^lODERN VETERINARY PRACTICE. 



THE ETB.] 



The cri/sfalJine lens is not exactly ft porfi-ct 

 IcMis, tlio ftiitoric^r siili- briiit; rather the flattest. 

 It is contained in a perfect capsule or bag, ft 

 fluid separating tlie eapsulo from tho lens, 

 which is called the liquor morgagni, from IM. 

 !Morgagni, the name of its first discoverer. 

 This capsule does not adhere to, but is con- 

 fined in its position by the tunica vitrea, being 

 reflected over its edge, at which part, also, it is 

 surrounded by the choroid coat, formed into 

 folds, called the ciliary processes. It was for- 

 merly doubted how these parts grew ; but 

 doubts were dispelled on its being discovered 

 that both arteries and veins are situated in 

 these parts. Tiie lens is not only flatter an- 

 teriorly, but is not of the same structure 

 throughout, the outer surface being like a 

 mucus or jelly; whilst within, it becomes harder, 

 even to the centre. From this arrangement 

 tlie rays of light are bent from one degree to 

 another, in proportion as they approach the 

 centre — the crystalline lens acting as a refract- 

 ing glass, receiving the rays, in some measure 

 refracted by the cornea and aqueous humour, 

 and bringing them to a focus on the retina. 



" When either the cornea or the crystalline 

 lens is too convex," says Mr. Spooner, "the 

 object is brought to a focus too soon, and vision 

 is indistinct, and the person near-sighted." 

 This, he adds, as we have already observed, is 

 a frequent defect in the eyes of horses that 

 shy, in which we often find full goggle eyes. 

 "When, however, the cornea or the lens is too 

 flat, a contrary effect is produced, and the 

 object is not converged soon enough. This is 

 often a complaint in old people, who are 

 relieved by convex glasses, whilst the former 

 defect is relieved by concave lenses. 



The iris has an involuntary power of 

 changing under difierent degrees of light. It 

 also has another power independent of the 

 efiects of light — namely, that of looking at a 

 minute object, though the light be weak, and 

 becoming larger as the pupil gets smaller, and 

 vice versd. The pupil being small, the rays 

 are confined to such as proceed from the 

 object alone. Thus there is no confusion of 

 images on the retina, but the smallest object is 

 seen ; and, as in the last instance, when the 

 object to be seen is large, the iris contracts, 

 that the rays may be admitted from all parts 

 for a perfect representation. 



[the eye. 



In examining tho ciliart/ processes, they are 

 nothing more than a continuation of tlie cho- 

 roid coat which surrounds tho capsule of tho 

 crystalline lens. They are called choroidea, 

 and are tho actual foldings of that membrane. 

 Their object is to prevent tho passai;(! of any 

 rays of light on the outside of tho lens, which 

 otherwise would render vision imperfect. 



The vitreous humour is of a peculiar com- 

 position. Although it seems to be a kind of 

 jelly, it is not; but nearly of the same fluidity 

 as the aqueous humour. This appearance 

 arises from its being contained in cells, and 

 not in a single capsular bag. A proof of this 

 is found in the breaking of the cells, when the 

 fluid drops freely. It is also admirably formed ; 

 for being in contact with the expansion of the 

 optic nerve, it cannot alter its position, or be 

 injured by pressure. 



The phenomenon of vision is both curious 

 and grand. In the first place, all objects to 

 be seen require a sufficient quantity of light to 

 render the optic nerve sensible. Although 

 different animals require difierent degrees, 

 man requires the most — a fact demonstrated 

 by the construction of his eye, which has a 

 small transparent cornea, and a black pigment. 

 The vision, to be perfect, must be painted on 

 the retina; and here is a curious fact, that 

 though we have two eyes, we see only one 

 object; yet there is an object painted on each 

 retina, but only one on the sensorium. Being 

 possessed of only one sensorium, so long as 

 the object is the same, there is only one im- 

 pression formed ; but immediately objects 

 change, there are difierent impressions fornied. 

 All objects on the retina being painted upside 

 down, the rays of light cross each other, and 

 meet in a point near the lens, when they pro- 

 ceed through the vitreous humour nearly in 

 the same right lines. 



Objects presented to the eye have their 

 images painted on the black part of the retina, 

 the rays of the incident pencils converging to 

 their proper foci there, by the refraction of tho 

 different humours; and for this office they are 

 admirably adapted ; for as the distance between 

 the back and front of the eye is very small, 

 and the rays of each of the pencils that form 

 the image fall parallel, or else diverging on the 

 eye, a strong refractive power is necessary for 

 bringing them to their foci at the retina: but 



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