EXTERNAL SUBSTANCE. 



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STRUCTURE OF THE EYE, 



WITH AN EXPLANATION OF THE PEINCIPAL ANATOMICAL TERMS. 



A supposed object viewed by the animal, and an inverted imag-e of which, a A 



is thrown on the retina at the back of the eye. 

 Points where the rays, iiaving- passed the cornea and the lens, converg by the 



refractive power of the lens. 

 Rays proceedinjr from the extremities of the object to the eye. 

 The cornea, or horny and transparent part of the eye, covered by the conjunctiva, 



uniting- different parts together. 

 The crystalline (crystal or glassy) lens, behind the pupil, and in front of the vitrooua 



humour. 

 Muscles of the eye. 

 Optic nerve, or nerve of sight. 

 The sclerotica (hard firm coat) covering the whole of the eye, except the portion occu 



pied by the cornea, and being- a seeming prolongation of the covering of thi 



optic nerve. 

 Tlic choroides (receptacle or covering), or choroid coat, covered with a black secro 



tion or paint. 

 The iris, or rainbow-coloured circular membrane, under the cornea, in front of th<i 



eye; and on which the colour of the eye depends. The duplicature behind is the 



urea, from beinsr coloured like a grape. The opening in tlie centre is the pupil. 

 The ciliary (hair-like) processes. 

 The retina, or net-like expansion of the optic nerve, spread over the whole of the 



choroides as far as the lens. 

 The vitreous (glass-like) humour filling the whole of the cavity of the eye behind 



the lens. 

 The aqueous (water-like) humour filling the space between the cornea and the lens 



The conjitdha, f, is that membrane which lines the lids, and covers tha 

 fore part of the eye. it covers all that we can see or feel of the eye, and 

 even its transparent part. It is itself transparent, and transmits the colour 

 of the parts beneath. It is very susceptible of inflammation, during which 

 the lining of the lid will become intensely red, and the white of the eye will 

 be first streaked with red vessels, and then covered with a complete mesh 

 of them, and the cornea will become cloudy and opaque. It is the seat of 

 various diseases, and particularly in it commences the sad inflammation of 

 the horse's eye, which bids defiance to the veterinary surgeon's skill, and, 

 almost invariably, terminates in blindness. 



The examination of the conjunctiva, by turning down the lid, will enable 

 us to form an accurate judgment of the degree of inflammation which exists 

 in the eye. Horsemen and farriers, however, seem to think that it likewise 

 indicates the degree of inflammation in almost every other part, or, at least, 

 of the general fever which may accompany local inflammation. There is 

 a part which much more clearly indicateo mis, and especially if the general 

 disturbance be accompanied or produced by any affection of the lungs — a 



