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CHAPTER XVII. 



Diseases of the Eyes. 



sketch of the anatomy op the eye simple ophthalmia pbbioukj 



ophthalmia, or moon blindness cataract turning in of 



the eyelashes worm in the eye worms under the eyelids 



amaurosis, or glass eye. 



Sketch of the Anatomy of the Eye. 



The surface of the eye (Fig. 140), and the inside of the eyelids are covered 

 by the conjunctiva, which is a delicate mucous membrane. The front of the 

 eye-ball consists of a strong transparent structure, the cornea, which is 

 somewhat of the form of a small watch-glass ; and the remainder of the part 

 consists of a strong fibrous coat, the sclerotica, which is lined by a dark 

 membrane, the choroid. Behind the cornea, and attached to the choroid, 

 hangs a thin contractile curtain, termed the iris, through the centre ot 

 which there is an elliptical opening known as the pupil of the eye. The 

 anterior portion of the sclerotica lies immediately under the so-called while 

 of the eye, which is the expansion of the tendons of muscles that move the 

 eye. The cornea may be regarded as the continuation of the sclerotica ; 

 and the iris, as that of the choroid. 



The pupil is dilated by means of radiating muscular fibres which are in 

 the iris ; and is closed by circular fibres placed round the margin of the 

 opening. The distinctive colour of the eye is derived from that of the iris, 

 which is variously coloured, but in the horse is brown, with more or 

 less of a yellow tinge ; sometimes, however, it is almost white or grey, 

 when the animal is said to be " wall-eyed." Behind the iris, which 

 is slightly convex, and suspended from the choroid, is a biconvex, trans- 

 parent, solid body — the crystalline lens — ^which is compared by Chauveau 

 to a rose diamond. The lens divides the eye into two compartments ; the 

 anterior, which is partially divided by the iris, and filled by a watery fluiJ 

 called the aqueous humour ; and the posterior which holds a very similar, 

 but denser, liquid — the vitreous humour. The crystalline lens is coveied 

 by a transparent membrane called the capsule. 



The optic nerve, proceeding from the brain, enters the eyeball at its 

 posterior part, and pierces the sclerotica and choroid and, by its expansion, 

 forms a membrane, the retina, which lines the choroid, and terminates at 

 the circumference of the crystalline lens. When rays of light, coming from 

 any object, fall on the eye, they enter the pupil, and passing through the 

 crystalline lens, form an image on the retina, which conveys, through the 

 optic nerve to the brain, an impression of the object seen. This optical 



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