Lesson · ABO / NCLE
Ocular anatomy
The eye from front to back, the five corneal layers, the three-layer tear film and the tear tests, with the refractive indices the exams ask about.
After this lesson you can
About 12 minutes- Trace light through the eye and name each structure it passes
- List the corneal layers in order and what each does
- Name the tear-film layers and the gland behind each
- Read a Schirmer or TBUT result
The eye from front to back
Light passes through the tear film, cornea, aqueous humour, pupil, crystalline lens and vitreous before it reaches the retina. Each medium bends light a little, and the cornea does the most.
| Structure | What to know | Index |
|---|---|---|
| Cornea | Clear front window, about 40 to 46 D; a contact lens rests on it | 1.37 |
| Aqueous humour | Watery fluid in the front chambers; keeps pressure and feeds the cornea and lens | 1.33 |
| Iris and pupil | The iris sizes the pupil: bigger in dim light, smaller in bright light | — |
| Crystalline lens | Changes shape to focus (accommodation), pulled by the ciliary muscle | 1.42 |
| Vitreous humour | Clear gel that holds the eye’s shape | 1.33 |
| Retina | Light-sensitive lining: rods for dim light, cones for colour and detail | — |
| Macula and fovea | The centre of the retina; the fovea gives the sharpest vision | — |
| Optic nerve | Carries the signal from the retina to the brain | — |
- Sclera: the tough white outer coat. The limbus is the roughly 1 mm border where cornea meets sclera.
- Choroid: the blood-vessel layer between sclera and retina that nourishes the retina.
- Aqueous drains through the canal of Schlemm; if it cannot drain, pressure rises (see glaucoma in the pathology lesson).
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The corneal layers
Remember the order from outside in with “Epi, Bo, Stro, Desi, Endo”.
| Layer | Key facts |
|---|---|
| Epithelium | Outer barrier against infection and injury |
| Bowman’s membrane | Tough layer under the epithelium that gives the cornea strength |
| Stroma | About 90 % of corneal thickness; mostly water and collagen, no blood vessels |
| Descemet’s membrane | Thin, strong sheet made by the endothelium |
| Endothelium | Single cell layer that pumps fluid out of the stroma to keep it clear; it hardly regenerates |
Lids and the tear film
The tear film has three layers, remembered as LAM from the outside in: lipid, aqueous, mucin.
| Layer | Made by | Job |
|---|---|---|
| Lipid (oil) | Meibomian glands in the lids | Slows evaporation and smooths the blink |
| Aqueous (water) | Lacrimal gland | The bulk of the tears; carries salts and proteins |
| Mucin | Goblet cells in the conjunctiva | Makes the tears stick to the cornea |
- Tears drain through the puncta at the inner (medial) canthus.
- A blocked Meibomian gland is a chalazion; an infected one is a stye.
- The eye needs at least 12 blinks a minute to keep tears flowing; an incomplete (false) blink spreads them poorly.
| Test | Measures | Normal |
|---|---|---|
| Schirmer | Quantity of tears | 10 mm or more of wetting |
| Tear break-up time (TBUT) | Quality of tears | 10 seconds or more |
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