9* OF STIMULUS AND EXERTION. SECT. XII. 



excrement ; and the pulfations of the heart of a 

 viper are faid to continue fome time after it is 

 cleared from its blood. 



In thefe cafes the violent contractions of the fibres 

 produce pain according to law 4 ; and this pain 

 conftitutes an additional kind or quantity of ex- 

 citement, which again induces the fibres into con- 

 traction, and which painful excitement is again 

 renewed, and again induces contractions of the 

 fibres with gradually diminifhing effect. 



4. A quantity of flimulus greater than the laft 

 mentioned, or longer continued, induces the an- 

 tagonift mufcles into fpafmodic action. This is 

 beautifully iiluftrated by the ocular fpectra de- 

 fcribed in Sect. XL. No. 6. to which the reader 

 is referred. From thofe experiments there is rea- 

 fon to conclude that the fatigued part of the re- 

 tina throws itfelf into a contrary mode of action 

 like ofcitation or pandiculation, as foon as the fti- 

 mulus, which has fatigued it, is withdrawn ; but 

 that it ftill remains liable to be excited into action 

 by any other colours except the colour with which 

 it has been fatigued. Thus the yawning and ftretch- 

 ing the limbs after a continued action or attitude 

 feems occalioned by the antagonift mufcles being 

 ftimulated by their extenfion during the contrac- 

 tions of thofe in action, or in the fituation in 

 which that action laft left them. 



5. A quantity of ftimulus greater than the laft, 

 or longer continued, induces variety of convulfions 

 or fixed fpafms either of the affected organ or of 

 the moving fibres in the other parts of the body. 

 In refpect to the fpectra in the eye, this is well ii- 

 luftrated in No. 7. and 3, of Sect XL. Epileptic 

 convulfions, as theemprofthotonos and opifthotonos, 

 with the cramp of the calf of the leg, locked jaw, 

 and other cataleptic fits, appear to originate from 

 pain, as fome of thefe patients fcream aloud be- 

 fore 



