ECT. III. 5- I- THE RETINA. 15 



the perceptions of founds he has alfo loft the ideas of them ; 

 though the organs of fpeech (till retain fomewhat of their ufual 

 habits of articulation. 



'This obfervation may throw fome light on the medical treat- 

 ment of deaf people ; as it may be learnt from their dreams 

 whether the auditory nerve be paralytic, or their deafnefs be 

 owing to fome defect of the external organ. 



It rarely happens that the immediate organ of vifion is per- 

 fectly deftroyed. The mod frequent caufes of blindnefs are 

 occasioned by defects of the external organ, as in cataracts and 

 obfufcations of the cornea. But I have had the opportunity of 

 converting with two men, who had been fome years blind ; one 

 of them had a complete gutta ferena, and the other had loft the 

 whole fubftance of his eyes. They both told me that they did 

 not remember to have ever dreamt of vifible objects fince the 

 total lofs of their fight. 



V. Another method of difcovering that our ideas are animal 

 motions of the organs of fenfe, is from confidering the great 

 analogy they bear to the motions of the larger mufcles of the 

 body. In the following articles it will appear that they are orig- 

 inally excited into action by the irritation of external objects 

 like our mufcles ; are aflbciated together like our mufcular mo- 

 tions ; act in fimilar time with them ; are fatigued by continu- 

 ed exertion like them ; and that the organs of fenfe are fubject 

 to inflammation, numbnefs, palfy, convulfion, and the defects 

 of old age in the fame manner as the mufcular fibres. 



1. All our perceptions or ideas of external objefts are uni- 

 verfally allowed to have been originally excited by the ftimulus 

 of thofe external objects ; and it will be (hewn in a fucceeding 

 fedtion, that it is probable that all our mufcular motions, as well 

 thofe that are become voluntary as thofe of the heart and glan- 

 dular fyftem, were originally in like manner excited by the ftim- 

 ulus of fomething external to the organ of motion. 



2. Our ideas are alfo aflbciated together after their produc- 

 tion precifely in the fame manner as our mufcular motions ; 

 which will likewife be fully explained in the fucceeding fection. 



3. The time taken up in performing an idea is likewife much 

 the fame as that taken up in performing a mufcular motion. A 

 mufician can prefs the keys of an harpfichord with his fingers 

 in the order of a tune he has been accuftomed to play, in as 

 little time as he can run over thofe notes in his mind. So we 

 many times in an hour cover our eye-balls with our eye- lids 

 without perceiving that we are in the dark ; hence the percep- 

 tion or idea of light is not changed for that of darknefs in ib 

 fmall a time as the twinkling of an eye ; fo that in this cafe the 



mufcular 



