Most animal species have eyes similar in anatomy to that of humans, though each species has their own variation.  Essentially the eye, regardless of the species, is built and functions much like a camera.

When a person or animal looks at an object, light is reflected off of that object and it passes through the clear outer window of the eye, the cornea. The cornea is a sensitive structure with many nerve endings and its health and clarity is essential for clear vision.  After light passes through the cornea, it then passes through the front chamber of the eye (anterior chamber) which is filled with clear fluid (aqueous humor). The correct volume of aqueous humor ensures that our eyes have normal internal pressure and its composition is essential to the health of the inner cornea and the lens. Light then passes through the pupil, the black hole in the center of the iris (colored part of the eye), the lens, the vitreous (a clear, gel-like substance), and onto the back wall of the eye. 

The lens is crystal clear and it is suspended and kept in position by lens zonules (little anchoring ligaments). The lens allows us to focus on the objects we look at and see them clearly.  As light passes through the pupil, the lens focuses light back through to the retina (the light-sensitive tissue in the back wall of the eye). The retina then converts the light energy into an electrical signal that gets carried back to the brain by way of a cable called the optic nerve. The brain then processes the information, producing a picture - or that which we "see."

The health of the outer part eye is provided by the protection of the eyelids and the tears (produced by tear glands) which provide nutrients and lubrication to the eye surface. The sclera is the white outer fibrous wall of the eye, and the conjunctiva is a relatively clear membrane covering the sclera. Internally, the eye is rich in blood supply - particularly in the uveal tract which includes the iris, ciliary body and choroid. If any portion of this internal vascular coat or uveal tract becomes compromised or diseased, severe inflammation can occur within the eye.  

Learn more about diseases of the eye