The eye is a complex organ composed of many small parts, each vital to normal vision. The ability to see clearly depends on how well these parts work together.
Light rays bounce off all objects. If a person is looking at a particular object, such as a tree, light is reflected off the tree to the person's eye and enters the eye through the cornea (clear, transparent portion of the coating that surrounds the eyeball).
Next, light rays pass through an opening in the iris (colored part of the eye), called the pupil. The iris controls the amount of light entering the eye by dilating or constricting the pupil. In bright light, for example, the pupils shrink to the size of a pinhead to prevent too much light from entering. In dim light, the pupil enlarges to allow more light to enter the eye.
Light then reaches the crystalline lens. The lens focuses light rays onto the retina by bending (refracting) them. The cornea does most of the refraction and the crystalline lens fine-tunes the focus. In a healthy eye, the lens can change its shape (accommodate) to provide clear vision at various distances. If an object is close, the ciliary muscles of the eye contract and the lens becomes rounder. To see a distant object, the same muscles relax and the lens flattens.
Behind the lens and in front of the retina is a chamber called the vitreous body, which contains a clear, gelatinous fluid called vitreous humor. Light rays pass through the vitreous before reaching the retina. The retina lines the back two-thirds of the eye and is responsible for the wide field of vision that most people experience. For clear vision, light rays must focus directly on the retina. When light focuses in front of or behind the retina, the result is blurry vision.
The retina contains millions of specialized photoreceptor cells called rods and cones that convert light rays into electrical signals that transmitted to the brain through the optic nerve. Rods and cones provide the ability to see in dim light and to see in color, respectively.
The macula, located in the center of the retina, is where most of the cone cells are located. The fovea, a small depression in the center of the macula, has the highest concentration of cone cells. The macula is responsible for central vision, seeing color, and distinguishing fine detail. The outer portion (peripheral retina) is the primary location of rod cells and allows for night vision and seeing movement and objects to the side (i.e., peripheral vision).
The optic nerve, located behind the retina, transmits signals from the photoreceptor cells to the brain. Each eye transmits signals of a slightly different image, and the images are inverted. Once they reach the brain, they are corrected and combined into one image. This complex process of analyzing data transmitted through the optic nerve is called visual processing.
The stabilization of eye movement is accomplished by six extraocular muscles that attach to each eyeball and perform their horizontal and vertical movements and rotation. These muscles are controlled by impulses from the cranial nerves that tell the muscles to contract or to relax. When certain muscles contract and others relax, the eye moves.
The six muscles and their function are listed here:
- Lateral rectus–moves the eye outward, away from the nose
- Medial rectus–moves the eye inward, toward the nose
- Superior rectus–moves the eye upward and slightly outward
- Inferior rectus–moves the eye downward and slightly inward
- Superior oblique–moves the eye inward and downward
- Inferior oblique–moves the eye outward and upward
There are five different types of eye movements:
- Saccades–looking from object A to object B
- Pursuit–smoothly following a moving object
- Convergence/divergence–both eyes turning inward/outward simultaneously
- Vestibular–eyes sensing and adjusting to head movement via connections with nerves in the inner ear
- Fixation maintenance–minute eye movements during fixation
Eyelids, Eyelashes, Conjunctiva
The eyelids are moveable folds of skin that protect the front surface of the eyes. They close the eyes and blink, which moistens the surface of the eyes and removes debris. The eyelashes (also called cilia) are hairs that grow at the edge of the eyelids and remove minute particles of debris away from the surface of the eyes. The conjunctiva is the thin, transparent, mucous membrane that lines the eyelids and covers the front surface of the eyeballs. The section that lines the eyelids appears red in color because it contains many blood vessels. The section that covers the cornea appears white because of the sclera behind it.
Tear Production and Elimination
Tears perform vitally important functions:
- Carry bacteria-fighting compounds to the eye
- Carry nutrients to and waste products away from the eye
- Keep the eye moist
- Provide a smooth refracting surface
- Remove debris from the eye
Tear components are produced by the lacrimal gland, several other small glands, and cells within the eyelid. As the eyelid closes, tears are swept downward, toward the nose, and enter the puncta (openings in the upper and lower lids, close to the nose). As the eyes blink, tears are forced through narrow channels into the lacrimal sac. Once the muscles relax and the eye opens, the tears move from the sac to the nasolacrimal duct and into the nose. This accounts for stuffy, runny noses when crying.
Aqueous Humor Production and Elimination
Aqueous humor is nutritive watery fluid produced by the ciliary body through the ciliary body processes and secreted into the posterior chamber (i.e., space between the iris and the lens). It maintains pressure and provides nutrients to the lens and cornea. Aqueous humor diffuses through the pupil into the anterior chamber (between the lens and cornea) and is reabsorbed into the venous system by two routes:
- Through the trabecular meshwork (collagen cords that form a spongelike, three-dimensional net) into the canal of Schlemm, which carries it into the venous system. Responsible for 80–90% of aqueous drainage.
- Through the anterior ciliary body directly into larger blood vessels (called uveal-scleral outflow pathway). Responsible for 10–20% of aqueous drainage.
What Can Go Wrong in the Eye?