62 OF STIMULUS Sect. XII. 4. 2. 



increafed fenfation ; the voluntary motions, and even the aflbci- 

 ate ones, become much impaired or diminiihed ; and delirium 

 and daggering fucceed. See Sect. XXI. on Drunkennefs. 

 And hence the great proftration of the ftrength of the locomo- 

 tive mufcles in fome fevers, is owing to the exhauftion of fenfo- 

 rial power by the increafed action of the arterial fyftem. 



In like manner a ftimulus greater than natural, applied to a 

 part of the fyftem, increafes the exertion of fenforial power in 

 that part, and diminifhes it in fome other part. As in the com- 

 mencement of fcarlet fever, it is ufual to fee great rednefs and 

 heat on the faces and breads of children, while at the fame time 

 their feet are colder than natural ; partial heats are obfervable 

 in other fevers with debility, and are generally attended with 

 torpor or quiefcence of fome other part of the fyftem. But 

 thefe partial exertions of fenforial power are fometimes attend- 

 ed with increafed partial exertions in other parts of the fyftem, 

 -which fympathize with them, as the flulhing of the face after a 

 full meal. Both thefe therefore are to be afcribed to fympathet- 

 ic aflbciations, explained in Sect. XXXV. and not to general 

 exhauftion or accumulation of fenforial power. 



2. A quantity of ftimulus 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 contractions of animal fibres being not fo eafily excited by a 

 lefs ftimulus after the organ has been fubjected to a greater. 

 Thus after looking at any luminous object of a fmall fize, as at 

 the fetting fun, for a lhort time, fo as not much to fatigue the 

 eye, this part of the retina becomes lefs fenfible to fmaller quan- 

 tities of light ; hence when the eyes are turned on other lefs lu- 

 minous parts of the fky, a dark fpot is feen refembling the fhape 

 of the fun, or other luminous object; which we laft behold. See 

 Sect. XL. No. 2. 



Thus we are fome time before we can diftinguifh object^ in 

 an obfeure 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 ftimulus of fermented or 

 fpirituous liquors, are not excited into due action by weaker ones. 



3. A quantity of ftimulus fomething greater than the laft 

 mentioned, or longer continued, induces the organ into fpaf- 

 modic 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 

 * »d recurs repeatedly before it entirely va nifties. See Sect. XL. 



No. 



