Eye Health: Anatomy of the Eye
About the Eye and How It Works
You can get a close-up look at different parts of the eye in this eye diagram from the National Institutes of Health.
Diagram of the eye, viewed from the side
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.
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.
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 does not contain any blood vessels, but instead contains nerve endings that make it extremely sensitive. That is why a scratch or a loose eyelash is so painful.
Aqueous humor is a clear, watery fluid contained in a chamber behind the cornea that helps bring nutrients to the ocular tissue. It is made behind the lens and flows to the front of the eye, where it is drained by a tissue called the trabecular meshwork. Problems with the flow of this fluid can lead to problems with the pressure inside the eye.
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 tissue inside the eye that has a hole 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.
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:
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.
Therefore, people with lighter-colored eyes should wear sunglasses outdoors, especially during the summer. According to Prevent Blindness America, extended exposure to ultraviolet (UV) light (such as sunlight) has been linked with cataracts and macular degeneration.
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 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 Fig. 1):
Fig. 1: More rounded lens can focus on near objects
More elongated (or stretched) to focus on objects that are far away (see Fig. 2):
Fig. 2: More elongated 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 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 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 an 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.
The Prevent Blindness website provides a view of the inside of the eye and its component parts at "The Eye and How We See."
You can get a close-up look at different parts of eye in this eye diagram from the National Institutes of Health.
What are Myopia, Hyperopia, Astigmatism, and Presbyopia?
With myopia, you can see close-up objects clearly, but distant objects are blurred. Myopia occurs when the eyeball is too long for light rays to focus correctly on your retina. The more myopic you are, the blurrier your vision will be at a distance and objects will have to be closer to you in order to see them clearly.
With hyperopia, you can see distant objects clearly, but close-up objects are blurred. Hyperopia occurs when the eyeball is too short for light rays to focus correctly on your retina.
If you have an astigmatism, the surface of your eye (the cornea) is not smoothly curved; instead, the surface is irregular. Normally, the surface of your cornea is rounded, much like a basketball; with astigmatism, however, your cornea is shaped more a like a football (American football, that is). This doesn't allow light rays to focus clearly on a single point on your retina. Astigmatism is usually accompanied by myopia or hyperopia and is usually correctable with eyeglasses.
Presbyopia makes it difficult to focus on close-up words or images. Most people are between 40 and 50 when they begin to realize that the letters of the telephone book are "too small" or that it's necessary to hold the newspaper further away in order to see clearly. At the same time, the ability to focus on objects that are far way remains intact.
Here is what you see with normal vision:
Here is what you see if you have a refractive error. The image appears blurred at a distance if you have myopia or appears blurred close-up if you have hyperopia or presbyopia.