The back of the eye is called the fundus. This term covers everything visible when a doctor looks inside your eye from the front: the retina, the macula, the optic disc, the fovea, and the blood vessels that supply them. In broader anatomical terms, the entire rear portion of the eye is known as the posterior segment, which includes all of those structures plus the choroid layer and the gel-like vitreous humor that fills the space inside.
Structures That Make Up the Fundus
The fundus isn’t a single structure. It’s a layered system where each part plays a distinct role in vision.
The retina is the innermost layer and the one doing the heavy lifting. It contains millions of light-sensitive cells that convert light into electrical signals your brain can interpret. Think of it as the film in an old camera, or the sensor in a digital one. It lines the interior back wall of the eye and connects to the brain through the optic nerve.
The choroid sits just behind the retina. It’s one of the most blood-vessel-dense tissues in the entire body, and its primary job is delivering oxygen and nutrients to the outer layers of the retina. When choroidal blood flow is impaired, the retina can’t function properly, which is one pathway to age-related macular degeneration.
The sclera is the tough, white outer coat of the eye. At the back, it has a small perforated area called the lamina cribrosa, a connective tissue mesh through which nerve fibers and blood vessels pass on their way to and from the brain.
The Macula and Fovea: Your Sharp Vision Center
Near the center of the retina sits the macula, a circular region roughly 5 to 6 millimeters across. This is the part of your eye responsible for high-resolution color vision. Everything you do that requires detail (reading, recognizing faces, driving) depends on the macula.
At the very center of the macula is the fovea, a specialized pit about 1.5 millimeters in diameter packed with the highest concentration of cone photoreceptors anywhere in the eye. Cones are the cells that handle color vision and fine detail, and the fovea’s dense packing of them is what gives you sharp central vision. Inside the fovea is an even smaller zone called the foveola, only about 350 micrometers across, where visual acuity peaks at its absolute sharpest. The foveola contains no rod cells at all, just cones.
How the Retina Converts Light Into Vision
The retina has two types of photoreceptor cells: rods and cones. Rods are extremely sensitive to light and handle vision in dim conditions. You rely on them when navigating a dark room or looking at the night sky. Cones work under normal and bright lighting, respond quickly to changes in light intensity, and are responsible for both color perception and sharp detail.
When light hits these cells, it triggers a chemical chain reaction. A light-sensitive molecule inside each photoreceptor changes shape, setting off a cascade that ultimately closes tiny channels in the cell membrane. This changes the cell’s electrical charge, which alters the chemical signal it sends to neighboring nerve cells. Those signals travel through several layers of processing cells within the retina itself before reaching the retinal ganglion cells, whose long fibers bundle together to form the optic nerve.
The Optic Disc and Your Blind Spot
About 1.2 million nerve fibers from retinal ganglion cells converge at a single point called the optic disc, where they exit the eye as the optic nerve. The optic nerve carries all visual information from that eye to the brain. It also controls the pupil’s reflexive response to light.
Because the optic disc is packed with nerve fibers rather than photoreceptors, it can’t detect light. This creates a small blind spot in each eye’s visual field. You don’t normally notice it because your brain fills in the gap using information from the surrounding retina and from the other eye.
The Vitreous Humor
The posterior segment isn’t just layers of tissue. The large interior space between the lens and the retina is filled with a transparent, gel-like substance called the vitreous humor. It makes up about 80% of the eye’s total volume and is roughly 98 to 99 percent water, with the remaining fraction being a scaffold of collagen and hyaluronan that gives it structure.
The vitreous keeps the eye’s spherical shape, provides structural support to the retina, and reduces the scattering of light passing through to the photoreceptors. As you age, this gel can shrink and pull away from the retina, sometimes causing floaters (those drifting specks or cobwebs in your vision).
Conditions That Affect the Back of the Eye
Most serious threats to the posterior eye involve the retina or the choroid. Common symptoms of retinal disorders include blurred or distorted vision, flashes of light, floating specks, loss of central or peripheral vision, difficulty seeing at night, and in severe cases, sudden vision loss. Risk factors include aging, smoking, obesity, diabetes, high blood pressure, eye injuries, and family history of retinal problems.
Age-related macular degeneration targets the macula specifically, gradually eroding central vision while leaving peripheral vision intact. Retinal detachment occurs when the retina pulls away from the choroid layer that nourishes it, which is a medical emergency. Diabetic retinopathy damages the small blood vessels in the retina, and glaucoma involves progressive damage to the optic nerve fibers at the optic disc.
How Doctors Examine the Back of the Eye
The most basic method is a dilated eye exam. Eye drops widen your pupils so the doctor can look through the lens and directly see the fundus, checking the retina, optic disc, macula, and blood vessels for signs of damage.
For more detailed views, optical coherence tomography (OCT) uses light waves to create cross-sectional images of the retina at near-microscopic resolution. The scan is quick and noninvasive. It works by splitting a beam of light into two paths, then analyzing the interference pattern when the beams recombine to map tissue depth. Modern OCT machines capture over 50,000 scans per second and can build three-dimensional images of the retina, allowing doctors to examine individual retinal layers and measure their thickness.
Newer variations include swept-source OCT, which uses longer wavelengths of light to penetrate deeper and image the choroid layer, and OCT angiography, which detects the movement of blood cells within vessels to map retinal and choroidal blood flow without injecting any dye. Fluorescein angiography, which does use an injected dye, remains another option for evaluating the retina’s blood vessels in detail.