SECT. XII. 4. 4. AND EXERTION. 63 



No. c. Thus the action of vomiting ceafes and is renewed by 

 intervals, although the emetic drug is thrown up with the firft 

 effort. A tenefmus continues by intervals fome time after the 

 exclufion of acrid 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 fibves produce 

 pain according to law 4 , and this pain conftitutes an additional 

 kind or quantity of excitement, which again induces the fibres 

 into contraction, and which painful excitement is again renew- 

 ed, and again induces contractions of the fibres with gradually 

 diminifhing effect. 



4. A quantity of flimulus greater than that laft mentioned, or 

 longer continued, induces the antagonift mufcles into fpafmodic 

 action. This is beautifully illuftrated by the ocular fpedtra de- 

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

 From thofe experiments there is reafon to conclude that the fa- 

 tigued part of the retina throws itfelf into a contrary mode of 

 action like ofcitation or pandiculation, as foon as the ftimulus, 

 which has fatigued it, is withdrawn , but that it ftill remains li- 

 able to be excited into action by any other colours except the 

 colour with which it has been fatigued. Thus the yawning and 

 ftretching the limbs after a continued action or attitude feems 

 occafioned by the antagonift mufcles being ftimulated by their 

 exteniion during the contractions of thofe in action or in the 

 fituation in which that action laft left them. 



5. A quantity of ftirnulus greater than the laft, or longer con- 

 tinued, 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 il- 

 luftrated in No. 7 and 8, of Sect XL. Epileptic convulfions, as 

 the emprofthotonos and opifthotonos, with a cramp of the calf 

 of the leg, locked jaw, and other cataleptic fits, appear to origi- 

 nate from pain, as fome of thefe patients fcream aloud before 

 the convulfion takes place , which ieems at firft to be an effort 

 to relieve painful fenfation, and afterwards an effort to prevent it. 



In thefe cafes die violent contractions of the fibres produce fo 

 much pain, as to conftitute a perpetual excitement 5 and that in 

 fo great a degree as to allow but fmall intervals of relaxation of 

 the contracting fibres as in convulfions, or no intervals at all as 

 in fixed fpafms. 



6 A quantity of ftimulus greater than the laft, or longer con- 

 tinued, produces a paralyfis of the organ. In many cafes this 

 paralyfis is only a temporary effect, as on looking long on a fmall 

 area of bright red filk placed on a fheet of white paper on the 



floor 



