Eyes

Function

Each eye is a living camera: the cornea and lens focus light onto the retina, while the iris's pupil adjusts exposure in fractions of a second and the ciliary muscle reshapes the lens for near or far (accommodation). Six extraocular muscles aim it with a precision of arc-minutes, guided by cortical feedback loops.

The retina's 120 million rods and 6 million cones transduce photons into electrical signals; ganglion cell axons — a million per optic nerve — carry the image to the lateral geniculate nucleus and on to the visual cortex, where seeing actually happens. The eye's gel vitreous and aqueous humour maintain its optical pressure; the aqueous's drainage angle defines glaucoma's risk.

Blood supply

Ophthalmic artery (internal carotid) — the retinal arterioles are the only blood vessels that can be viewed directly, through an ophthalmoscope.

Nerve supply

CN II (optic) for vision; CN III, IV, VI move the eye; V1 senses the cornea; parasympathetics constrict pupil and curve lens.

Clinical relevance

Cataract

Lens proteins cloud with age or UV — the world's leading cause of blindness, cured in 15 minutes by phacoemulsification.

Glaucoma

Optic nerve damage usually from raised intraocular pressure — silent peripheral loss until the centre goes.

Diabetic retinopathy

Microvascular damage leaks and proliferates on the retina — screening photography saves sight.

Related conditions

• Cataract
• Glaucoma
• Age-related macular degeneration
• Diabetic retinopathy
• Conjunctivitis (pink eye)