5$ OF STIMULUS Sect. XII. 3. i. 



violent in degree ; and for weeks in fome fevers, from the ftim- 

 ulus of contagious matter. 



But if a fecond dofe of opium be exhibited before the fibres 

 have regained their natural quantity of fenforial power, its ef- 

 fect will be much lefs than the former, becaufe the fpirit of an- 

 imation or fenforial power is in part exhaufted by the previous 

 excefs of exertion. Hence all medicines repeated too frequent- 

 ly gradually lofe their effect, as opium and wine. Many things 

 of difagreeable tafte at firft ceafe to be difagreeable by frequent 

 repetition, as tobacco ; grief and pain gradually diminilh, and 

 at length ceafe altogether, and hence life itfelf becomes toler- 

 able. 



Befides the temporary diminution of the fpirit of animation 

 or fenforial power, which is naturally ftationary or refident in 

 every living fibre, by a fingle exhibition of a powerful ftimulus, 

 the contractile fibres themfelves, by the perpetual application of 

 a new quantity of ftimulus, before they have regained their nat- 

 ural quantity of fenforial power, appear to fuffer in their capa- 

 bility of receiving fo much as the natural quantity of fenforial 

 power ; and hence a permanent deficiency of fpirit of anima- 

 tion takes place, however long the ftimulus may have been 

 withdrawn. On this caufe depends the permanent debility of 

 thofe, who have been addicted to intoxication, the general weak- 

 nefs of old age, and the natural debility or inirritability of thofe, 

 who have pale fkins and large pupils of their eyes. 



There is a curious phenomenon belongs to this place, which 

 has always appeared difficult of folution •, and that is, that opi- 

 um or aloes may be exhibited in fmall dofes at firft, and gradu- 

 ally increafed to very large ones without producing ftupor or 

 diarrhoea. In this cafe, though the opium and aloes are given 

 in fuch fmall dofes as not to produce intoxication or catharfis, 

 yet they are exhibited in quantities fufficient in fome degree to 

 cxhauft the fenforial power, and hence a ftronger and a ftrong- 

 er dofe is required ; otherwife the medicine would foon ceafe 

 to aft at all. 



On the contrary, if the opium or aloes be exhibited in a large 

 dofe at firft, fo as to produce intoxication or diarrhoea ; after a 

 few repetitions the quantity of either of them may be diminifh- 

 ed, and they will ftill produce this effect. -For the more pow- 

 erful ftimulus difievers the progreflive catenations of animal mo- 

 tions, defcribed in Sect. XVII. and introduces a new link be- 

 tween them ; whence every repetition ftrengthens this new af- 

 fociation or catenation', and the ftimulus may be gradually de- 

 creafed, or be nearly withdrawn, and yet the effect fhall con- 

 tinue y becaufe the fenforial power of aflbciaiion. or catenation 



being 



