SECT. XXXV. 2. i. OF ASSOCIATION. 349 



to the whole aflbciated train of motions ; and in confequence 

 the motions of the primary part, though increafed by the ftim- 

 ulus of an extraneous body, ceafe to be accompanied with pain 

 or fenfation. 



If this mode of reafoning be juft it explains a curious fact, 

 why when two parts of the body are ftrongly ftimulated, the 

 pain is only felt in one of them, though it is poflible by volunta- 

 ry attention it may be alternately perceived in them both. In 

 the fame manner, when two new ideas are prefented to us from 

 the ftimulus of external bodies, we attend to but one of them at 

 a time. In other words, when one fet of fibres, whether of the 

 mulcles or organs of ienfe, contract fo ftrongly as to excite 

 much fenfation ; another fet of fibres contracting more weakly 

 do not excite fenfation at all, becaufe the fenforial power of fen- 

 fation is pre-occupied by the firft fet of fibres. So we cannot 

 will more than one effect at once, though by aflbciations previ- 

 oufly formed we can move many fibres in combination. 



Thus in the inftances above related, the termination of the 

 bile-duct in the duodenum, and the exterior extremity of the 

 urethra, are more fenfible than their other terminations. When 

 thefe parts are deprived of their ufual motions by deficiency of 

 fenforial power, as above explained, they become painful ac- 

 cording to law the fifth in Section IV. and the lefs pain orig- 

 inally excited by the ftimulus of concreted bile, or of a (tone at 

 their other extremities ceafes to be perceived. Afterwards, how- 

 ever, when the concretions of bile, or the ftone in the urinary 

 bladder, become more numerous or larger, the pain from their 

 increafed ftimulus becomes greater than the aflbciated pain ; and 

 is then felt at the neck of the gall-bladder or urinary bladder ; 

 and the pain of the glans penis, or at the pit of the ftomach, 

 ceafes to be perceived. 



2. Examples of the fecond mode, where the increafed action 

 of the primary part of a train of fenfitive aflbciation ceafes, when 

 that of the fecondary part commences, are alfo not unfrequent ; 

 as this is the ufual manner of the tranflation of inflammations 

 from internal to external parts of the fyftem, fuch as when an 

 inflammation of the liver or ftomach is tranflated to the mem- 

 branes of the foot, and forms the gout ; or to the fkin of the 

 face, and forms the rofy drop ; or when an inflammation of the 

 membranes of the kidneys is tranflated to the fkin of the loins, 

 and forms one kind of herpes, called (hinglcs 5 in thefe cafes by 

 whatever caufe the original inflammation may have been pro- 

 duced, as the fecondary part of the train of fenfitive aflbciation 

 is more fenfible, it becomes exerted with greater violence than 

 the firft part of it ; and by both its increufed pain, and the in- 



creafed 



