Anatomy of the Eye: A Clear Guide to Every Part and What It Does

The human eye is roughly the size of a large grape, about 24 mm from front to back in an adult, yet it contains an astonishing number of specialised structures. Some bend light, some control how much light gets in, some convert light into nerve signals, and others protect, lubricate and move the eye with remarkable precision. Each part has a job, and when one part fails, the symptoms often reveal exactly which structure is affected.

This guide walks through the anatomy of the eye from the outside in: first the protective outer layers, then the focusing system, then the light-sensitive retina and optic nerve, and finally the supporting structures around the eyeball, including the eyelids, tear system and the six muscles that move each eye. Along the way, we note which common conditions affect each part. If you want to know how all these parts work together to create sight, read our companion guide on how vision works.

The three layers of the eyeball

Anatomists describe the wall of the eyeball as three concentric layers, rather like the layers of an onion:

Inside these layers are the chambers filled with fluid and gel, and the crystalline lens. The front of the eye contains the anterior chamber (between cornea and iris) and the posterior chamber (between iris and lens), both filled with aqueous humour. Behind the lens, the large vitreous chamber holds the vitreous gel.

The outer layer: cornea, sclera and conjunctiva

Cornea

The cornea is the clear, dome-shaped window at the very front of the eye. It is only about half a millimetre thick in the centre, yet it provides roughly two-thirds of the eye's focusing power. To stay transparent, the cornea has no blood vessels; it receives oxygen mainly from the air through the tear film and nutrients from the aqueous humour.

The cornea has five main layers: the outer epithelium, which heals rapidly from scratches; Bowman's layer; the thick stroma, made of precisely arranged collagen fibres; Descemet's membrane; and the endothelium, a single layer of cells that pumps fluid out of the cornea to keep it clear. The cornea is also one of the most densely innervated tissues in the body, which is why even a tiny scratch or eyelash feels so painful.

Problems include abrasions, infections and ulcers, dry eye, corneal swelling and conditions that change its shape, such as keratoconus. An irregularly curved cornea is the most common cause of astigmatism.

Sclera

The sclera is the white of the eye. It is a tough, fibrous coat that covers about five-sixths of the eyeball, maintains its shape and provides attachment points for the eye muscles. At the front it joins the cornea at a boundary called the limbus, which contains stem cells that renew the corneal surface. Inflammation of the sclera (scleritis) causes deep, boring pain and needs medical care, while inflammation of the thin layer on top of it (episcleritis) is usually milder.

Conjunctiva

The conjunctiva is a thin, transparent mucous membrane that covers the white of the eye and lines the inside of the eyelids. It contains small blood vessels and cells that produce mucus for the tear film. Because it folds back on itself at the top and bottom, a contact lens cannot get lost behind the eye.

Inflammation of the conjunctiva, or conjunctivitis, makes the eye pink or red, gritty and watery. A small burst blood vessel under the conjunctiva (subconjunctival haemorrhage) looks alarming but is usually harmless. Growths such as pinguecula and pterygium also arise from the conjunctiva, often linked to sun exposure.

The middle layer: iris, ciliary body and choroid

Iris and pupil

The iris is the coloured part of the eye. Its colour comes mainly from the amount of melanin pigment and the way the tissue scatters light, explained in our guide to eye colour. The iris contains two muscles: the sphincter pupillae, which encircles the pupil and narrows it in bright light, and the dilator pupillae, which runs radially and widens it in dim light.

The pupil is simply the opening in the centre of the iris. It looks black because light entering it is absorbed inside the eye. Normally both pupils are round, equal in size and react briskly to light. Unequal pupils (anisocoria) can be harmless, but a newly unequal pupil, especially with headache, droopy eyelid or double vision, needs urgent evaluation.

Ciliary body

Behind the iris lies the ciliary body, a ring of tissue with two main jobs. First, its ciliary muscle changes the shape of the lens to focus on near objects (accommodation) via fine fibres called zonules. Second, its ciliary processes produce aqueous humour, the clear fluid that nourishes the lens and cornea and maintains pressure inside the eye.

Aqueous humour flows from the posterior chamber, through the pupil, into the anterior chamber and drains out through the trabecular meshwork, a sieve-like tissue in the angle where the iris meets the cornea. If drainage is impaired, eye pressure can rise and damage the optic nerve, which is a major mechanism in glaucoma.

Choroid

The choroid is a layer packed with blood vessels between the sclera and retina. It supplies oxygen and nutrients to the outer retina, including the photoreceptors, and its pigment absorbs stray light. The iris, ciliary body and choroid together form the uvea, and inflammation of any part of it is called uveitis.

The lens and the vitreous

Lens

The crystalline lens is a transparent, biconvex structure about 9–10 mm across, sitting just behind the iris. It is made of tightly packed protein fibres arranged so precisely that light passes through without scattering. The lens is enclosed in a thin elastic capsule and held in place by the zonules.

The lens keeps growing slowly throughout life by adding new fibres at its outer edge, so it becomes larger, denser and less flexible with age. This loss of flexibility causes presbyopia from around the mid-40s. Over time, the proteins can also clump and lose transparency, causing a cataract. In cataract surgery, the cloudy lens contents are removed and replaced with an artificial intraocular lens placed inside the natural capsule.

Vitreous

The vitreous humour is a clear, jelly-like substance that fills the space between the lens and retina, accounting for about two-thirds of the eye's volume. It is mostly water, held in a framework of collagen fibres and hyaluronic acid. In childhood it is a firm gel; with age, it gradually liquefies and can shrink and peel away from the retina, an event called posterior vitreous detachment.

