SECT. XII. 4. OF STIMULUS AND EXERTION. 93 



fore the convulfion takes place; which feems at 

 firft to be an effort to relieve painful fenfation, and 

 afterwards an effort to prevent it. 



In thefe cafes the violent contractions of the 

 fibres produce fo much pain, as to conftitute a per- 

 petual excitement ; and that in fo great a degree 

 as to allow but fmall intervals* of relaxation of the 

 contracting fibres as in convulfions, or no inter- 

 vals at all as in fixed fpafms. 



6. A quantity of (limulus greater than the laft, 

 or longer continued, produces a paralyfis of the 

 organ. In many cafes this paralyfis is only a tem- 

 porary effect, as on looking long on a fmall area 

 of bright red (ilk placed on a meet of white paper 

 on the floor in a (Iron g 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'fti- 

 mulus of that colour. Thus cathartic medicines, 

 opiates, poifons, contagious matter, ceafe to in- 

 fluence our fyftem after it has been habituated to 

 the ufe of them, except by the exhibition of in- 

 creafed quantities of them ; our fibres not only be- 

 come unaffected by ftimuli, by which they have 

 previoufly been violently irritated, as by the mat- 

 ter of the fm all-pox or mealies ; but they alfo be- 

 come unaffected by fenfation, where the violent 

 exertions, which difobled them, were in confe- 

 quence of too great quantity of fenfation. And 

 laftly, the fibres, which become difobedient to vo- 

 lition, 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 fucceeds, whence the intervals of all muf- 

 cular contractions, as mentioned in No. 3 and 4 of 

 this Section ; the immediate caufe of thefe more 

 permanent kinds of paralyfis is probably owing in 



H 2 the 



