OTHER INVOLUNTARY MOTIONS. 129 i. Stimuli applied to the mufcles of animals, when laid bare, in« (lead of only one contraction lading for a condderabie time, pro¬ duce feveral contractions and relaxations alternately fucceeding each other, which become gradually weaker, and are repeated after longer intervals, as the force of the irritating caufe is diminifhed*. Now, thefe alternate contractions are eafily accounted for, if we fuppofe them to proceed from a fendent pkinciple, which, in order to the getting rid of the pain or uneafy fenfation that arifes from the irri¬ tation of the mufcle, determines the influence of the nerves into its fibres more ftrongly than ufual. For, if by one or two contractions 4 the irritating caufe be thrown off, and, with it, the difagreeable fenfation removed, the mufcle will return to its former date of red ; Æ otherwife, it will continue for a longer time to be agitated by al¬ ternate convulfive motions, which will be more or lefs forcible, and repeated after fhorter or longer intervals, in proportion as the fiimu- lus and painful fenfation hence enfuing are dronger or weaker. The titillation of a dighter ftimulus will be fo much weakened by the drd contraCiion of the mufcle, that fome fpace of time mud intervene before it will be able to produce a fécond : whereas the fmart pain which follows a drong irritation, affeCts the fendent principle fo powerfully, that no fooner is the mufcle relaxed, than a new contrac¬ tion fucceeds. Thus a gentle irritadon of the left orifice of the do¬ rn a cli occadons only a dighter hiccup or convuldve contraction of the diaphragm, which, too, is not repeated till after condderabie paufes ; while a greater irritation, not only excites dronger convul- iions of this mufcle, but alfo a quicker repetition of them. 'Why the fendent principle, in confequence of a painful fenfation, does not keep dich mufcles as are irritated in a continued date of contraction, but differs them to be alternately relaxed, fhall be after¬ wards explained. If the contraction of an irritated mufcle were owing to the aCtion of theßimulus upon it as a mere mechanical organ, then, fo long as the fiimulus continued to aCt equably, the mufcle ought to remain equally contracted, and, upon its ceadng, the mufcle ought to be re- R laxedy ? Seft. i. No, io, and u. above.