SECT. XII. 3. OF STIMULUS AND EXERTION. 87 



3. When a ftimulus is repeated at uniform in- 

 tervals of time with fuch diftances between them, 

 that the expenditure of fenforial power in the act- 

 ing fibres becomes completely renewed, the effect is 

 produced with greater facility or energy. For the 

 fenforial power of allbciation is combined with the 

 fenforial power of irritation, or, in common lan- 

 guage, the acquired habit aflifts the power of the 

 ilimulus. 



This circumftance not only obtains in the annual 

 and diurnal catenations of animal motions explain- 

 ed in Sect. XXXVI. but in every lefs circle of ac- 

 tions or ideas, as in the burthen of a fong, or the 

 iterations of a dance ; and conftitutes the pleafure 

 we receive from repetition and imitation ; as treat- 

 ed of in Sett. XXII. 2. 



4. When a ftimulus has been many times repeat- 

 ed at uniform intervals, fo as to produce the com- 

 plete action of the organ, it may then be gradual- 

 ly diminifhed, or totally withdrawn, and the ac- 

 tion of the organ will continue. For the fenforial 

 power of affociation becomes united with that of 

 irritation, and by frequent repetition becomes at 

 length of fufficient energy to carry on the new link 

 in the circle of actions, without the irritation which 

 at firft introduced it. 



Hence, when the bark is given at dated inter- 

 vals for the cure of intermittent fevers, if fixty 

 grains of it be given every three hours for the 

 twenty-four hours preceding the expected pa- 

 roxyfm, fo as to ftimulate the defective part of the 

 fyftem into action, and by that means to prevent 

 the torpor or quiefcence of the fibres, which con- 

 ititutes the cold fit ; much lefs than half the quan- 

 tity, given before the time at which another pa- 

 roxyfm of quiefcence would have taken place, will 

 be fufficient to prevent it ; becaufe now the fenfo- 

 rial power, termed affociation, acts in a twofold 



manner, 



