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MANUAL OF MODERN FARRIERY. 
part of the eye, from the optic-nerve to the cornea. This 
is intended to absorb the stray rays of light which might 
dazzle and confuse the animal, and is not found on any 
portion which may be called the field of vision ; but in its 
stead a bright green substance is spread, which extends more 
over the upper than the lower portion, because the objects 
which are most necessary for him to see are below the level 
jf the head. This in some way, yet undiscovered, enables 
the animal to see even when it is comparatively dark. In 
the dusk this beautiful sea-green colour may be seen in 
the eye of a horse. 
Cream-coloured, or perfectly white horses, have not this 
dark pigment; so that the ordinary appearance of the 
oupil is red instead of black. In looking into their eyes 
we do not see the covering, but the choroid coat itself. 
w, m. The iris, or rainbow-coloured circular membrane, situated 
under the cornea in front of the eye, and on which the 
colour of the eye depends. The duplicature behind is the 
uvea, so called from its colour resembling a grape. See also 
plate iii. fig. 5, i The iris acts as a curtain, and floats in 
the aqueous humour. 
In horses the colour of the iris is subject to little variation, 
but for the most part has some analogy with that of the 
nide, varying in different degrees from hazel to dark brown. 
n. The pupil is placed in the centre of the cornea, and through 
which all the rays of objects pass to the retina or mirror of 
the eye. See also plate iii. fig. 5, k. 
In the horse it is of an oblong form, (see plate iii. fig. 4.) 
It is variable in size, depending upon the intensity of the light 
in which it is viewed. In the open air it will be much con¬ 
tracted, so as to prevent too large a quantity of light being 
thrown upon the retina, as it is painful and injurious to that 
object to receive too great a portion of it, as well as pre- 
