6o OF STIMULUS Sect. XII. 3. 4. 



ure we receive from repetition and imitation ; as treated of in 

 Seel. XXII. 2. 



4. When a ftimulus has been many times repeated at uni- 

 form intervals, fo as to produce the complete action of the or- 

 gan, it may then be gradually diminifhed, or totally withdrawn, 

 and the action of the organ will continue. For the fenforial 

 power of afTociation becomes united with that of irritation, and 

 by frequent repetition becomes at length of fumcient energy to 

 carry on the new link in the circle of actions, without the irri- 

 tation which at firft introduced it. 



Hence, when the bark is given at ftated intervals for the 

 cure of intermittent fevers, if fixty grains of it be given every 

 three hours for the twenty-four hours preceding the expected 

 parbxyfm, fo as to ftimulate the defective part of the fyftem in- 

 to action, and by that means to prevent the torpor or quiefcence 

 of the fibres, which conftitutes the cold fit ; much lefs than half 

 the quantity, given before the time at which another paroxyfm 

 of quiefcence would have taken place, will be fumcient to pre- 

 vent it ; becaufe now the fenforial power, termed afTociation, 

 acts in a twofold manner. Firft, in refpect to the period of 

 the catenation in which the cold fit was produced, which is now 

 difTeyered by the ftronger ftimulus of the firft dofes of the bark ; 

 and, fecondly, becaufe each dofe of bark being repeated at peri- 

 odical times, has its effect increafed by the fenforial faculty of 

 afTociation being combined with that of irritation. 



Now, when fixty grains of Peruvian bark are taken twice a 

 day, fuppofe at ten o'clock and at fix, for a fortnight, the irrita- 

 tion excited by this additional ftimulus becomes a part of the 

 diurnal circle of actions, and will at length carry on the increaf- 

 ed action of the fyftem without the afliftance of the ftimulus of 

 the bark. On this theory the bitter medicines, chalybeates, 

 and opiates in appropriated dofes, exhibited for a fortnight, give 

 permanent ftrength to pale feeble children, and other weak 

 conftkutions. 



5. When a defect of ftimulus, as of heat, recurs at certain 

 diurnal intervals, which induces fome torpor or quiefcence of 

 a part of the fyftem, the diurnal catenation of actions becomes 

 difordered, and a new afTociation with this link of torpid action 

 i i formed - 3 on the next period the quantity of quiefcence will be 

 incr I, fuppofe the fame defect of ftimulus to recur, becaufe 

 now the new afTociation confpires with the defective irritation 

 in introducing the torpid action of this part of the diurnal cat- 

 enation. In this manner many fever-fits commence, where the 

 patient is for fome clays indifpofed at certain hours, before the 



cold 



