Figure: Schematic diagram of the human eye showing all major structures.
Numbered Labels and Functions
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Cornea — Transparent, avascular. Provides ~2/3 of total refractive power (~43 diopters). Fixed, cannot adjust. Main refracting surface.
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Pupil — Opening in the iris controlling light entry. Constricts (miosis) in bright light / parasympathetic stimulation. Dilates (mydriasis) in dim light / sympathetic stimulation.
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Iris — Coloured diaphragm. Contains circular muscles (parasympathetic → constriction) and radial muscles (sympathetic → dilation).
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Lens — Transparent, biconvex, elastic. Provides ~1/3 refractive power (~20 diopters). ADJUSTABLE via ciliary muscles for accommodation. Loses elasticity with age → presbyopia. Clouding = cataract.
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Ciliary body/muscles — Smooth muscle. Contract for near vision (lens rounds up). Relax for distant vision (lens flattens). Connected to lens via suspensory (zonular) ligaments.
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Vitreous humour — Gel-like, fills posterior chamber. Maintains eye shape.
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Aqueous humour — Clear fluid in anterior chamber. Nourishes cornea and lens. Produced by ciliary body. Drains via Canal of Schlemm. Increased pressure → glaucoma.
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Retina — Innermost layer. Contains photoreceptors (rods and cones), bipolar cells, ganglion cells. Where the image forms (inverted, reduced).
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Fovea centralis — Point of highest visual acuity. Densely packed cones (no rods). Used for central/colour vision.
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Optic disc (Blind spot) — Optic nerve exit. No photoreceptors. Papilloedema here indicates raised intracranial pressure.
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Choroid — Highly vascular layer between retina and sclera. Supplies blood to outer retina. Contains melanin (absorbs stray light).
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Sclera — Tough, white outer coat. Protects the eye.
Light Path Summary
Cornea → Aqueous humour → Pupil (iris) → Lens → Vitreous humour → Retina (fovea for sharp vision) Image formed: inverted and reduced on the retina.