SENSATION. 265 



ing apprized, by the increasing loudness of the sound of 

 falling waters, as he advances in a particular direction, that 

 he is coming nearer and nearer to the cataract. Yet how 

 much is really implied in all these apparently simple phe- 

 nomena! Science has taught us that these perceptions of 

 external objects, far from being direct or intuitive, are only 

 the final results of a long series of operations, produced by 

 agents of a most subtle nature, which act by curious and 

 complicated laws, upon a refined organization, disposed in 

 particular situations in our bodies, and adjusted with admi- 

 rable art to receive their impressions, to modify and com- 

 bine them in a certain order, and to convey them in regular 

 succession, and without confusion, to the immediate seat of 

 sensation. 



Yet this process, complicated as it may appear, constitutes 

 but the first stage of the entire function of perception: for 

 ere the mind can arrive at a distinct knowledge of the pre- 

 sence and peculiar qualities of the external object which 

 gives rise to the sensation, a long series of mental changes 

 must intervene, and many intellectual 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 ana- 

 lysis of the phenomena, however, as I shall afterwards at- 

 tempt to show, we find that no less than twelve distinguish- 

 able kinds of changes, or rather processes, 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 system, so as to produce sen- 

 sation, are of different kinds, and are governed by laws pe- 

 culiar to themselves. The structure of the organs must, 

 accordingly, be adapted, in each particular case, to receive 



VOL. II. 34 



