SECT. XII. 4. OF STIMULUS AND EXERTION. 9? 



not to general exauftion or accumulation of fenfo- 

 rial power. 



2 A quantity of (limul us greater than natural, 

 producing an increafed exertion of fenforial power 

 in any particular organ, diminifhes the quantity 

 of it in that organ. This appears from the con- 

 tractions of animal fibres being not fo eafily excit- 

 ed by a lefs itimulus after the organ has been fub- 

 jecled to a greater. Thus after looking at any 

 luminous object of a fmall fize, as at the fetting fun, 

 for a fhort time, fo as not much to fatigue the eye, 

 this part of the retina becomes lefs fenfible to fmal- 

 ler quantities of light ; hence when the eyes are 

 turned on other lefs luminous parts of the Iky, a 

 dark fpot is feen refembling the fhape of the fun, 

 or other luminous object which we laft behold. 

 See Seel. XL. No. 2. 



Thus we are fome time before we can diftinguifh 

 objects in an obfcure room after coming from 

 bright day-light, though the iris prefently contracts 

 itfelf. We are not able to hear weak founds after 

 loud ones. And the ftomachs of thofe who have 

 been much habituated to the ftronger flimulus of 

 fermented or fpirituous liquors, are not excited 

 into due action by weaker ones, 



3. A quantity of iiimulus fomething greater than 

 the laft mentioned, or longer continued, induces 

 the organ into fpafmodic action, which ceafes and 

 recurs alternately. Thus on looking for a time 

 on the fetting fun, fo as not greatly to fatigue the 

 fight, a yellow fpectrum is feen when the eyes are 

 clofed and covered, which continues for a time, 

 and then difappears and recurs repeatedly before 

 it entirely vaniihes. See Sect. XL. No. 5. Thus 

 the action of vomiting ceafes and is renewed by 

 intervals, although the emetic drug is thrown up 

 with the firft effort. A tenefmus continues by 

 intervals fome time after the exclufion of acrid 



H excrement ; 



