Bauhaus-Universität Weimar

7 6 
OF THE VITAL AND 
contracting the pupil, in order to the more diftinCl vifion of near 
objeCts, which feems, like the uniform motions of the eyes, to be 
partly acquired by habit» The caufes, therefore, to which the con¬ 
traction of the pupil are owing, being weaker in new-born infants 
than in adults, it is no wonder that this paffage appears more di¬ 
lated in the former than in the latter. 
In old people, the pupil becomes lefs moveable, becaufe the re¬ 
tina grows lefs fenfible of the ßimulus of light, and the mufcular fi¬ 
bres of the iris lofe in part their contraCiile power : further, in old 
age, the cornea, on account of the decreafe of the aqueous humour, 
lofes its brilliancy ; whence the retina will be lefs affeCled by light, 
and confequently the pupil would become wider, were it not that 
this is more than compenfated by the diminifhed elaflicity of the 
longitudinal fibres of the uvea, and by the eye becoming lefs, in 
confequence of a diminution of the quantity of its humours, whence 
the diameter of the circulas albus, to which the uvea is attached, 
being leffened, the aperture of the pupil muft become narrower. 
The motions of the pupil from light differ from thofe which 
are performed in order to render vifion lefs indiftinCt ; fince the for¬ 
mer are owing to an uneafy fenfation affeCting the retina -, whereas 
the latter arife from an aCl of volition, or effort of the will in or¬ 
der to the more diftinCt vifion of objeCls at certain diftances. The 
former are of the fame kind with the contraction of the diaphragm 
in the hiccup and in vomiting, of the mufcles of refpiration in 
freezing, and of the acceleratores urina in expelling the fernen. The 
latter agree with the motions of the cryflalline, by which the eye 
is adapted to fee diftinCtly at different diftances, and with the uni¬ 
form motions of the eyes in looking at objeCls. The firft are al¬ 
ways neceffary and independent of the will ; but the fécond are 
plainly voluntary, and can be reftrained if we pleafe, though they 
are often not attended with confcioufnefs of volition. 
In looking at near objeCls, the pupil is leffened, at the fame time 
that the cryflalline humour is brought forward towards the cornea 
by the contraction of the ciliary proceffes j but when we contem¬ 
plate more diftant ones, the contraction of the ciliary proceffes and 
orbicular
        

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