SENSATION. 373 



disposed in particular situations in our bodies, 

 and adjusted with admirable art to receive their 

 impressions, to modify and combine them in a 

 certain order, and to convey them in regular 

 succession, and without confusion, to the imme- 

 diate seat of sensation. 



Yet this process, complicated as it may ap- 

 pear, constitutes but the first stage of the entire 

 function of perception : for before the mind can 

 arrive at a distinct knowledge of the presence 

 and peculiar qualities of the external object 

 which gives rise to the sensation, a long series of 

 mental changes must intervene, and many intel- 

 lectual operations must be performed. All these 

 take place in such rapid succession, that even 

 when we include the movement of the limb, 

 which is consequent upon the perception, and 

 which we naturally consider as part of the same 

 continuous action, the whole appears to occupy 

 but a single instant. Upon a careful analysis of 

 the phenomena, however, as I shall afterwards 

 attempt to show, we find that no less than twelve 

 distinguishable kinds of changes, or rather pro- 

 cesses, some of which imply many changes, must 

 always intervene, in regular succession, between 

 the action of the external object on the organ of 

 sense, and the voluntary movement of the limb 

 which it excites. 



The external agents, which are capable of 

 affecting the different parts of the nervous sys- 



