88 OF STIMULUS AND EXERTION. SECT. XII. 3- 



manner. Firfl, in refpect to the period of the ca- 

 tenation in which the cold fit was produced, which 

 is now diflevered by the ftronger ftimulus of the 

 firft dofes of the bark ; and, fecondly, becaufe each 

 dofe of* bark being repeated at periodical times, has 

 its effect increafed by the fenibrial faculty of affo- 

 ciation 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 irritation excited by this 

 additional ttimulus becomes a part of the diurnal 

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

 increafed action of the fyftem without the afliftance 

 of the flimulus of the bark. On this theory the 

 bitter medicines, chalybeates, and opiates in ap- 

 propriated dofes, exhibited for a fortnight, give 

 permanent ftrength to pale feeble children, and 

 other weak conftitutions. 



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 aflbciation with this link of torpid ac- 

 tion is formed \ on the next period the quantity 

 of quiefcence will be increafed, fuppofe the fame 

 defect of ftimulus to recur, becaufe now the new 

 aflbciation confpires with the defective irritation 

 in introducing the torpid action of this part of the 

 diurnal catenation. In this manner many fever- 

 fits commence, where the patient is for fome days 

 indifpoied at certain hours, before the cold pa- 

 roxyfm of fever is completely formed. See Sect. 

 XVII. 3. 3. on Catenation of Animal Motions. 



6. If a ftimulus, which at firft excited the affect- 

 ed organ into fo great exertion as to produce fen- 

 fation, be continued for a certain time, it will ceafe 

 to produce fenfation both then and when repeated, 



though 



