SECT. XII. 2. OF STIMULUS AND EXERTION. 77 



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 ftimulus ; yet as the 

 immediate caufe, which excites the fenforial powers 

 of fenfation, or of aflbciation into exertion, have 

 obtained no general name, we iliall ufc the word 

 ftimulus for them all. 



Hence the quantity of motion produced in any 

 particular part of the animal fyftem will be as the 

 quantity of ftimulus and the quantity of fenforial 

 power, or fpirit of animation, redding in the con- 

 tracting fibres. Where both thefe quantities are 

 great ftrength is produced, when that word is ap- 

 plied to the motions of animal bodies. Where 

 either of them is deficient, iveaknefs is produced, as 

 applied to the motions of animal bodies. 



Now as the fenforial power, or fpirit of animati- 

 on, is perpetually exhaufted by the expenditure of 

 it in fibrous contractions, and is perpetually re- 

 newed by the fecretion or production of it in the 

 brain and fpinal marrow, the quantity of animal 

 ftrength muft be in a perpetual (late of fluctuation 

 on this account ; and if to this be added the un- 

 ceafmg variation of all the four kinds of ftimulus 

 above dcfcribed, which produce the exertions of 

 the fenforiai powers, the ceafelefs viciffitude of 

 animal ftrength becomes eafiiy comprehended. 



If the quantity of fenforial power remains the 

 fame, and the quantity of ftimulus be leiTened, a 

 weaknefs of the fibrous contractions enfues, which 

 may be denominated debility from defeft of ftimulus. 

 If the quantity of ftimuius remains the lame, and 

 the quantity of fenforial power be leflened, another 

 kind of weaknefs enfues, which may be termed 

 debility frojn defefl of fenforial power ; the former of 



G 2 thefe 



