The human eye is made of water, collagen, specialized proteins, and a surprisingly small number of other materials arranged into distinct layers and fluid-filled chambers. An adult eyeball measures about 22 to 25 millimeters front to back and is built from three concentric tissue layers: a tough outer shell, a blood-vessel-rich middle layer, and a light-sensitive inner layer. Between and within these layers sit transparent fluids, a flexible lens, and roughly 97 million light-detecting cells.
The Outer Shell: Sclera and Cornea
The outermost coat of the eye has two parts that merge at a ring called the limbus. The sclera is the white portion, making up more than 80% of the eye wall. It gets its strength and opacity from densely packed collagen fibers, about 90% of which are type I collagen, the same structural protein found in tendons and bone. These fibers are embedded in a hydrated gel of proteoglycans, large molecules that act like a water-holding cushion between the collagen strands. The irregular arrangement of these fibers is what makes the sclera opaque rather than clear.
The cornea, by contrast, is the transparent window at the front. It has five distinct layers stacked in a disc roughly 320 micrometers thick (about a third of a millimeter). From outside in: a protective epithelium, Bowman’s layer (a thin sheet of collagen), the stroma (the thickest layer, also collagen but arranged in perfectly uniform sheets that allow light through), Descemet’s membrane, and the endothelium, a single layer of cells that pumps water out to keep the cornea from swelling and turning cloudy. The cornea’s transparency comes entirely from that precise collagen spacing. If the fibers swell or shift, vision blurs.
The Middle Layer: Blood Vessels and Pigment
Beneath the sclera sits the uvea, a pigmented, blood-vessel-dense layer with three zones. The choroid lines the back of the eye and supplies oxygen and nutrients to the retina. It is packed with melanin, the same pigment found in skin, which absorbs stray light that could otherwise bounce around inside the eye and blur your vision.
Toward the front, the choroid transitions into the ciliary body, a ring of muscle and tissue that produces aqueous humor (the fluid filling the front of the eye) and controls the shape of the lens for focusing. The iris, the colored part you see, is the most anterior portion of this middle layer. Its color comes from melanocytes, pigment-producing cells scattered through a connective tissue stroma that also contains blood vessels, nerves, and immune cells. Brown eyes have a high density of melanin in these stromal melanocytes; blue eyes have very little, allowing the collagen structure to scatter shorter wavelengths of light.
The Inner Layer: The Retina
The retina is a thin sheet of neural tissue lining the back of the eye. It contains two types of photoreceptor cells: approximately 92 million rods, which detect dim light and movement, and about 4.6 million cones, which handle color vision and fine detail. Rods dominate the peripheral retina, while cones are concentrated in the fovea, the tiny central pit responsible for sharp reading vision.
Behind the photoreceptors sits the retinal pigment epithelium (RPE), a single layer of melanin-rich cells that absorbs photons the rods and cones miss, recycles visual pigment molecules, and nourishes the photoreceptors from the back. When the RPE degenerates, as in age-related macular degeneration, the photoreceptors lose their support system and vision deteriorates.
All the signals from the rods and cones funnel through retinal ganglion cells, whose long fibers bundle together to form the optic nerve. A healthy eye contains roughly 1 to 1.2 million of these nerve fibers, each carrying a piece of the visual image to the brain.
The Lens
Suspended just behind the iris by tiny fibers attached to the ciliary body, the crystalline lens is one of the most protein-dense tissues in the body. It is made primarily of water and specialized proteins called crystallins, which are packed so tightly and uniformly that light passes through without scattering. The lens has no blood supply and no nerve connections. It receives all its nutrients from the aqueous humor surrounding it.
Crystallin proteins are unusual because they are never replaced. The ones you’re born with are the same ones you have at 80. Over decades, these proteins accumulate chemical damage, gradually clumping and yellowing. That slow degradation is the root cause of cataracts.
The Two Fluids Inside the Eye
The eye’s interior is filled with two very different fluids. The aqueous humor is a thin, watery liquid that fills the small chambers between the cornea and the lens. It is continuously produced by the ciliary body and drained through a mesh-like channel at the junction of the iris and cornea. This fluid delivers glucose and oxygen to the lens and cornea, which lack their own blood vessels. Glucose levels in the aqueous humor run lower than in the bloodstream, roughly 81 mg/dL compared to around 110 mg/dL in blood, with a delay of less than five minutes as levels adjust.
The vitreous humor is the much larger gel that fills the space between the lens and the retina. It is 99% water. The remaining 1% is a scaffolding of collagen fibers and hyaluronic acid, a sugar-based molecule that traps water and gives the vitreous its gel-like consistency. This transparent gel holds the retina in place and maintains the eye’s spherical shape. With age, the collagen network breaks down, the gel liquefies in patches, and clumps of collagen cast shadows on the retina. Those shadows are the “floaters” many people start noticing in middle age.
How These Materials Work Together
Light enters through the cornea, which performs about two-thirds of the eye’s focusing. It passes through the aqueous humor, the pupil (the opening in the iris), and the lens, which fine-tunes focus by changing shape. The light then travels through the vitreous humor and lands on the retina, where photoreceptors convert it into electrical signals. Those signals travel along about a million nerve fibers in the optic nerve to the brain.
Every material in the eye serves one of three purposes: maintaining transparency so light can reach the retina, bending light to focus an image, or converting that image into neural signals. The cornea and lens are transparent because their proteins are arranged with nanometer-level precision. The vitreous and aqueous humors are transparent because they are almost entirely water. The sclera is opaque because its collagen is deliberately disordered, blocking light from entering anywhere except the front. Even the melanin lining the back of the eye exists to prevent internal reflections. The entire structure is an exercise in controlling where light goes and where it doesn’t.