THE EYE AND ITS VISUAL STRUCTURE. 209 



judicial to vision ; Avliile 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 versa. 

 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 beloAv ; 

 wliile 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-hke humour, which fills the whole 

 of the cavity of the eye behind the lens. It is a clear 



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