128 THE SENSORIAL FUNCTION. 



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 secretion or 

 paint. 



m m Th 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 (hair-like) processes. 



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

 roides 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 conjunctiva, /, is that membrane which lines the lids, and covers the fore 

 part of the eye. It spreads over 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 lids 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 that 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. 



Covering the back part of the eye, and indeed four-fifths of the globe of it, 

 is the sclerotica, k. It is an exceedingly strong membrane, composed of 

 fibres interweaving with each other, and almost defying the possibility of separa- 

 tion. An organ so delicate and so important as the eye requires secure protection. 



It is a highly elastic membrane. It is necessary that it should be so, when 

 it is considered that the eye is surrounded by several and very powerful muscles, 

 which must temporarily, and even for the purposes of vision, alter its form. 

 The elasticity of the sclerotica is usefully exhibited by its causing the globe of 

 the eye to resume its former and natural shape, as soon as the action of the 

 muscle ceases. 



The sclerotica has very few blood-vessels is scarcely sensible and its dis- 

 eases, except when it participates in general disturbance or disorganisation, are 

 rarely brought under our notice. 



The cornea is, or we should wish it to be, the only visible part of the horse's 

 eye, for the exhibition of much white around it is a sure symptom of 

 wickedness. The cornea fills up the vacuity which is left by the sclerotica in 

 the fore part of the eye, and, although closely united to the sclerotica, may be 

 separated from it, and will drop out like a watch-glass. It is not round, but 

 wider from side to side than from the top to the bottom ; and the curve rather 

 broader towards the inner than the outer corner of the eye, so that the near eye 

 may be known from the off one after it is taken from the head. 



The convexity or projection of the cornea is a point of considerable import- 

 ance. The prominence of the eye certainly adds much to the beauty of the 

 animal, but we shall see presently, when we consider the eye as the organ of 

 sight, that by being too prominent the rays of light may be rendered too con- 

 vergent, and the vision indistinct ; or, if the cornea is small and flat, the rays 

 may not be convergent enough, and perfect vision destroyed. In either case 

 the horse may unpleasantly start, or suddenly and dangerously turn round. An 

 eye neither too prominent nor too flat will be nearest to perfection. 



It should be perfectly transparent. Any cloudiness or opacity is the conse- 

 quence of disease. It is an exceedingly firm and dense membrane, and can 



