CHAPTER XIII. 



DISEASES OF THE EYE. 



The eye, or organ of vision, is composed of three tunics or 

 coats, and of the same number of humours. To the external 

 coat (sclerotic and cornea) it owes its form. The middle 

 tunic is made up of the choroid, or vascular coat, of the iris, 

 or the thin curtain suspended in the aqueous humour, and 

 perforated in the centre by an opening called the pupil, or 

 pupillary opening, which in the horse is of an elliptical form ; 

 in man it is round. The inner coat is called the retina, or 

 nervous covering, and is the terminal expansion of the optic 

 nerve. 



The humours are three in number, and they serve as re- 

 flectors of the light. They are — the aqueous humour, crys- 

 talline lens, and vitreous humour. The latter is the largest, 

 and occupies about four-fifths o^ the whole interior of the 

 globe or eyeball. The appendages of the eye are — the eye- 

 lids, the eyelaslies, and the membrana nictitans, generally 

 called the haw, which is situated in the inner or lower angle 

 of the eye. It is connected with the different muscles of the 

 eyeball. By the contraction of the straight muscle of the 

 eye, the haw is forced outwards, and is one of the beautiful 

 arrangements that nature has provided for the protection of 

 so delicate and sensitive an organ. The eye is whoUy covered 

 by a thin membrane called the conjunctiva. 



