SECT. XII. 2. 2. AND EXERTION. 55 



defire or averfion, which excites their volition into action. At 

 the fame time in other cafes the ftimulus of pain or pleafure, 

 and the ftimulus or external bodies, may excite into aclion the 

 fenforial powers of ienfation and irritation, and thus add great- 

 er force to their mufcular actions. 



2. The application of the ftimulus, whether that ftimulus be 

 fome quality of external bodies, or pleafure or pain, or defire or 

 averfion, or a link of aflbciation, excites the correfpondent fenfo- 

 rial power into action, and this caufes the contraction of the fi- 

 bre. On the contraction of the fibre a part of the fpirit of ani- 

 mation becomes expended, and the fibre ceaies to contract, 

 though the ftimulus continues to be applied ; till in a certain 

 time the fibre having received a fupply of fenforial power is 

 ready to contract again if the ftimulus continues to be applied. 

 If the ftimulus on the contary be withdrawn, the fame quanti- 

 ty of quiefcent fenforial power becomes refident in the fibre as 

 before its contraction ; as appears from the readinefs for action 

 of the large locomotive mufcles of the body in a ihort time af- 

 ter common exertion. 



But in thofe mufcular fibres, which are fubject to conftant 

 ftimulus, as the arteries, glands, and capillary veflels, anothet 

 phenomenon occurs, if their accuftomed ftimulus be withdrawn ; 

 which is, that the fenforial power becomes accumlated in the: 

 contractile fibres, owing to the want of its being perpetually 

 expended, or caried away, by their ufual unremitted contrac- 

 tions. And on this account thofe mufcular fibres become af- 

 terwards excitable into th^ir natural actions by a much weaker 

 ftimulus ; or into unnatural violence of action by their accuf- 

 tomed ftimulus, as is feen in the hot fits of intermittent fevers, 

 which are in confequence of the previous cold ones. Thus the 

 minute veflels of the fkin are conftantly ftimulated by the 

 fluid matter of heat ; if the quantity of this ftimulus of heat be 

 a while diminifhed, as in covering the hands with fnow, the 

 veflels ceafe to act, at> appears from the palenefs of the fkin ; if 

 this cold application of fnow be continued but a fhort time, the 

 fenforial power, which had habitually been fupplied to the fi- 

 bres, becomes now accumulated in them, owing to the want of 

 its being expended by their accuftomed contractions. And 

 thence a lefs ftimulus of heat will now excite them into violent 

 contractions. 



If the quiefcence of fibres, which had previouily been fubjecT: 

 to perpetual ftimulus, continues a longer time j or their accuf- 

 tomed ftimulus be more completly withdrawn ; the accumula- 

 tion of fenlorial power becomes ftill greater, as in thofe expofed 

 to cold and hunger ; pain is produced, and the organ gradually 



dies 



