THE EYE AND ITS VISUAL STRUCTURE. 
209 
judicial to vision; while in a dark stable it expands, in 
consequence of a deficient portion of light reaching the 
retina. 
This contraction and expansion of the pupil is of much 
service in enabling purchasers to judge of the state of the 
horse’s sight. There is a description of blindness, in which 
the crystalline lens and cornea continue quite transparent, 
but the retina of the eye is palsied, and consequently not 
affected by light; so that the pupils are hardly if at all 
altered by a change from light to darkness, or vice versd. 
In purchasing a horse, the size of both pupils should be 
strictly observed, to see that they both expand and con¬ 
tract to the same extent by the change of light. The hand 
ought to be held over the eyes for a short time, and then 
notice if both pupils expand alike, when removed. 
Suspended from the upper edge of the pupil of the eye 
two or three black bodies may be discovered on close 
inspection, the size of millet-seeds. When the horse is 
suddenly brought into an intense light, the pupil suddenly 
contracts ; these little globes are pressed out from between 
the edges of the iris, and an equal number, but of much 
smaller size, are attached to the lower edge of the iris. 
Their use probably is to intercept portions of light which 
would be injurious to the eye. But their chief function is 
called into action while grazing, and perform the duties 
which are attributed to the eyelashes, namely, to obstruct 
the rays of light in those directions in which it would 
come with the greatest force both from above and below ; 
while at the same time the field of view is quite open, 
so far as respects the pasture on which the horse is grazing 
o, o. The ciliary, or hair-like processes. 
p , The vitreous, or glass-like humour, which fills the whole 
of the cavity of the eye behind the lens. It is a clear 
