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DISEASES OF THE EYE. 165 



o The relina^ or net-like expansion of the optic nerve, spread over the whole 



of the choroides as far as the lens. 

 p The vitreous (glass-like) humor filling the whole of the cavity of the eye 



behind the lens. 

 q The aqueous (water-like) humor filling the space between the cornea 



and the lens. 



"We shall be. better able to understand tbe various diseases 

 of the horse's eje, if we can firsf get a correct idea of its 

 structure and the functions of its different parts. With the 

 help of the foregoing cut, this will not be difficult. 



The eye has three distinct membranes, or coats, and also 

 three humors. The sclerotic coat (see k) is that upon the 

 outside, covering about four-iifths of the globe of the eye. 

 It is strong, firm, and inelastic, by which qualities it is ad- 

 liiirably adapted to its office of protecting from external 

 injury the delicate organ which it incloses. It is of a white 

 color, and constitutes that membrane which is seen when 

 the " white of the eye " is exposed. In the human being, 

 this is very conspicuous ; but in the horse it is rarely visible, 

 unless the animal meditates mischief, when he turns his 

 glance outward or backward as far as he can, and thus shows 

 a little patch of the sclerotic coat. 



In front — this being the part over which the sclerotica is 

 not extended — is the cornea (see /). This is a perfectly trans- 

 parent coat, set in like a watch-crystal in its case, under the 

 circular edges of the sclerotica. It is the outer membrane 

 upon the front of the eye. 



The choroid coat, (see J) comes next, nearly similar in its ex- 

 tension to the sclerotic. It is an exceedingly fine membrane, 

 filled with a net-work of blood-vessels, and covered upon the 

 inside with a peculiar secretion, a black pigment, or paint, 

 which absorbs such wandering rays of light received within 

 the eye as might dazzle or confuse the vision. Opposite the 

 pupil (which will be described shortly), it has a beautiful, 

 greenish -white lining, whose reflection, although not visible 

 in the glare of day, may be seen quite plainly in the gray of 

 twilight. 



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