CHAPTER XII 

 DISEASES OF THE EYE 



General Discussion. — The eye is situated in the orbital 

 cavity, to which it is attached by muscles that rotate it in dif- 

 ferent directions. The orbit is lined by fibro-fatty tissues that 

 form a cushion for the eye. Anteriorly it is protected by the 

 eyelids, and in birds by a third eyelid that corresponds to the 

 membrana nictitans of quadrupeds. The lachrymal gland which 

 secretes the tears keeps the above parts moist. 



The eye is the essential organ of vision. It is formed by a 

 spherical shell which encloses fluid or semisolid parts. The 

 shell is anteriorly made up of a transparent convex membrane, 

 the cornea, while the remainder of its wall is formed by three 

 opaque layers or tunics. 



The external tunic is the sclerotic. It is a white, solid mem- 

 brane, forming about four-fifths of the external shell. Its ex- 

 ternal face is related to the muscles and fatty cushion. It 

 receives posteriorly, a little lower than its middle portion, the 

 insertion of the optic nerve, which passes through the shell and 

 spreads out to form a very thin membrane, the retina or internal 

 coat. 



The retina lines about two-thirds of the posterior portion of 

 the shell of the eye. It is made up of seven layers. The essential 

 layer is named from its appearance, rods and cones. 



The paiddle coat is the choroid. This is a dark, pigmented, 

 vascular and muscular membrane. The posterior portion is in 

 contact with the retina. Anteriorly it forms the ciliary processes 

 and the iris. 



The media of the eye are the crystalline lens, vitreous and 

 aqueous humors. The crystalline lens is a transparent, biconvex 

 body sustained by the ciliary processes. The vitreous humor is 



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