EXTERNAL SUBSTANCE. 



SO 



STRUCTURE OF THE EYE, 



WITH AN EXPLANATION OF THE PSINCIPAL ANATOMICAL TEEMS. 



A B. A supposed object viewed by the animal, and an inverted image of which, o l> 

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



e «. Points where the rays, having passed the cornea and the lens, converge by the 

 refractive power of the lens. 



d e Rays proceeding from the extremities of the object to the eye. 



f The coi-nea, or horny and transparent part of the eye, covered by the conjunctiva, 

 uniting different parts together. 



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

 humour. 



h h Muscles of the eye. 



i Optic nerve, or nerve of sight. 



k 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 the 

 optic nerve. 



I The choroides (receptacle or covering), or choroid coat, covered with a black secre- 

 tion or paint. 



m m The iris, or rainbow-coloured circular membrane, under the cornea, in front of the 

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

 uvea, from being coloured like a grape. The opening in the centre is the pupil. 



n n The ciliary (hair4ike) processes, 



The retina, 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) humour filling the whole of the cavity of the eye behind 

 the lens. 



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



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

 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 

 tiie 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 i.s 

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

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

 G 



