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The eye is the organ of sight. This complex structure works by capturing light and transforming it into impulses that the brain can interpret as images.

To understand visual perception, it is important to know the functions of the parts of the eye. The eye includes the eyeball and all structures within and surrounding its almost spherical mass. This delicate organ is nestled within the bony socket of the skull. A layer of fat cushions the socket; the eyebrow, eyelashes, and eyelid provide a barrier against incoming irritants.

Lining the inside of the eyelid and continuing over the exposed surface of the eyeball is the conjunctiva, a thin protective membrane. Tears released from the lacrimal glands in the upper eyelid moisten the conjunctiva and keep the eye clean. The sclera is the tough, white, outer layer of the eyeball. The sclera covers the entire eyeball, except for the circular area in front that admits light, which is covered by the transparent cornea. The choroid layer contains blood vessels that nourish the eye.



Light enters the eye through the cornea. The curved cornea helps to focus the light inward. Behind the cornea is a pigmented structure called the iris. The iris surrounds an opening known as the pupil. The iris changes the size of the pupil, depending on the amount of light present in the environment: If the surroundings are relatively dark, the pupil is enlarged to admit more light; if the environment is bright, the pupil is made smaller.

Behind the iris is the lens, a transparent structure held in place by elastic, muscular-type tissue. The tissue can change the shape of the lens to finely focus the incoming light rays onto the light-sensitive cells that line the back of the eye.

Between the cornea and the lens is a space, the anterior chamber, which is filled with a fluid called aqueous humor. Aqueous humor contains nutrients that nourish the cornea and the lens. The fluid also allows light rays to pass through easily.

The chamber of the eyeball behind the lens holds a clear jelly called vitreous humor. In the retina (the layer of light-sensitive cells that lines the back of the eyeball) are the specialized cells, called rods and cones, that convert light focused from the cornea and lens into electrical impulses. Sensitive nerve endings then transmit these impulses to the brain via the optic nerve, which extends from the rear of the eyeball to the brain.

posted by Martin Crosbie @ 6:46 AM