Anatomy and Physiology of the Eye
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Direct answer
The eyeball is an optical globe about 2.5 cm in diameter formed of three coats — the fibrous coat (cornea and sclera), the vascular coat (iris, ciliary body and choroid) and the nervous coat (retina) — enclosing the aqueous humour, lens and vitreous. The cornea supplies roughly two-thirds of the eye's total refractive power of about +58 to +60 dioptres, the lens the remainder, and light is focused on the fovea where phototransduction begins. Aqueous humour secreted by the ciliary processes nourishes the avascular cornea and lens and drains through the trabecular meshwork.
What you must remember
- Dimensions and contents: the adult globe is about 24 mm anteroposteriorly, weighs about 7 g and holds roughly 6.5 ml; the anterior chamber holds about 0.25 ml of aqueous and is about 2.5 mm deep at its centre.
- Cornea: avascular and transparent, with five layers — epithelium, Bowman's membrane, stroma (about 90 per cent of thickness), Descemet's membrane and endothelium; transparency rests on avascularity, the relative dehydration maintained by the endothelial pump, and the uniform lattice arrangement of stromal collagen; sensory supply is from the ophthalmic division of the trigeminal nerve.
- Uveal tract: the iris sphincter is parasympathetic (oculomotor) and the dilator sympathetic; the ciliary body secretes aqueous and its muscle effects accommodation; the choroid nourishes the outer retina through the choriocapillaris.
- Retina: ten layers; the macula with its central fovea lies about 3.5 mm temporal to and slightly below the optic disc; the inner two-thirds of the retina is supplied by the central retinal artery and the outer third, including the photoreceptors, by the choriocapillaris.
- Optic nerve: carries about 1.2 million axons, is myelinated by oligodendrocytes and sheathed by meninges, which is why it behaves as central nervous system tissue and does not regenerate after division.
- Extraocular muscles: the superior oblique is supplied by the fourth and the lateral rectus by the sixth nerve, and all others by the third; the superior rectus elevates and intorts, the inferior oblique elevates and extorts, the superior oblique depresses and intorts.
- Adnexal physiology: the precorneal tear film has an outer lipid layer from meibomian glands, a middle aqueous layer and an inner mucin layer from goblet cells; tears drain from the puncta through canaliculi, sac and nasolacrimal duct to the inferior meatus, and the near reflex couples accommodation, convergence and miosis.
Common confusion
Students most often mix the two retinal blood supplies: the central retinal artery, an end artery from the ophthalmic artery, serves the inner retina, while the short posterior ciliary arteries feeding the choriocapillaris serve the outer retina and the photoreceptors — the reason central retinal artery occlusion spares a tiny island fed by cilioretinal vessels in some eyes, and why outer-layer ischaemia follows choroidal disease. Do not confuse the endothelium (a metabolic pump, non-replicating) with the epithelium (regenerating barrier), nor the fovea (avascular, cone-rich, site of sharpest vision) with the optic disc (blind spot, about 1.5 mm across).
Exam-focused takeaway
In theory, prepare a labelled cross-section of the globe with coats, chambers and refractive media, and a visual pathway diagram with field defects, as both earn easy marks. In viva, expect rapid-fire questions on extraocular muscle actions and nerve supply, corneal layers, tear film and the drainage route of aqueous; answer in the ordered way examiners expect. In the clinical posting, apply the anatomy while learning direct ophthalmoscopy — identifying the disc, macula and vessel arcades is anatomy made visible, and the same map explains diabetic and hypertensive fundus changes later.
Frequently asked questions
Why is the cornea avascular, and how is it nourished?
Avascularity aids transparency; the cornea draws nutrition from the aqueous humour, the tear film and the perilimbal vessels at its periphery, and the endothelium keeps it relatively dehydrated.
Which layers of the retina are supplied by the choriocapillaris?
The outer third — the retinal pigment epithelium and the photoreceptors — depends on the choriocapillaris, while the inner layers are served by the central retinal artery and its branches.
What keeps the cornea transparent?
Avascularity, the uniform lattice arrangement of stromal collagen fibrils, the endothelial fluid pump maintaining relative dehydration, and an intact smooth epithelial surface.
Which cranial nerves supply the extraocular muscles?
The fourth nerve to the superior oblique, the sixth to the lateral rectus and the third to the remaining four muscles, the levator palpebrae superioris and the sphincter pupillae.
Why does the optic nerve not regenerate after injury?
It is a tract of the central nervous system: its axons are myelinated by oligodendrocytes and it is invested by the meningeal sheaths, so divided fibres do not regrow functionally.
Where do tears finally drain?
Through the puncta and canaliculi into the lacrimal sac, then down the nasolacrinal duct to open in the inferior meatus of the nose beneath the inferior turbinate.