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.