Clumps of collagen in the vitreous cast shadows on the retina, which we see as floaters. A sudden increase in floaters or flashing lights may mean the vitreous is pulling on the retina, which can cause a tear or detachment.

The retina, macula and fovea

The retina is a thin layer of neural tissue, around a quarter of a millimetre thick, lining the inside of the back of the eye. It is actually an outgrowth of the brain during development. It contains about 6 million cones and over 90 million rods, plus layers of nerve cells that begin processing visual information.

The main retinal layers, from the outside in, include:

• Retinal pigment epithelium (RPE): a single layer of pigmented cells that supports and nourishes the photoreceptors.
• Photoreceptors: rods for dim-light and peripheral vision; cones for detail and colour.
• Bipolar, horizontal and amacrine cells: intermediate processing.
• Ganglion cells and nerve fibre layer: their long fibres form the optic nerve.

The retina receives a double blood supply: the central retinal artery nourishes its inner layers, while the choroid supplies the outer layers. Blockage of the retinal arteries or veins can cause sudden vision loss.

Macula and fovea

The macula is an area about 5 to 6 mm across near the centre of the retina, slightly yellow because of the pigments lutein and zeaxanthin. At its centre lies the fovea, a small pit containing tightly packed cones and no rods, which gives us our sharpest vision for reading, driving and recognising faces. Diseases of the macula, such as age-related macular degeneration and diabetic macular oedema, specifically threaten central vision.

The optic nerve

About 1.2 million nerve fibres from the retinal ganglion cells gather at the optic disc, located slightly towards the nose from the macula, and leave the eye as the optic nerve. Because there are no photoreceptors on the optic disc, it creates a natural blind spot in each eye's visual field.

The optic nerve runs backwards through the eye socket to the optic chiasm, where fibres from the nasal half of each retina cross over. Eye doctors examine the optic disc during every full eye exam: a pale disc, an enlarged central cup or a swollen disc can signal glaucoma, inflammation, raised pressure in the skull or other conditions. Nerve fibres in the optic nerve do not regenerate, so damage is generally permanent, which makes early detection so important.

The eye socket and eyelids

Orbit

The eyeball sits in the orbit, a bony cavity formed by seven skull bones. The orbit is cushioned by fat and contains the eye muscles, nerves, blood vessels and the lacrimal gland. The rim of the orbit is strong and protects the eye from many blunt injuries, but the floor and inner wall are thin and can fracture after a blow. Swelling of tissues within the orbit, as in thyroid eye disease, can push the eye forward.

Eyelids

The eyelids are thin folds of skin, muscle and connective tissue that protect the front of the eye. Each blink spreads the tear film evenly across the cornea and helps pump tears towards the drainage system. Key structures include:

• Orbicularis oculi: the circular muscle that closes the eyelids.
• Levator palpebrae superioris: the main muscle that lifts the upper eyelid, aided by the smaller Müller's muscle. Weakness or stretching of this system causes a droopy eyelid (ptosis).
• Tarsal plates: firm connective tissue that gives the lids their shape.
• Meibomian glands: about 20–40 tiny oil glands in each lid that secrete the oily layer of the tear film. Their blockage is a leading cause of evaporative dry eye and of chalazia.
• Eyelashes: sensitive hairs that trigger a blink when touched and help keep debris out.

Inflammation of the eyelid margins (blepharitis) is very common and often causes burning, crusting and gritty eyes.

The tear system

The lacrimal system produces and drains tears. Tears keep the cornea moist, smooth and clear, wash away debris, supply oxygen and contain antibacterial proteins.

Tear production

The main lacrimal gland sits in the upper outer part of the orbit and produces reflex tears, for example when you cut onions or get dust in your eye. Small accessory glands in the conjunctiva and eyelids produce the steady basal tears that keep the eye moist all day. The tear film has three intertwined components: a mucous layer that helps tears stick to the eye, a watery (aqueous) layer and an outer oily (lipid) layer from the meibomian glands that slows evaporation.

Tear drainage

Tears drain through two tiny openings called puncta on the inner edges of the upper and lower eyelids. From there they flow through small channels (canaliculi) into the lacrimal sac and down the nasolacrimal duct into the nose. A blocked tear duct, common in newborns and also in older adults, causes a constantly watery eye and sometimes infection. An imbalance between tear production and quality leads to dry eye disease.

The six extraocular muscles

Each eye is moved by six extraocular muscles attached to the sclera. Most of them originate from a ring of tendon at the back of the orbit (the annulus of Zinn). They are controlled by three cranial nerves and must work in perfect coordination with the muscles of the other eye so that both eyes point at the same target.

When these muscles or their nerves are weakened, the eyes become misaligned, causing double vision in adults or a squint in children. Childhood misalignment can lead to amblyopia, so it should always be assessed promptly. Eye movement exercises can be a pleasant way to relax tired eyes, and specific exercises prescribed by an orthoptist can help certain problems such as convergence insufficiency, but they do not cure most types of strabismus or any refractive error. You can try gentle movement routines such as figure-eight tracking for comfort.

When to see a doctor

Most changes in the eye develop slowly and are best detected through routine eye examinations, which allow a professional to check every structure described above, from the cornea to the optic nerve. Book a routine appointment if you notice gradually blurring vision, persistent irritation, a watery eye that does not settle, a lump on the eyelid that lasts more than a few weeks, or eyelids that seem to droop.