, we continue to see a 

 bright light after the eyes have been closed against it. 

 This is evidently due to the current of nervous energy 

 that flows along the optic nerve, and produces the sensa- 

 tion of light in the optic ganglia, continuing to fl.ow after 

 its exciting cause has ceased to act ; and the nerves of 

 consciousness, as I suppose, possess, in a higher degree 

 than those of sensation, this property of continuing to 

 transmit their currents after the exciting cause of those 

 currents has ceased. This property may, perhaps, be a 

 purely physical one, and may be comparable to the fact 

 that some elastic bodies, when struck, vibrate longer than 

 others. It is to be remembered that nervous currents are 

 not currents of fluid, but currents of energy, like electric 

 currents. That higher kind of memory which consists not 

 in the continuation, but in the revival of the consciousness EecoUec- 

 of sensation — as, for instance, when we recall what we ^^^^' 

 heard or saw yesterday — is to be explained in a parallel 

 manner. The revival of the consciousness of sensation 

 — or, in a word, recollection — is due to the reproduction due to tlie 



of a current in the nerves of consciousness, exactly similar reproduc- 



' -^ _ tion of a 



(except generally in intensity) to the current to which the current of 

 original consciousness of that sensation was due. But ^°gg_* 

 currents cannot produce or reproduce themselves ; there 

 must be an exciting cause for the currents of reproduced 

 consciousness, or recollection, as well as for those of 

 original consciousness, or consciousness of sensation. I . 



conscious- 



