PHYSIOLOGY. 51 



to restrain her ; and, very accidentally, in attempting to shut out 

 the light, they found, that upon covering one eye the spec- 

 tres disappeared. Now there is no sort of doubt that there is 

 a constitution of the sentient extremities of the organ of the 

 eye itself, which can give this false imagination ; and we have 

 some slender analogy here, as upon shutting my eye and rub- 

 bing it, I have the appearance of lucid stars ; so that, from a 

 motion excited in the retina the imagination can be produced. 

 The muscae volitantes, also, which patients sometimes see float- 

 ing in the air, in the shape of the darting of lightning, of lucid 

 ribbons, &c. and which in some persons are always descending, 

 and in other cases traversing the eyes, depend upon a particu- 

 lar constitution of the organ of the eye, and often upon some 

 degree of inflammation which we can cure by the proper re- 

 medies of inflammation applied to the part ; there is no doubt, 

 therefore, that these false imaginations can arise from external 

 impressions. So in the case of the ear ; a man imagines that he 

 hears the sound of a bell, but from nobody else hearing it, he 

 comes to learn that it is the tinnitus aurium, which does occur 

 probably from motions excited in the organ itself: and it is 

 proper to observe, that like deceptions may arise with regard 

 to the other organs. Thus we have perceptions of odours, 

 which we have no reason to believe to exist ; but we are liable 

 to impute this to the fluids of the nose, and never impute it to 

 the organs of smell, or to any motions made upon it. So with 

 regard to the false taste of the mouth, we are ready to impute 

 it to the saliva, or to matters exhaled from the stomach ; but 

 we have reason to believe that it may be entirely from a change 

 of motion in the organ. I will add another illustration with 

 regard to the power of imagination in exciting the same state of 

 the part formerly made by an impression upon it. If you 

 bring a feather to tickle a man's upper lip, you can throw him 

 into convulsions ; and if you have done so once or twice, you 

 need only for the third time approach him with the feather ; for 

 before you come within an inch, the person feels the same tre- 

 mor in his lip as before ; so that from the imagination he has 

 this false perception, that he thinks the feather touches his nose 

 when it does not. So with regard to the case of touch, it 

 is not necessary to speak of the case of pruritus, which may, 



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