Class IV. 2. i. OF ASSOCIATION. 399 



ORDO II. 



Decreafed AJJbciate Alotions. 



GENUS I. 



Catenated with irritative Motions, 



As irritative mufcular motions are attendad with pain, when 

 they are exerted too weakly, as well as when they are exerted 

 too ftrongly ; fo irritative ideas become attended with feniation 

 when they are exerted too weakly, as well as when they are ex- 

 erted too ftrongly. Which accounts for thefe ideas being at- 

 tended with fenfation in the various kinds of vertigo defcribed 

 below. 



There is great difficulty in tracing the immediate caufe of the 

 deficiencies of action of fome links of the aflbciations of irrita- 

 tive motions ; firft, becaufe the trains and tribes of motions, 

 which compofe thefe links, are fo widely extended as to embrace 

 almofl the whole animal fyftem ; and fecondly, becaufe when 

 the firft. link of an arTbciated train of actions is exerted with too 

 great energy, the fecond link by reverfe fympathy may be affected 

 with torpor. And then this fecond link may tranfmit, as it 

 were, this torpor to a third link, and at tjie fame time regain its 

 own energy of action ; and it is poflible this third link may 

 in like manner tranfmit its torpor to a fourth, and thus regain 

 its own natural quantity of motion. 



-I {hall endeavour to explain this by an example taken from 

 fenfitive aflbciated motions, as the origin of their difturbed ac- 

 tions is more eafily detected. This morning I faw an elderly 

 perfon, who had gradually loft all the teeth in his upper jaw, 

 and all of the under except three of the molares ; the laft of 

 thefe was now loofe, and occafionally painful •, the fangs of 

 which were almoft naked, the gums being much wafted both 

 within and without the jaw. He is a man of attentive obfer- 

 vation, and affured me, that he had again and again noticed, 

 that, when a pain commenced in the membranes of the alveolar 

 procefs of the upper jaw oppofite to the loofe tooth in the under 

 one (which had frequently occurred for feveral days paft) the 

 pain of the loofe tooth ceafed. And that, when the pain after- 

 wards extended to the ear and temple on that fide, the pain in 

 the membranes of the upper jaw ceafed. In this cafe the mem- 

 branes of the alveolar procefs of the upper jaw became torpid, 

 and confequently painful, by their reverie fympathy with the 



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