The white part of the eye is called the sclera. It’s a tough, opaque shell made primarily of collagen fibers that covers about 80% of the eye’s surface, extending from the cornea (the clear window at the front) all the way around to the back where the optic nerve exits. The sclera is what gives the eyeball its shape and protects the delicate structures inside.
What the Sclera Is Made Of
The sclera is built from densely packed collagen fibers, the same protein found in your skin, bones, and connective tissues throughout the body. These fibers are arranged in irregular, overlapping bundles, which is actually what makes the sclera opaque. The cornea at the front of your eye contains similar collagen, but its fibers are arranged in perfectly uniform rows that allow light to pass through. The sclera’s disorganized structure scatters light instead of transmitting it, producing its characteristic white appearance.
The sclera has four distinct layers stacked on top of each other. The outermost is the episclera, a thin, clear tissue with its own blood supply. Beneath that sits the stroma, the thickest layer, packed with collagen-producing cells and the fibers they generate. The lamina fusca is a transitional layer that connects the sclera to the iris and other internal structures. The endothelium forms the innermost lining.
The sclera isn’t uniform in thickness. It’s thinnest near the middle of the eye (around 0.4 to 0.5 millimeters) and thickest at the back, near the optic nerve, where it reaches about 1 millimeter. For reference, that thinnest point is roughly the thickness of four sheets of paper.
What the Sclera Does
The sclera serves as the eye’s structural backbone. Its rigid collagen shell maintains the eye’s round shape, which is essential for focusing light correctly onto the retina. Without that consistent shape, your vision would distort. The sclera also acts as armor, protecting the interior of the eye from physical injury. Six small muscles attach directly to the sclera’s outer surface, pulling the eye in different directions so you can look around without turning your head.
Why Human Eyes Look Different From Other Animals’
If you’ve ever noticed that most animals seem to have dark eyes with little or no visible white, you’re not imagining it. Humans have unusually large, exposed sclera compared to other species. One prominent explanation, known as the cooperative eye hypothesis, proposes that our bright white sclera evolved to make it easier for other people to follow our gaze. When someone glances to the left, you can track that movement because the dark iris shifts against the white background.
This would have been a significant advantage for early humans who needed to coordinate silently during hunting or alert each other to threats without making a sound. Interestingly, while it was long assumed that white sclera was unique to humans among primates, more recent research has found that visible white sclera does appear in some non-human primates as well, though not as prominently or consistently.
What Sclera Color Changes Can Mean
A healthy sclera appears white or slightly off-white. When the color shifts noticeably, it can signal something worth paying attention to.
Yellow sclera is one of the earliest visible signs of jaundice, a buildup of bilirubin in the blood. Bilirubin is a yellow pigment produced when the body breaks down old red blood cells. When the liver can’t process it efficiently, levels rise, and the sclera picks up the yellow tint before the skin does. This can point to liver conditions, gallbladder problems, or certain blood disorders.
Blue or gray sclera can appear when the sclera is abnormally thin, allowing the darker tissue underneath (the uvea and its blood vessels) to show through. In children, this is sometimes a sign of a genetic connective tissue condition. Osteogenesis imperfecta, commonly called brittle bone disease, is the most well-known cause. Other conditions linked to blue sclera include Marfan syndrome, Ehlers-Danlos syndrome, and brittle cornea syndrome.
Red patches on the sclera usually come from a broken blood vessel just beneath the surface, called a subconjunctival hemorrhage. These look alarming but are typically harmless and clear up on their own within a couple of weeks.
Conditions That Affect the Sclera
Two inflammatory conditions target the sclera directly, and telling them apart matters because one is mostly a nuisance while the other can threaten your vision.
Episcleritis affects only the outermost layer. It causes localized redness, mild irritation, and sometimes a watery discharge, but no real pain and no vision changes. It often resolves on its own. Scleritis, on the other hand, involves the deeper layers and produces a deep, dull ache that typically worsens at night and can wake you from sleep. The affected area may take on a bluish or violet tint rather than bright red. Because the eye muscles attach to the sclera, moving the eye hurts. Vision can be affected, and the condition often requires treatment to prevent lasting damage.
The sclera also plays a role in nearsightedness. In people who develop high myopia, the sclera gradually thins and stretches as the eyeball elongates. Research has shown that this process involves a reduction in collagen production, increased activity of enzymes that break down collagen, and a shift toward thinner collagen fibers. The result is a sclera that becomes more flexible and extensible than normal, allowing the eye to continue elongating and worsening the myopia over time.
The Sclera in Eye Surgery
The sclera’s toughness makes it a useful anchoring point for certain surgical procedures. In scleral buckling, a treatment for retinal detachment, a surgeon places a silicone band around the outside of the sclera like a belt. This pushes the wall of the eye inward, pressing it back against the detached retina so it can reattach. A freezing device seals the retina to the inner wall. The buckle typically stays in place permanently, and the sclera heals around it.