72 OF STIMULUS AND EXERTION. SECT.*!/. i<* 



hundred and twenty pulfations in a minute ; unlefs 

 under peculiar cir cum fiance, as the great additional 

 ftimuli of wine or of external heat. 



5. After a mufcle or organ of fenfe has been ex- 

 cited into contraction, and the fenforial power ceafes 

 to act, the laft fituation or configuration of it con- 

 tinues ; unlefs it be diftributed by the action of 

 fome antagonift fibres, or other extraneous power* 

 Thus in weak or languid people, wherever they 

 throw their limbs on their bed or fofa, there they 

 lie, till another exertion changes their attitude ; 

 hence one kind of ocular fpectra feems to be pro- 

 duced after looking at bright objects ; thus when a 

 fire-flick is whirled round in the night, there ap- 

 pears in the eye a complete circle of fire 5 the ac- 

 tion or configuration of one part of the retina not 

 ceafing before the return of the whirling fire. 



Thus if any one looks at the fetting fun for a 

 fiiort time, and then covers his clofed eyes with his 

 hand, he will for many feconds of time perceive 

 the image of the fun on his retina. A fimilar image 

 of all other bodies would remain fome time in the 

 eye, but is effaced by the eternal change of the 

 motions of the extremity <,f this nerve in our at- 

 tention to other objects. See Sect. XVII. i. 3. on 

 Sleep. Hence the dark fpots, and other ocular 

 fpectra, are more frequently attended to, and re- 

 main longer in the eyes of weak people, as after 

 violent exercife, intoxication, or want of fleep. 



6. A contraction of the fibres fomewhat greater 

 than ufual introduces pleafurable feniation into the 

 fyftern, according to the fourth law of animal cau- 

 fation. Hence the pleafure in the beginning of 

 drunkenneis is owing, to the incrcafed action of the 

 fyftem from the flimulus of vinous fpirit or of 

 opium. If the contractions be ftill greater in ener- 

 gy or duration, painful fenfations are introduced, 



as 



