An introduction to eyes and how they work
Vision is an incredibly complex process that works so well, we never need to give it much thought.
The work of the visual system can be summarized as follows: light enters our pupil and is focused onto the retina at the back of the eye. The retina converts the light signal into electrical impulses. The optic nerve then carries the impulses to the brain where the signals are processed.
To understand how this amazing feat occurs, we will start with a glimpse at the anatomy of the eye.
Below is a 3D model of the eye, which is fully interactive.
Explore the 3D model, using your mouse pad or touchscreen, to understand more about the eye.
Anatomy of the eye
The tissues of the eye can be split into three types:
- refracting tissues that focus light
- light-sensitive tissues
- support tissues
We will look at each of these in turn.
Refracting tissues focus incoming light onto the light-sensitive tissues, to give us a clear, sharp image. If they are the wrong shape, misaligned, or damaged, vision can be blurry.
The refracting tissues include:
The pupil: This is the dark spot in the center of the colored part of your eye, which, in turn, is called the iris. The pupil expands and shrinks in response to light, acting similarly to the aperture on a camera.
In very bright conditions, the pupil constricts or shrinks to around 1 millimeter (mm) in diameter to protect the sensitive retina from damage. When it is dark, the pupil can dilate or widen up to 10 mm in diameter. This dilation allows the eye to take in as much light as possible.
Iris: This is the colored portion of the eye. The iris is a muscle that controls the size of the pupil and, therefore, the amount of light reaching the retina.
Lens: Once light has traveled through the pupil, it reaches the lens, which is a transparent convex structure. The lens can change shape, helping the eye to focus light accurately onto the retina. With age, the lens becomes stiffer and less flexible, making focusing more difficult.
Ciliary muscle: This muscular ring is attached to the lens and, as it contracts or relaxes, it changes the shape of the lens. This process is called accommodation.
Cornea: This is a clear, dome-like layer that covers the pupil, iris, and anterior chamber or fluid-filled area between the cornea and the iris. It is responsible for the majority of the eye's focusing power. However, it has a fixed focus so cannot adjust to different distances.
The cornea is densely populated with nerve endings and incredibly sensitive. It is the eye's first defense against foreign objects and injury. Because the cornea must remain clear to refract light, it has no blood vessels.
Two fluids circulate throughout the eyes to provide structure and nutrients. These fluids are:
Vitreous fluid: Found in the back section of the eye, vitreous fluid is thick and gel-like. It makes up the majority of the eye's mass.
Aqueous fluid: This is more watery than vitreous fluid and circulates through the front of the eye.
Light-sensitive tissues: Retina
Photograph showing the retina, including the macula (dark patch) and optic disc (pale region).
The retina is the innermost layer of the eye. It houses more than 120 million light-sensitive photoreceptor cells that detect light and convert it into electrical signals.
These signals are sent on to the brain for processing.
Photoreceptor cells in the retina contain protein molecules called opsins that are sensitive to light.
The two primary photoreceptor cells are called rods and cones. In response to particles of light, the rods and cones send out electrical signals to the brain.
Cones: These are found in the central region of the retina called the macula, and they are particularly dense in a small pit at the center of the macula known as the fovea. Cones are essential for detailed, color vision. There are three types of cones, normally called:
• short or blue
• middle or green
• long or red
Cones are used to see in normal light conditions and allow us to distinguish colors.
Rods: These are mostly found around the edges of the retina and are used for seeing in low light levels. Although they cannot distinguish colors, they are extremely sensitive and can detect the lowest amounts of light.
Optic nerve: This thick bundle of nerve fibers transmits signals from the retina to the brain. In all, there are around 1 million thin, retinal fibers called ganglion cells that carry light information from the retina to the brain.
The ganglion cells leave the eye at a point called the optic disc. Because there are no rods and cones, it is also referred to as the blind spot.
Different subsets of ganglion cells register different types of visual information. For instance, some ganglion cells are sensitive to contrast and movement, others to shape and details. Together, they carry all the necessary information from our visual field.
The brain allows us to see in 3-D, giving us depth perception, by comparing the signals from both eyes.
The signals generated in the retina end up in the visual cortex, a part of the brain that is specialized for processing visual information. Here, the impulses are stitched together to create images.
Sclera: This is commonly referred to as the white of the eye. It is fibrous and provides support for the eyeball, helping it keep its shape.
Conjunctiva: A thin, transparent membrane that covers most of the white of the eye, and the inside of the eyelids. It helps lubricate the eye and protect it from microbes.
Choroid: A layer of connective tissue between the retina and sclera. It contains a high concentration of blood vessels. It is just 0.5 mm thick and contains light-absorbing pigment cells that help reduce reflections in the retina.
Ishihara plates are used to test for colorblindness.
As with any part of the body, problems with our sight can arise from illness, injury, or age. Below are just some of the conditions that can affect the eyes:
Age-related macular degeneration: The macular slowly breaks down, producing blurry vision and, sometimes, loss of vision in the center of the visual field.
Amblyopia: This begins in childhood and is often called lazy eye. One eye does not develop properly because the other, stronger eye dominates.
Anisocoria: This occurs when pupils are an unequal size. It can be a harmless condition or a symptom of a more serious medical problem.
Astigmatism: The cornea or lens is incorrectly curved so that light is not focused properly on the retina.
Cataracts: Clouding of the lens causes cataracts. They lead to blurred vision and, if untreated, blindness.
Colorblindness: This occurs when cone cells are absent or do not work correctly. Someone who is colorblind finds it difficult to distinguish between certain colors.
Conjunctivitis or pink eye: This is a common infection of the conjunctiva, which covers the front of the eyeball.
Detached retina: A condition when the retina comes loose. It requires urgent treatment.
Diplopia or double vision: This can be caused by several conditions that are often serious and should be checked by a doctor, as soon as possible.
Floaters: These are specks that drift across a person's visual field. They are normal but can also be the sign of something more serious, such as retinal detachment.
Glaucoma: Pressure builds up inside the eye and can eventually damage the optic nerve. It can eventually lead to loss of sight.
Myopia: This is otherwise known as nearsightedness. With myopia, it is difficult to see things that are far away.
Optic neuritis: The optic nerve becomes inflamed, often due to an overactive immune system.
Strabismus: The eyes point in different directions; it is particularly common among children.
In a nutshell
The eyes and our visual system work hard every second we are awake, weaving a seamless visual reality from a dizzying array of light-based impulses.
We take vision for granted, but our eyes are one of the most amazing feats of evolutionary engineering.