DISEASES OF THE EYE. 897 



and their causes, or the treatment required in the various cases. As far 

 as possible all minute anatomical description will be avoided. 



817. Shape and size of the Eye. 



The Eye is nearly globular, except at its anterior part, which, probably 

 for the purpose of better converging the rays of light, is more convex. 



The size of the eye is nearly the same in all horses. Any apparent 

 difference arises chiefly from the depth at which it is placed in its socket 

 in different animals. 



818. Outer coverings or Coats. 



The outer surface of the anterior portion of the eye is formed by the 

 Cornea. The remainder of the globe is invested by three coats, namely 

 the Sclerotic or fibrous outer coat, the Choroid or vascular middle coat, 

 and the nervous or inner lining called the lletina. 



ai9. The Cornea. 



The cornea covers about one fifth of the globe of the eye. It is firm 

 in substance, elastic, and capable of resisting considerable pressure, and 

 therefore well calculated to protect the eye from external injury ; whilst 

 its perfect transparency allows the free passage of light through it to the 

 interior. In form it is convex, so as to act as a lens for transmitting the 

 rays of light. 



The cornea consists of two layers, but is denser in the centre than on 

 the sides. By this arrangement greater converging power is gained, and 

 better protection is also afforded to the more exposed parts. Its struc- 

 ture is laminated, and its anterior surface is covered with the epithelial 

 part of the conjunctival membrane. There are no blood vessels in the 

 cornea itself, only nutrient canals; but the conjunctival layer under 

 inflammation becomes very vascular. This is the only layer of the Cornea 

 which is sensitive. The posterior or inner layer is epithelial. 



820. The Sclerotic or outer coat. 



The Sclerotic or outer coat is composed of white fibrous tissue inter- 

 woven in every direction. Its use is to give strength to the globe, and 

 it also serves to give attachment to the various muscles which move the 

 eyeball. It is itself loosely invested by a very fine tunic called the vagi- 

 imlis, which allows the eye more easily to glide over the soft cushion of 

 fat on which it rests. 



821. The Choroid or middle coat. 



The Choroid or middle coat is highly vascular and supplies the nutri- 

 ment required by the internal structures. It is covered with a brown 

 pigment, except at its upper back part, which is invested by a bright 

 green pigment, termed the tapetum lucidum. This peculiarity is found 

 in all animals intended to see in a diminished light. 



