[ 129 3 
of thefe motions on our minds, cannot be explained 
from this analogy, which does not fubfift in faCt. 
This objection therefore can have no force againft 
our doCtrine, however it may recoil upon another ; 
as all voluntary mufcles whatever may be excited to 
contractions by irritations exceftively painful or plea- 
fing, the contraction in fuch cafes being involuntary, 
neceffary, and uncontroulable. But the ftimuli that 
affeCt the heart and other parts, whole motions are 
naturally involuntary, are not of this clafs and ftrength, 
and indeed are fo little perceived by the mind, as to 
lay it under no fuch neceffity, as* that Jlimulus which 
renders the action of the acceleratores urince involun- 
tary in expelling the femen. 
5. Laftly it has been objected, that, though the 
motions of the uvea are involuntary from light af- 
feCting the eye; they are truly voluntary when it 
contracts in order to the diftinCt vifion of an objeCt 
placed near the eye, whofe minute parts we want to 
obferve accurately. 
It leems on the contrary certain, that the contrac- 
tions of the uvea , in order to diftinCt vifion, equally 
arife from different impreffions of light on the retina > 
and are equally involuntary, and folely dependant 
upon the impreffions of different degrees of light in 
all cafes: the diftinCtion indeed is chiefly verbal; 
as it is granted, that the motions of the uvea are in- 
voluntary from light affeCting the eye, that is, differ- 
ent degrees of light (hiking upon the retina necefla- 
rily occafion more or lets of contraction in the pupil. 
But as vifion, confidered as diftinCt or indiftinCt, is 
occafioned by various impreffions of light upon the 
retina, the contraction of the pupil neceffarily fol- 
Vol. LVII. S lows. 
