SECT. XII. 2. OF STIMULUS AND EXERTION. 75 



When this variation of the exertion of the fen- 

 forial power becomes much and permanently above 

 or beneath the natural quantity, it becomes a 

 cifeafe. If the irritative motions be too great or too 

 little, it fhews that the flimulus of external things 

 affects this fenforial power too violently or too in- 

 ertly. If the fenfitive motions be too great or too 

 little, the caufe arifes from the deficient or exube- 

 rant quantity of fenfation produced in confequence 

 of the motions of the mufcuhr fibres or organs of 

 fenfe; if the voluntary aclions are difeafed, the 

 caufe is to be looked for in the quantity of volition 

 produced in confequence of the defire or aversion 

 occaiioned by the painful or pleafurable fenfations 

 above mentioned. And the difeafes of afTociations 

 probably depend on the greater or lefs quantity of 

 the other three fenforial powers by which they 

 were formed. 



From whence it appears that the propenfity to 

 action, whether it be called irritability, fenfibility, 

 voluntarity, or aflbciability, is only another mode 

 of expreffion for the quantity of fenforial power re- 

 fiding in the organ to be excited. And that on the 

 contrary the words inirritability and inienfibility, 

 together with inaptitude to voluntary and aflbciate 

 motions, are fynonymous with deficiency of the 

 quantity of fenforial power, or of the fpirit of 

 animation, refiding in the organs to be excited. 



II. Of fenforial Exertion. 



i. There are three circumflances to be attended 

 to in the production of animal motions, ift. The 

 ftimulus. 2d. The fenforial power. 3d. The con- 

 tractile fibre, i ft. A ftimulus, external to the or- 

 gan, originally induces into action the fenforial fa- 

 culty termed irritation ; this produces the contrac- 

 tion of the fibres, which if it be perceived at all, 



G introduces 



