64 OF STIMULUS Sect. XII. 5. 1. 



floor in a ftrong light, the red filk gradually becomes paler, and 

 at length difappears ; which evinces that a part of the retina, by 

 being violently excited, becomes for a time unaffected by the 

 itimulus of that colour. Thus cathartic medicines, opiates, poi- 

 fons, contagious matter, ceafe to influence our fyftem after it has 

 been habituated to the ufe of them, except by the exhibition of 

 increafed quantities of them ; our fibres not only become unaf- 

 fected by ftimuli, by which they have previoufly been violently 

 irritated, as by the matter of the fmaJl-pox or meafles ; but they 

 alfo become unaffected by fenfation, where the violent exertions, 

 which difabled them, were in confequence of too great quantity 

 of fenfation. And laftly the fibres, which become difobedient 

 to volition, are probably difabled by their too violent exertions 

 in confequence of too great a quantity of volition. 



After every exertion of our fibres a temporary paralyfis fuc- 

 ceeds, whence the intervals of all mufcular contractions, as men. 

 tioned in No. 3 and 4 of this Section ; the immediate caufe of 

 thefe more permanent kinds of paralyfis is probably owing in the 

 f-ime manner to the too great exhauftion of the fpirit of anima- 

 tion in the affected part ; fo that a ftronger ftimulus is required, 

 or one of a different kind from that, which occafioned thofe too 

 violent contractions, to again excite the affected organ into ac- 

 tivity ; and if a ftronger ftimulus could be applied, it mult 

 again induce paralyfis. 



For thefe powerful ftimuli excite pain at the fame time, that 

 they produce irritation ; and this pain not only excites fibrous 

 motions by its ftimulus, but it alfo produces volition ; and thus 

 all thefe ftimuli acting at the fame time, and fometimes with 

 the addition of their affociations, produce fo great exertion as to 

 expend the whole of the fenforial power in the affected fibres. 



V. Of Stimulus lefs than natural. 



I. A quantity of ftimulus lefs than natural, producing a de- 

 creafed exertion of fenforial power, occafions an accumulation 

 of the general quantity of it. This circumftance is obfervable 

 in the hemiplegia, in which the patients are perpetually mov- 

 ing the mufcles, which are unaffected. On this account we 

 awake with greater vigour after lleep, becaufe during fo many 

 hours, the great ufual expenditure of fenforial power in the per- 

 formance of voluntary actions, and in the exertions of our or- 

 gans of fenfe, in confequence of the irritations occafioned by ex- 

 ternal objects had been fufpended, and a coniequent accumula- 

 tion had taken place. 



In like manner the exertion of the fenforial power lefs than 



natural 



