Sect. XII. 3. 2. AND EXERTION. 59 



being united with the ftimulus, increafes in energy with every 

 repetition of the catenated circle ; and it is by thefe means that 

 all the irritative alTociations of motions are originally produced. 



Thus if the Peruvian bark be given in the intervals between 

 the fits of intermittent fever in fuch fmall dofes, as not to pre- 

 vent the returns of fever, the conftitution ceafes to obey its 

 ftimulus, and the difeafe cannot be cured even by the largefl 

 dofes of bark, unlefs the patient ceafes to take any for a few 

 days previous to the exhibition of larger dofes. But if large 

 dofes be at firft exhibited, fo as to prevent the return of fever, 

 fmall ones taken afterwards will continue to prevent the return 

 of it. 



. 2. When a ftimulus is repeated at fuch diftant intervals of 

 time, that the natural quantity of fenforial power becomes com- 

 pletely reftored in the acting fibres, it will act with the fame en- 

 ergy as when firft applied. Hence thofe who have lately accuf- 

 tomed themfelves to large dofes of opium by beginning with 

 fmall ones, and gradually increafing them, and repeating them 

 frequently, as mentioned in the preceding paragraphs ; if they 

 intermit the ufe of it for a few days only, muft begin again with 

 as fmall dofes as they took at firft, other wife they will experi- 

 ence the inconveniences of intoxication. 



On this circumftance depend the conftant unfailing effects of 

 the various kinds of ftimulus, which excite into action all the 

 vafcular fyftems in the body ; the arterial, venous, abforbent, 

 and glandular vefTels, are brought into perpetual unwearied ac- 

 tion by the fluids, which are adapted to ftimulate them ; but 

 thefe have the fenforial power of affociation added to that of ir- 

 ritation, and even in fome degree that of fenfation, and even of 

 volition, as will be fpoken of in their places; and life itfelf is thus 

 carried on by the production of fenforial power being equal to its 

 wafte or expenditure in the perpetual movement of the vafcu- 

 lar organization. 



3. When a ftimulus is repeated at uniform intervals of time 

 with fuch diftances between them, that the expenditure of fen- 

 forial power in the acting fibres becomes completely renewed, 

 the effect is produced with greater facility or energy. For the 

 fenforial power of aflbciation is combined with the fenforial 

 power of irritation, or, in common language, the acquired hab- 

 it aflifts the power of the ftimulus. 



This circumftance not only obtains in the annual and diur- 

 nal catenations of animal motions explained in Seel:. XXXVI. 

 but in every lefs circle of actions or ideas, as in the burthen of 

 a fong, or the iterations of a dance 5 and conftitutes the pleas- 

 ure 



