CHAP. XII.] AND PERCEPTION. 179 



been called into action when an object of the same kind 

 had been presented. It is, therefore, by the simultaneous 

 consciousness and fusion, as it were, of the subjective 

 sides of various new and old impressions that a present 

 object is 'perceived,' or recognized. It could only be 

 by the previous establishment of structural communica- 

 tions between the several related sensory cells, that the 

 excitation of those of any one order would suffice to revive 

 more or less strongly in other groups just such molecular 

 changes as like objects had on previous occasions excited. 

 And it may be easily understood that the molecular 

 movements initiated by any one or two ingoing sense 

 impressions, may start from such groups of cells and 

 thence flow over into all communicating channels be- 

 tween them and the cells of other related groups just 

 as outpoured water from some overfull lake or reservoir 

 would flow easily through any set of connected channels 

 which might have become established around it. The 

 more definite the nervous paths, and the more frequently 

 they have been traversed by stimuli, the easier will it be 

 for molecular movements (as it would be for water, in the 

 illustration given) to flow along such channels when the 

 next occasion arises. 



Some such process as is above indicated would seem 

 to correspond physically with what is known as an act 

 of Perception. As the writer has elsewhere* pointed 

 out, one of the principal features of such an act is that it 

 tends to associate, as it were, into one state of conscious- 

 ness much of the knowledge which has been derived at dif- 

 ferent times and in different ways concerning any particular 

 external object. When impressions from such an object are 

 made upon any sensory nuclei, they strike first upon the 

 corresponding ' perceptive centres ' in the cerebral hemi- 



* " The Physiology of Thinking."" Brit. Med. Journ.," May, 1869. 



N 2 



