Eye and vision care

Anatomy of the eye

Often called the most complex organ in our body, the eye allows us to see things around us. Yet, the ability to see is dependent on the actions of several structures both inside the eye and around it. Some support the main activity of sight by carrying fluid such as tears and blood. Some structures such as the lids and the epithelium of the cornea protect the eye from injury. Others still, such as the pain-sensing nerves in the cornea and the optic nerve, send sensory information to the brain. Muscles allow the eye to move. All of these structures must work well together in order to allow optimal vision.

How does the eye work? How do we see?

The eye allows us to see and interpret the shapes, colors, and dimensions of objects by processing the light they reflect or emit. Working like a camera, the eye allows light to enter through the cornea, a structure that works much like a camera's aperture. The cornea is a transparent dome that bulges forward and that functions as the window of the eye. It's made up of connective tissue with a thin layer of epithelium on the surface. The cornea is nourished by the aqueous humor, a thin, watery fluid that fills the space between the cornea and the iris. The aqueous humor, produced by the ciliary body, nourishes the cornea and gives the front of the eye its form and shape.

The transparency of the cornea is due to the fact that it has no blood supply and contains very few cells. The cornea, however, contains the highest concentration of nerve fibers of any structure in the body. As anyone who's ever had a grain of sand, a particle of dust, or another foreign body in their eye can tell you, irritation of the cornea can be extremely painful.

The amount of light that is allowed to enter is controlled by the pupil, the circular hole in the center of the colored disc inside the eye known as the iris. The pupil opens and closes much in the same way a shutter does.

In low light situations, the dilator muscles pull away from the center of the iris, causing the pupil to dilate, or grow larger. This allows more light to reach the retina. When a situation is too bright, sphincter muscles pull toward the center of the pupil, constricting it and allowing less light to reach the retina.

As seen, light first converges on the cornea and then passes through the pupil, to further converge to a nodal point just behind the surface of the crystalline lens. At the nodal point, the image is inverted. Light continues to pass through the gelatinous vitreous humor, a clear gel occupying the rear chamber of the eye. Once the image is inverted at the nodal point, it ideally reaches a clear focus on the retina.

The retina, the eye's innermost layer, can easily be compared to the film inside a camera. It's composed of nerve tissue and photoreceptors (called rods and cones, respectively) which sense light entering the eye.

When light passes by these photoreceptors, a chemical reaction is initiated, stimulating nerve cells and enabling the transmission of electrical impulses. These impulses travel from the photoreceptors to bipolar cells and from there to ganglion cells. Ganglion cells then receive the impulses and send them to the brain via the optic nerve. It's in fact our brain that allows us to see by turning the inverted image focused on the retina right-side up and interpreting the information our eyes collect.

What about other structures? What do they do?

Sclera

The eye is protected by a thick white coating called the sclera. This tough, opaque tissue acts as the eye's protective outer coat.

Extraocular muscles—rectus and oblique muscles

Along with various ligaments, tendons, and fascial expansions, six extraocular muscles connect to the sclera and hold it in place. The extraocular muscles control eye movement. The superior (top) and inferior (bottom) rectus muscles control the eye's movment up and down. The medial rectus and lateral rectus muscles (not shown in the diagram above) control the eye's movment from side to side. The superior oblique and inferior oblique muscles (not shown in the diagram above) help rotate the eyes inward and outward. All six of these extraocular muscles work in unison to move the eye. They coordinate so that the eyes are always aligned. There are conditions, however, in which eye movements are not synchronous and in which a person may appear cross-eyed. Strabismus is one such condition.

Choroid

Composed of layers of blood vessels, the choroid lies between the retina and the sclera. It helps nourish the back of the eye and is attached to the edges of the optic nerve. Toward the front of the eye, the choroid connects with the ciliary body.

Optic nerve

The optic nerve carries electrical impulses from the retina to the brain. It connects to the back of the eye near a region called the macula.

