Eye anatomy: understanding how your vision works

The eye is an optical organ about 24 mm across that captures light, focuses it and turns it into a nerve signal sent to the brain. Light passes in turn through the cornea, the aqueous humour, the pupil, the lens and the vitreous before forming a sharp image on the retina. Understanding the anatomy of the eye means understanding why certain vision defects appear and where the main eye conditions arise.

OVERVIEW

How is the eye organised?

The eye works like a sophisticated camera: an optical system at the front focuses the light, a sensitive sensor at the back records it, and a cable carries it to the brain. This whole structure, housed within the bony orbit, is on average 24 mm long and weighs barely more than 7 grams. Each part plays a precise role.

The eye is classically described as three layers, or “tunics”, stacked from the outside in. This arrangement helps to locate each element and to understand where vision defects and eye diseases begin.

• The outer tunic: the sclera (the “white of the eye”) and the transparent cornea at the front, which protect the eyeball and give it its shape.
• The middle tunic, or uvea: the choroid, the ciliary body and the iris, richly supplied with blood vessels and pigment.
• The inner tunic: the retina, the nervous tissue that captures light and initiates vision.

Inside these layers lie the transparent media that light must cross: the aqueous humour at the front, the lens in the centre and the vitreous humour at the back. The perfect transparency of these media is essential for clear vision. Any loss of transparency, such as a cataract of the lens, has a direct impact on sight.

AT THE FRONT OF THE EYE

What is the role of the cornea and the aqueous humour?

The cornea is the eye’s first lens: this transparent, domed membrane provides about two thirds of the optical power needed to focus. The aqueous humour, a clear fluid located just behind it, nourishes the cornea and the lens and maintains the internal pressure of the eye. Together, they form the eye’s gateway for light.

The cornea, the first lens

The cornea is the thin, transparent dome that covers the front of the eye. Free of blood vessels so that it stays perfectly clear, it is richly supplied with nerves, which explains how sensitive it is to the slightest speck of dust. Its even curvature is essential: it is what bends and concentrates the light rays towards the inside of the eye.

When the curvature of the cornea is not perfectly spherical, light no longer converges to a single point: this is astigmatism. When the cornea thins and bulges into a cone shape, we speak of keratoconus, a condition that often begins in young adults. The cornea is also the structure reshaped during refractive laser surgery to correct short-sightedness, long-sightedness and astigmatism.

To explore these topics further, see our dedicated pages on keratoconus in young adults and on vision correction with LASIK.

The aqueous humour and the pressure inside the eye

Behind the cornea lies the anterior chamber, filled with aqueous humour. This transparent fluid is produced continuously by the ciliary body, then drained away through a system located at the angle of the eye. The balance between production and drainage determines the intraocular pressure.

When the aqueous humour drains poorly, the pressure rises and can damage the optic nerve: this is the mechanism of glaucoma. That is why measuring eye pressure is part of every routine ophthalmology examination. The outer surface of the eye, for its part, is protected by the tear film; when it is insufficient, the result is dry eye, a common problem when working at screens.

THE DIAPHRAGM

What do the iris and the pupil do?

The iris is the coloured membrane of the eye that regulates how much light comes in, while the pupil is the central black opening through which that light passes. Like the diaphragm of a camera, the iris constantly adjusts the diameter of the pupil according to the surrounding brightness, between roughly 2 and 8 millimetres.

The iris owes its colour to the amount of pigment (melanin) it contains: little pigment gives a light-coloured eye, a lot gives a dark eye. Two tiny muscles are housed within it. One tightens the pupil in bright light, the other widens it in dim conditions. This pupillary reflex, automatic and rapid, protects the retina from glare and optimises vision in low light.

The pupil is therefore not a structure as such, but simply an opening: it appears black because the inside of the eye absorbs light. It is this same opening that the ophthalmologist dilates with eye drops to examine the back of the eye. After dilation, vision remains blurred and dazzled for a few hours, until the pupillary reflex takes over again.

FOCUSING

How do the lens and the ciliary body focus?

The lens is a transparent, flexible lens that fine-tunes the focusing of light onto the retina. Thanks to the ciliary body, a ring-shaped muscle that changes its shape, the eye adjusts its vision both far and near: this is accommodation. This ability naturally declines with age, which explains presbyopia after 45.

The lens, a living lens

The lens is a transparent, biconvex lens, suspended just behind the iris by fine fibres called the zonule. It completes the work of the cornea by focusing light precisely onto the retina. Its distinctive feature is that it is deformable: it can bulge or flatten to match its power to the distance of the object being looked at.

Over time, the lens loses its flexibility, then its transparency. The loss of flexibility hampers near vision: this is presbyopia, which makes reading difficult from around the age of forty. The gradual clouding of the lens, meanwhile, corresponds to cataract, the leading cause of reversible loss of vision after 65. Our page on cataract surgery sets out how it is treated.

The ciliary body and accommodation

The ciliary body is a muscular ring located at the base of the iris. It performs two vital functions. First, it produces the aqueous humour that nourishes the front of the eye. Second, by contracting or relaxing, it tightens or loosens the zonule and so changes the shape of the lens.

This mechanism, known as accommodation, makes it possible to switch effortlessly from distance vision to near vision. When you look at a nearby object, the ciliary body contracts, the zonule relaxes and the lens bulges to increase its power. From around the age of 45, the lens becomes too rigid to change shape: near vision deteriorates, marking the onset of presbyopia.

THE INTERNAL GEL

What is the vitreous humour?

