74 OF STIMULUS AND EXERTION. SECT. XII. i. 



and that thofe weak conftitutions, which are at. 

 tended with cold extremities and large pupils of the 

 eyes, may poiTefs lefs mobility of the contractile 

 fibres, as well as lefs quantity of exertion of the 

 fpirit of animation. 



In anfwer to this mode of reafoning it may be 

 fufficient to obferve, that the contractile fibres con- 

 fift of inert matter, and when the fenforial power 

 is withdrawn, as in death, they poffefs no power of 

 motion at all, but remain in their laft ftate, whe- 

 ther of contraction or relaxation, and muft thence 

 derive the whole of this property from the fpirit of 

 animation. At the fame time it is not improbable, 

 that the moving fibres of ttrong people may pof- 

 fefs 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 expen- 

 diture of the icnforial power, or fpirit of animati- 

 on ; 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 propor- 

 tionally lefiened ; which is to be afcribed to the ex- 

 hauftion or diminution of its quantity. On the 

 contrary, where there has been lefs fibrous contrac- 

 tion than ufual for a certain time, the fenforial 

 power or animation becomes accumulated in the in- 

 active part of the fyftem. Hence vigour fucceeds 

 reft, and hence the propenfity to action of all our 

 organs of (enfe and mufcles is in a ftate of perpe- 

 tual fluctuation. The irritability for inftance of 

 the retina, that is, its quantity of fenforial power, 

 varies every moment according to the brightnefs or 

 obfcurity of the object laft beheld compared with 

 the prefent one. The fame occurs to our fenfe of 

 heat, and to every part of our fyftem, which is 

 capable of being excited into action. 



When 