Macula

Located roughly in the center of the retina, the macula is a small and highly sensitive part of the retina responsible for detailed central vision. The macula is what determines our visual acuity—that is, our ability to appreciate details and to perform tasks such as reading that require central vision. With age-related macular degeneration, the macula loses its ability to clearly discern details and central vision may become blurry.

Eyelids

The eyelids help protect the eye from the environment, injury, and light. By spreading tears evenly over the surface of the eye, they help maintain a smooth corneal surface. Eyelids consist of an outer layer of skin, a middle layer of muscle, and an inner conjunctiva. The muscles are what give the eyelids their form. Lazy or weakened muscles may make eyelids droop, giving someone the appearance of having a "lazy eye".

The orbicularis oculi is a circular muscle that closes the lids. Mueller's muscle gives the eyelids tone and helps maintain elasticity. These muscles work not only to protect our eyes from bright light and from debris, sand, and other harmful particles in our environment, but they also play a role in facial expressions, allowing us to express emotions and feelings.

Conjunctiva

Beginning at the outer edge of the cornea and covering the visible part of the sclera, the conjunctiva is a thin, transparent tissue that covers the outer surface of the eye and that also lines the inside of the eyelids. The conjunctiva is part of a complex system that works to lubricate the eye. Tiny meibomian glands in the conjunctiva secrete oils and mucus to moisten and lubricate the eye. The meibomian glands line the inner edges of the eyelids.

Tear film

As mentioned, the conjunctiva is part of a more complex system that works to keep the eye moist. Tears help in this. Not only do they help keep they eye moist, but tears also help nourish the front of the eye. They provide protection from injury and infection by flushing away debris and foreign particles, and they create a smooth surface for light to pass through the eye.

Tears are produced by tiny lacrimal glands located toward the outer edge of the eye, just below the eyebrow. Tear film is composed of three layers: oil, water, and mucus. The bottom mucus layer, produced by goblet cells in the conjunctiva, anchors the tear film by helping it adhere to the eye. The top oil layer acts as a sealant and prevents evaporation of the water layer beneath.

Each time you blink, your eyelids spread the tear film evenly across the surface of your eye. Blinking forces tears into tiny drains (called puncta) found at the inner corners of the upper and lower eyelids. From the puncta, the tear film travels via the upper and lower canaliculus and empties into the lacrimal sac. The lacrimal sac, which connects to the nasal passage, drains into the nasolacrimal duct. It's because of this connection between the eye's tear production system and the nose that your nose runs when you cry and that you can sometimes taste eye drops as they drain from your nasal passage into your throat.

When people develop problems with the quality of the tear film that lubricates the eye, they may develop a condition known as Dry Eye Syndrome.

Lens

Located just behind the iris, the crystalline lens helps focus light on the retina. The innermost part of the lens, or nucleus, is surrounded by softer material called the cortex. The lens is encased in a capsular bag and held in place by zonules, tiny fibers extending from the ciliary body which act as guy wires, suspending the lens inside the eye.

While the lens is normally clear, some people may develop cataracts, a condition in which the lens becomes clouded, resulting in blurry vision. When the lens loses its normal elasticity and gradually hardens, people are said to have presbyopia.

Angle structures

The area at which the cornea and iris join is known as the drainage angle or simply, the angle. The angle is made up of several complex structures that comprise the eye's drainage system—structures including the outermost part of the iris, the front of the ciliary body, the trabecular meshwork, and Schlemm's Canal.

Aqueous fluid formed in the ciliary body behind the iris flows through the pupillary space into the aqueous chamber at the front of the eye. From this front (anterior) chamber, fluid then travels into the angle structures and drains from the eye, passing through a filter known as the trabecular meshwork and then through a tiny channel in the sclera called the Canal of Schlemm or Schlemm's Canal.

Production and drainage of aqueous fluid determines the eye's intraocular pressure. When the angle at which the cornea meets the iris is too narrow or in cases where scar tissue or other tissue obstructs the drainage angle, drainage of aqueous fluid slows and pressure within the eye increases. When this happens, a person may develop glaucoma.