52 OF STIMULUS SECT. XIL 2. i. 



II. Of fenforial Exertion^ 



i. There are three circumftances to be attended to hi the 

 production of animal motions, id. The flimulus. ad. The 

 fenforial power. 3d. The contractile fibre, ift. A ftimulus, 

 external to the organ, originally induces into action the fenfo- 

 rial faculty termed irritation ; this produces the contraction of 

 the fibres, which, if it be perceived at all, introduces pleafure or 

 pain ; which in their active (tare are termed fenfation ; which 

 is another fenforial faculty, and occafionally produces contrac- 

 tion of the fibres ; this pleafure or pain is therefore to be con- 

 fidered as another ftimulus, which may either act alone or in 

 conjun&ion with the former faculty of the fenforium termed 

 irritation. This new ftimulus, of pleafure or pain either induces 

 into action the fenforial faculty termed fenfation, which then 

 produces the contraction of the fibres ; or it introduces defire 

 or averfion, which excite into action another fenforial faculty, 

 termed volition, and may therefore be confidered as another 

 ftimulus, which either alone or in conjunction with one or both 

 of the two former faculties of the fenforium produces the con- 

 traction of animal fibres. There is another fenforial power, 

 that of a aflbciation, which perpetually, in conjunction with one 

 or more of the above, and frequently fingly, produces the con- 

 traction of animal fibres, and which is itfelf excited into action 

 by the previous motions of contracting fibres. 



Now as the fenforial power, termed irritation, refiding in any 

 particular fibres, is excited into exertion by the (timulus of ex- 

 ternal bodies ating on thofe fibres ; the fenforial power, termed 

 fenfation, refiding in any particular fibres is excited into exertion 

 by the ftimulus of pleafure or pain a6ting on thofe fibres ; the 

 fenforial power, termed volition, refiding in any particular fibres 

 is excited into exertion by the ftimulus of defire or averfion ; 

 and the fenforial power, termed aflbciation, refiding in any par^ 

 tidar fibres, is excited into adtion by the ftimulus of other fi- 

 brous motions, which had frequently preceded them. The 

 word ftimulus may therefore be ufed without impropriety of 

 language, for any of thefe four caufes, which excite the four 

 fenforial powers into exertion. For though the immediate 

 caufe of volition has generally been termed a motive ; and that 

 of irritation only has generally obtained the name of Jlimulus ; 

 yet, as the immediate caufe, which excites the fenforial powers 

 of fenfation, or of aifociation, into exertion, have obtained no 

 general name, we (hall ufe the word ftimulus for them all. 



Hence the quantity of motion produced in any particular 

 part of the animal iyftem will be as the quantity of ftimulus, 



a,nd 



