Eye Health: Anatomy of the Eye



About the Eye and How It Works

side-view diagram of the eye

Diagram of the eye, viewed from the side. National Eye Institute

To understand this diagram of the eye, try to picture it as being split in two, like an apple that's been cut in half. Imagine yourself looking into the eye from the cut side.

The Cornea

The cornea is a transparent dome-shaped tissue that forms the front part of your eye.

It functions as a window and allows light to enter your eye. It also begins the process of focusing light rays that allow you to see words and images clearly. The cornea provides 65-75% of your eye's focusing power.

The cornea does not contain any blood vessels, but instead contains many nerve endings that make it extremely sensitive. That is why a scratch or a loose eyelash is so painful.

Jake Whalen: Living and Coping with Keratoconus

Head shot of Jake Whalen

Jake Whalen is a freelance copywriter who has struggled for most of his life with keratoconus.

Keratoconus is a degenerative condition of the cornea, a transparent dome-shaped tissue that forms the front part of the eye. Keratoconus gradually causes the cornea to thin, bulge/protrude outward, and become cone-shaped. This creates an abnormal curvature of the eye that can cause blurred vision, glare problems, light sensitivity, and even extreme pain.

Learn more about the basics of effective eye care, including:

Aqueous Humor

Aqueous humor is a clear, watery fluid, contained in two chambers behind the cornea, that helps to bring nutrients to the eye tissues. It is produced by the ciliary body, a ring of tissue that sits behind the iris.

As it circulates, the aqueous fluid flows to the front part of the eye, where it is drained by the trabecular meshwork, a sponge-like filtering system located where the cornea and iris meet. After draining through the trabecular meshwork, the aqueous fluid then passes through a small duct, called the canal of Schlemm, and is absorbed into the bloodstream.

The health of your eye depends upon a continuous process of production, flow, and drainage of this aqueous fluid. Any interruption of this process can lead to problems with increased pressure inside the eye, such as glaucoma.

The Sclera

The sclera is a tough white outer coating of fibrous tissue that covers your entire eyeball (all the way around) except for the cornea. The muscles that move the eye are attached to the sclera. The name sclera comes from the Greek word "skleros," which means "hard."

The Iris and the Pupil

The iris is a ring-shaped membrane inside the eye that surrounds an opening in the center, called the pupil. The iris contains muscles that allow the pupil to become larger (open up or dilate) and smaller (close up or constrict). The iris regulates the amount of light that enters your eye by adjusting the size of the pupil opening.

In bright light, the iris closes (or constricts) and makes the pupil opening smaller to restrict the amount of light that enters your eye.

constricted pupil

The iris in bright light

In dim light, the iris opens (or dilates) and makes the pupil opening larger to increase the amount of light that enters your eye:

dilated pupil

The iris in dim light

In addition, it is the iris that determines your eye color. People with brown eyes have heavily pigmented irises, while people with blue or lighter-colored eyes have irises with less pigment.

The Lens

The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. It is the second part of your eye, after the cornea, that helps to focus light and images on your retina.

Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance. The lens provides 25-35% of your eye's focusing power.

The ciliary muscles, which are part of the ciliary body, are attached to the lens and contract or release to change the lens shape and curvature.

The lens becomes more rounded to focus on near objects (see Figure 1):

eye near

Fig. 1: A more rounded lens can focus on near objects.

The lens becomes more elongated (or stretched) to focus on objects that are far away (see Figure 2):

eye distance

Fig. 2: A more elongated/stretched lens can focus on far objects

Over time, the lens loses some of its elasticity and therefore loses some of its ability to focus on near objects. This is called presbyopia and explains why people need reading glasses as they become older.

The Choroid

The choroid is a dark brown membrane that is rich with blood vessels, located between the sclera and the retina. It supplies blood and nutrients to the retina and nourishes all of the other structures within the eye.

The Vitreous

The vitreous is the jelly-like substance that fills the inside of the back part of the eye. Over time, the vitreous becomes more liquid and can detach from the back part of the eye, which can create floaters. If you notice new floaters or flashing lights, it is important to see your eye doctor, because a detached vitreous can cause a hole (a condition called a macular hole) to develop in the retina.

The Retina and Optic Nerve

The retina is the light-sensitive tissue that lines the inside surface of the eye, much like wallpaper. Cells in the retina convert incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which resembles your television cable) to the brain, which finally interprets them as visual images.

The macula is the small sensitive area in the center of the retina that provides clear central vision. The fovea is located in the center of the macula and provides the sharpest detail vision.

Some Facts about the Retina

The retina is the light-sensitive tissue that lines the inside surface of the eye.

The retina contains photoreceptor cells that convert (or process) incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which resembles your television cable) to the brain, which finally interprets them as visual images.

There are two types of photoreceptors: rods and cones, which are the light-processing cells responsible for peripheral (side) and central (straight-ahead) vision.

Rods

  • The specialized, highly light-sensitive retinal processing cells that are able to function in low light levels. They provide peripheral (or side) vision, are responsible for dark adaptation, and are most sensitive to movement/motion. They are less sensitive to color perception.
  • A normal retina contains approximately 120-150 million rods, primarily in the peripheral, or outer, retina.
  • Rods provide scotopic vision which refers to eyesight in low light conditions.

Cones

  • The specialized retinal processing cells that function in bright light levels and provide central (or straight-ahead) vision, along with sharp visual acuity, detail, and color vision. They require bright light to function and are not sensitive to lower light levels.
  • A normal retina contains approximately 6-7 million cones, primarily in the macula, the small area in the center of the retina that provides clear central vision. Cones are the most concentrated in the fovea, which is located in the center of the macula and provides the sharpest detail vision.
  • Cones provide photopic vision, which refers to eyesight in daylight conditions.

Additional Eye Diagrams

You can get a closer look at different parts of eye with this interactive eye diagram from the National Eye Institute.

Healthline.com provides an interactive Human Eye in 3D online tool that can help you understand how the parts of the eye work in relation to each other.

More Information

You can learn more about the parts of the eye and refractive errors, including myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (both near and far blurriness) at Refractive Error and Astigmatism and A Guide to Eye Conditions.

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