24 MOTH3NS OF THE RETINA. S*CT. III. 4. 



a rofe, are exact refemblanccs of the fame fimplc 

 ideas from perception ; and in confequence muft be 

 a repetition of thofe very motions. 



3. The difagreeable fenfation called the tooth- 

 edge is originally excited by the painful jarring of 

 the teeth in biting the edge of the glafs, or porce- 

 lain cup, in which our food was given us in our in- 

 fancy, as is further explained in the Section XVI. 

 10, on Inftinft. This di {agreeable fenfation is after- 

 wards excitable not only by a repetition of the 

 found, that was then produced, but by imaginati- 

 on alone, as I have myfelf frequently experienced ; 

 in this cafe the idea of biting a china cup, when I 

 imagine it very diftinclly, or when I fee another 

 perfon bite a cup or glafs, excites an actual pain in 

 the nerves of my teeth. So that this idea and pain 

 ieem to be nothing more than the reiterated motions 

 of thofe nerves, that were formerly fo difagreeably 

 affected. 



Other ideas that are excited by imagination or ' 

 recollection in many inftances produce flmilar ef- 

 fects on the conftitution, as our perceptions had 

 formerly produced, and are therefore undoubtedly 

 a repetition of the fame motions. A ftory which 

 the celebrated Baron Van Swieten relates of him- 

 felf is to this purpose. He was prefent when the 

 putrid carcafe of a dead dog exploded with prodi- 

 gious ftench ; and fome years afterwards, acciden- 

 tally riding along the fame road, he was thrown 

 into the fame ficknefs and vomiting by the idea of 

 the ftench, as he had before experienced from the 

 perception of it. 



4. Where the organ of fenfeis totally deftroyed, 

 the ideas which were received by that organ feem 

 to peri Hi along with it, as well as the power of per- 

 ception. Of this a fatisfactory inftance has fallen 

 under my obfervation. A gentleman about fixty 

 years of age had been totally deaf for near thirty 



years : 



