Sect. XII. i. 7. AND EXERTION. * 51 



of this property from the fpirit of animation. At the fame time 

 it is not improbable, that the moving fibres of ftrong people may 

 poflefs a capability of receiving or containing a greater quantity 

 of the fpirit of animation than thofe of weak people. 



In every contraction of a fibre there is an expenditure of the 

 fenforial power, or fpirit of animation ; and where the exertion 

 of this fenforial power has been for fome time increafed, and the 

 mufcles or organs of fenfe have in confequence acted with 

 greater energy, its propenfity to activity is proportionally leffen- 

 ed ; which is to be afcribed to the exhauftion or diminution of 

 its quantity. On the contrary, where there has been lefs fibrous 

 contraction than ufual for a certain time, the fenforial power or 

 fpirit of animation becomes accumulated in the inactive part of 

 the fyftem. Hence vigour fucceeds reft, and hence the propen- 

 fity to action of all our organs of fenfe and mufcles is in a ftate 

 of perpetual fluctuation. The irritability for inflance of the 

 retina, that is, its quantity of fenforial power, varies every mo- 

 ment according to the brightnefs or obfcurity of the object lad 

 beheld compared with the prefent one. The fame occurs to 

 our fenfe of heat, and to every part of our fyftem, which is ca- 

 pable of being excited into action. 



When this variation of the exertion of the fenforial power be- 

 comes much and permanently above or beneath the natural 

 quantity, it becomes a difeafe. If the irritative motions be too 

 great or too little, it (hews that the ftimulus of external things 

 affects this fenforial power too violently or too inertly. If the 

 fenfitive motions be too great or too little, the caufe arifes from 

 the deficient or exuberant quantity of fenfation produced in 

 confequence of the motions of the mufcular fibres or organs of 

 fenfe ; if the voluntary actions are difeafed the caufe is to be 

 looked for in the quantity of volition produced in confequence 

 of the defire or averfion occafioned by the painful or pleafurable 

 fenfations above mentioned. And the difeafes of aflbciation 

 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, wheth- 

 er it be called irritability, fenfibility, vohintarity, or aflociability, 

 is only another mode of expreflion for the quantity of fenforial 

 power' refiding in the organ to be excited. And that on the 

 contrary the words inirritability and infenfibility, 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 



