So OF STIMULUS AND EXERTION. SECT. XII. 2 



excited by two kinds of fiimulus. Thefe are the 

 fenforial power of irritation excited by the ilimulus 

 of bodies external to the moving fibres, and the 

 fenforial power of fenfation excited by the pain in 

 confequence of the increafed contractions of thofe 

 moving fibres. 



And in infane people in fome cafes the force of 

 their mufcular actions will be in proportion to the 

 quantity of fenforial power, which they poilefs, and 

 the quantity of the fiimulus of deiire or averfion, 

 which excites their volition into action. At the 

 fame time in other cafes the famulus of pain or 

 pleafure, and the fiimulus of external bodies, may 

 excite into action the fenforial powers of fenfation 

 and irritation, arid thus add greater force to their 

 mufcular actions. 



2. The application of the ilimulus, whether that 

 fiimulus be fome quality of external bodies, or 

 pleafure or pain, or defire or averfion, or a link of 

 affociation, excites the correfpondent fenforial power 

 into action, and this cauies the contraction of the 

 fibre. On the contraction of the fibre a part of 

 the fpirit of animation becomes expended, and the 

 fibre ceafes to contract, though the ilimulus conti- 

 nues to be applied ; till in a certain time the fibre 

 having received a fupply of fenforial power is ready 

 to contract again, if the ilimulus continues to be 

 applied. If the ilimulus on the contrary be with- 

 drawn, the fame quantity of quiefcent fenforial 

 power becomes refident in the fibre as before its 

 contraction ; as appears from the readinefs for ac- 

 tion of the large locomotive mufcles of the body in 

 a (hort time after common exertion. 



But in thofe mufcuiar fibres, which are fubject 

 to conilant itimulus, as the arteries, glands, and 

 capillary veflels, another phenomenon occurs, if 

 their accuftomed ilimulus be withdrawn ; which is, 

 that the fenforial power becomes accumulated in the 



contractile 