The vitreous humour, or vitreous, is the transparent gel that fills the large cavity of the eye, between the lens and the retina. Made up of more than 98% water, it occupies about 80% of the volume of the eyeball and gives it its shape. Light passes through it before reaching the retina, so its transparency determines how sharp the image is.

The vitreous is made of water, collagen fibres and hyaluronic acid. Firm and uniform in childhood, it gradually liquefies with age and loses its consistency. This change is a natural part of the ageing of the eye, not a disease in itself.

As it liquefies, the vitreous can form condensations that cast moving shadows on the retina: these are floaters, sometimes called “flying flies”. Later, the gel can pull away from the retina, a phenomenon known as posterior vitreous detachment. Usually harmless, it should nevertheless be examined, as explained in our article on floaters.

THE SENSOR

How does the retina turn light into an image?

The retina is the nervous tissue that lines the back of the eye and converts light into electrical signals. It contains millions of light-sensitive cells, the cones and rods, which capture the image projected by the optical system. It is the eye’s true “sensor”, without which no vision would be possible.

Cones and rods

The retina is home to two main families of photoreceptors. The rods, very numerous in the periphery, are sensitive to low levels of light and provide night and peripheral vision, though without colour or fine detail. The cones, concentrated at the centre of the retina, allow colour and detailed vision in bright light.

These cells transform the light they receive into an electrical signal. This signal is then relayed by other retinal cells, then gathered towards the optic nerve. The retina therefore does not merely record the image: it already begins processing the visual information before sending it to the brain.

The macula and the fovea, the area of fine vision

At the centre of the retina lies the macula, a small area of about 5 mm that is particularly rich in cones. At its heart, the fovea is the point of sharpest vision: it is what you use to read, recognise a face or fix on a detail. A few millimetres of retina concentrate most of our visual acuity there.

This strategic area is also the most vulnerable. Age-related macular degeneration (AMD) affects the macula and impairs central vision, while sparing peripheral vision. In people with diabetes, damage to the small blood vessels of the retina, diabetic retinopathy, can also threaten the macula. Our pages on diabetic retinopathy and monitoring AMD look at these conditions in more depth.

Retinal detachment and fragility

The retina normally adheres to the back of the eye. Sometimes it tears and then lifts away: this is retinal detachment, an emergency that threatens vision. Certain signs should raise the alarm: a sudden shower of floaters, repeated flashes of light, or a dark veil spreading across part of the visual field call for a consultation without delay.

A dark veil or curtain = an emergency — the sudden appearance of a shadow, a black curtain or a burst of floaters in the field of vision may indicate a tear or a detachment of the retina. These signs call for an urgent ophthalmology examination, without waiting.

TO THE BRAIN

How is the image sent to the brain?

The optic nerve is the cable that connects the eye to the brain: it gathers about a million nerve fibres and carries the visual signal produced by the retina. This signal then travels to the visual cortex, at the back of the brain, where the image is finally interpreted. Vision therefore arises from the cooperation between the eye and the brain.

The optic nerve leaves the eye at a precise point on the retina, the optic disc. At this spot there are no photoreceptors: this creates a small blind area, the “blind spot”, which the brain fills in without our being aware of it. The fibres from the two eyes partly cross over at the back, which in particular makes depth perception possible.

The optic nerve is especially fragile in the face of eye pressure. In glaucoma, an intraocular pressure that is too high slowly damages its fibres, first reducing peripheral vision, in a way that long remains silent and insidious. Regular screening is therefore essential, particularly in people with diabetes, as detailed on our page about glaucoma and screening in people with diabetes.

THE OUTER LAYERS

What is the role of the choroid and the sclera?

The choroid and the sclera form the layers that surround and nourish the eye. The choroid is a richly vascularised layer, tucked between the retina and the sclera, that supplies oxygen and nutrients to the retina. The sclera, the “white of the eye”, is the rigid protective shell that maintains the shape of the eyeball and to which the eye muscles attach.

The choroid belongs to the uvea, the middle pigmented tunic of the eye. Its abundant network of vessels supplies the outer layers of the retina, which are very demanding in oxygen. Its pigmentation also absorbs stray light, much like the matte interior of a camera, which improves the contrast of the image.

The sclera is the white, tough, opaque membrane that envelops most of the eyeball. It protects the internal structures and serves as the anchoring point for the six extraocular muscles that direct the gaze. At the front, the sclera continues into the transparent cornea: this is where light can finally enter the eye, completing the visual pathway.

FAQ

Frequently asked questions

Sources

1. French Society of Ophthalmology (SFO). Anatomy and physiology of the eye and the visual pathways. Report and educational documents of the SFO.
2. College of University Ophthalmologists of France (COUF). Ophthalmology — Anatomy and physiology of the eye. National reference syllabus of the College.
3. Inserm. Eye and vision: information dossiers on the structure of the eye and eye diseases (AMD, glaucoma, cataract).
4. French National Health Insurance (Ameli.fr). Information sheets on eye health, cataract, glaucoma and diabetic retinopathy.

Further reading

• Cataract: when the lens clouds over
• LASIK: correcting vision with laser
• Diabetic retinopathy: protecting your retina
• Glaucoma and diabetes: the importance of screening
• Book an appointment at the practice

Cachan practice · Tel. +33 1 45 47 08 11
Paris 13 practice · Tel. +33 1 89 31 30 60

Disclaimer

This article is for information purposes. A personalised ophthalmology opinion remains essential for any treatment decision.

This article is intended for general information and is not a substitute for a medical consultation. Understanding the anatomy of the eye does not remove the need for a complete ophthalmology examination, the only way to check each structure of the eye and to detect any possible condition. In the event of reduced vision, visual discomfort or the sudden appearance of symptoms, consult an ophthalmologist.

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