Bauhaus-Universität Weimar

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.
        

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