The eyes are complex sensory organs. They are designed to optimize vision under conditions of varying lighting. The basic elements are similar to those of an average photography camera.
The white outer layer of the eyeball is called the sclera. The sclera is covered by a fine tissue containing tiny blood vessels called the conjunctiva. (It is the dilation and proliferation of these blood vessels that accounts for the pinkness of conjunctivitis or "pinkeye.")
The iris of the eye is the color portion behind the cornea. Our eye color is a function of the amount of pigment within the iris (brown eyes have the most pigment, while blue eyes have the least). The iris contains muscles that open and close its central opening called the pupil in response to decreases and increases in light exposure (exactly like a camera).
The central core of the eyeball is a gel-filled area called the vitreous. Behind the vitreous, at the back of the eye is the retina. The retina is composed of 10 fine layers of specialized nervous tissues that respond to light exposure by initiating chemical reactions that are translated into electrical signals which are then transmitted to the back of the brain through a large nerve in the back of the eye (optic nerve).
Six different muscles located around the eyeball are capable of directing the eyes toward the subject to be seen.
The eyes are protected by the tears normally produced by the tear glands (lacrimal glands) located above the outer portion of each eye. The tears drain from the eyes through the tear ducts, located on the inner portion of the eyes. These ducts drain into the nose. This is why our noses run when we weep!