Eye muscles
Six muscles move each eye, working in pairs so both eyes point at the same thing. This section shows the two that turn the eye up and down.
- Per eye
- Six
- Shown here
- Superior and inferior rectus
- Attach to
- The sclera
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Optic nerve
Carries the signals from the retina to the brain. It leaves the back of the eye at the optic disc.
- Carries
- About 1.2 million nerve fibres
- Leads to
- The brain's visual cortex, via the optic chiasm
Sclera
The white of the eye: a tough outer coat that protects it and keeps its shape. The eye muscles pull on it.
- Made of
- Dense connective tissue
- Continues as
- The cornea, at the front
Choroid
A layer rich in blood vessels between the sclera and the retina. It nourishes the retina, and its dark pigment soaks up stray light.
- Supplies
- Oxygen and nutrients to the retina
- Pigment
- Absorbs scattered light
Retina
The light-sensitive lining at the back of the eye. Its rods and cones turn light into nerve signals.
- Rods
- About 120 million, for dim light
- Cones
- About 6 million, for colour and detail
Vitreous humour
A clear jelly filling the space behind the lens. It holds the eye's round shape and lets light pass straight through to the retina.
- Fills
- About four fifths of the eye
- Made of
- Mostly water, with collagen fibres
Aqueous humour
A watery fluid filling the front of the eye. It feeds the cornea and the lens, which have no blood supply, and keeps the eye's pressure steady.
- Made by
- The ciliary body
- Replaced
- Continuously
Cornea
The clear dome at the front. Light bends most as it enters the eye here, so the cornea does most of the focusing before the lens fine-tunes it.
- Focusing
- About two thirds of the eye's power
- Blood vessels
- None, so it stays clear
Ciliary body
A ring of muscle that holds the lens on fine ligaments. Tightening it lets the lens round up to focus on something near.
- Holds
- The lens, on suspensory ligaments
- Also makes
- Aqueous humour
Lens
A clear, flexible lens behind the iris that changes shape to focus near or far. It stiffens with age, which is why reading glasses arrive in the forties.
- Focuses by
- Changing shape (accommodation)
- Clouding
- Is called a cataract
Iris
The coloured ring of muscle that sets the size of the pupil, letting in more light in the dark and less in the bright.
- Controls
- The size of the pupil
- Colour comes from
- Melanin
Pupil
The opening in the middle of the iris that light passes through. It looks black because the inside of the eye absorbs the light.
- Size
- About 2–8 millimetres across
- Narrows in
- Bright light
Fovea
A small pit at the centre of the retina, packed with cones and nothing else. It is where vision is sharpest, so the eye turns to put whatever you look at here.
- Contains
- Cones only, tightly packed
- Gives
- The sharpest, most detailed vision
Optic disc
Where the optic nerve leaves the eye. There are no rods or cones here, so it is the blind spot - the brain fills the gap in.
- Also called
- The blind spot
- Has
- No light-sensitive cells
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