A healthy human eye is a roughly one-inch sphere, but only about one-sixth of it is visible between your eyelids. What you can see is a white outer surface surrounding a colored ring with a black circle at its center. Each of these parts has a distinct look and function, and subtle changes in their appearance can reveal a lot about what’s happening inside your body.
The Parts You Can See
The most obvious feature of the eye is the sclera, the white portion that forms the outer wall of the eyeball. It’s a tough, opaque tissue that gives the eye its shape and provides a stark background for everything else. A thin, clear membrane called the conjunctiva sits over the sclera and the inner surface of your eyelids, keeping the front of the eye moist.
At the center of the visible eye is the iris, the colored part. The iris is actually a ring of muscle fibers that controls how much light enters the eye. Its color comes from pigment cells in the tissue, and the amount and type of pigment determine whether your eyes appear brown, blue, green, or somewhere in between. Over 50% of the world’s population has brown eyes. In the United States, a 2014 survey of more than 2,000 people found 45% had brown eyes, 27% blue, 18% hazel, 9% green, and about 1% another color. Green is the rarest globally, found in roughly 2% of people.
The pupil is the black opening at the center of the iris. It looks black because light entering the eye is absorbed by the tissues inside rather than reflected back out. Pupils constantly adjust in size. In normal indoor (fluorescent) lighting, the average pupil measures about 3.6 millimeters across. In bright light, it shrinks to around 2.6 millimeters. In darkness, pupils can dilate to 7 or 8 millimeters. Both pupils normally stay close to the same size, though slight differences of up to a millimeter can be normal in some people.
Covering the iris and pupil is the cornea, a clear, dome-shaped surface that acts as the eye’s first lens. You can’t see the cornea directly because it’s transparent, but you can notice its presence when light reflects off the front of the eye as a bright spot. The cornea is responsible for about two-thirds of the eye’s focusing power.
What the Inside Looks Like
During an eye exam, a doctor can look through the pupil to see the back of the eye, called the fundus. The view is striking and very different from the eye’s outer appearance. The retina, the light-sensitive layer lining the back wall, appears as an orange-red surface with a network of blood vessels branching across it. A healthy retina has vessels that follow a predictable branching pattern without any unusual swelling, narrowing, or leaking.
The optic disc, where the optic nerve connects to the retina, looks like a small pinkish-yellow oval with sharp, well-defined edges. A healthy disc has a small central depression called the “cup” that takes up roughly a third of the disc’s diameter. Radiating outward from the disc, there’s often a subtle sheen, a sign of a healthy nerve fiber layer. Near the center of the retina sits the macula, the area responsible for sharp central vision. At its very center is the fovea, which appears as a tiny bright point of reflected light. In a healthy eye, the macula and surrounding area are free of scars, spots, or unusual pigment changes.
How Eyes Change With Age
Eyes don’t look the same throughout life. One of the most visible age-related changes is arcus senilis, a white, blue, or gray arc that forms around the outer edge of the cornea. It starts as a crescent shape at the top or bottom of the cornea and can eventually extend into a full ring. The ring is made of fatty deposits that settle in the corneal tissue. It’s extremely common in older adults and is generally harmless, though when it appears in someone under 40, it can sometimes signal high cholesterol.
The sclera can also change with age, becoming slightly less bright white and taking on a faint yellowish or grayish tint. The iris may lose some pigment over decades, making eye color appear lighter. Pupils tend to get smaller with age and may respond more slowly to changes in light.
What Color Changes Can Mean
One of the most well-known changes in eye appearance is yellowing of the whites. This is called scleral icterus (or more precisely, conjunctival icterus), and it happens when bilirubin, a yellow waste product from the breakdown of red blood cells, builds up in the blood and deposits in the thin conjunctival membrane. Because the sclera acts like a white background, even small increases in bilirubin make the yellowing easy to spot. Yellowing of the eyes is often associated with liver problems, gallbladder disease, or conditions that cause excessive red blood cell breakdown. One thing worth knowing: warm-toned indoor lighting can make the whites of your eyes appear more yellow than they actually are, so checking under neutral or natural light gives a more accurate picture.
Redness in the white of the eye is another common change. A bright red patch that appears suddenly and doesn’t hurt is usually a subconjunctival hemorrhage, a small broken blood vessel under the conjunctiva. It looks alarming but typically clears on its own within a week or two. More diffuse redness with irritation or discharge points to conditions like conjunctivitis or allergies.
Unusual Things You Might See in the Eye
Two conditions that cause visible fluid to collect inside the front chamber of the eye (the space between the cornea and the iris) are worth distinguishing. A hyphema is a pool of blood trapped in that chamber, almost always caused by an eye injury, especially sports injuries. It appears as a visible layer of red settling at the bottom of the iris. A hypopyon looks similar in shape but white instead of red. It’s a collection of white blood cells that pools in the same space and can make the eye look like there’s a layer of white liquid floating in front of the colored part. Hypopyons are caused by infections or inflammatory conditions rather than trauma.
Other visible abnormalities include a pterygium, a fleshy, wing-shaped growth on the conjunctiva that can creep onto the cornea, and pinguecula, small yellowish bumps on the white of the eye near the cornea. Both are associated with UV exposure and dry or dusty environments. A cloudy or milky appearance to the pupil area, where it normally looks black, can indicate a cataract, a condition where the eye’s internal lens becomes opaque.