Stc-r.XXXV. 2. DISEASES OF ASSOCIATION. 511 



lus of an extianeousbody, ceafe to be accompanied 

 with pain or fecfation. 



If this mode of reafoning bejuft, it explains a 

 curious fact, why when two parts of the body are 

 ftrongly ftimulated, the pain is felt only in one of 

 them, though it is polEble by voluntary attention 

 it may be alternately perceived in them both. In 

 the fame manner, when two new ideas are prefent- 

 ed to us from the IHmulus of external bodies, we 

 attend to but one of them at a time. In other 

 words, when one fet of fibres, whether of the muf- 

 cles or organs of fenfe, contract fo ftrongly as to 

 excite much fenfation ; another fet of fibres con- 

 tracting more weakly do not excite fenfaiion at all, 

 becaufe the fenforial power of fenfation is pre-occu- 

 pied by the firft fet of fibres. So we cannot will 

 more than one effect at once, though by aflbciations 

 previoufly formed we can inove many fibres in corn- 

 bin a tiofi. 



Thus in the inftances above related, the termina- 

 tion of the bile duel: in the duodenum, and the ex- 

 terior 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 according to law the fiah in Section IV. and 

 the lefs pain, originally excited by the ftimulus of 

 concreted bile, or of a ftone at their other extre- 

 mities, ceafes to be perceived. Afterwards, howe- 

 ver, when the concretions of bile, or the ftone on 

 the urinary bladder, become more numerous or 

 larger, the pain from their increafed flimulus be- 

 comes gieater than the aflbciated pain ; and is then 

 felt at the neck of the gall bladder or urinary blad- 

 der ; and the pain of the glans penis, or at the pit 

 of the nomach ceafes to be perceived. 



2. Examples of the fecond mode, where the in- 

 creafed action of the primary part of a train of 



fenfnive 



