Page 2 of 3

AN41.1-3 | Eyeball — Self-Directed Learning

Part 1: Coats of the Eyeball (AN41.1)

The eyeball is approximately 24 mm in diameter and consists of three concentric coats (tunics):

1. Outer Fibrous Coat (Tunica Fibrosa)

Two parts:
• Sclera (posterior 5/6) — opaque, white, tough fibrous coat; provides structural support; site of attachment of extraocular muscles; the optic nerve pierces it posteriorly at the lamina cribrosa (a sieve-like area — the weakest point, site of glaucomatous cupping)
• Cornea (anterior 1/6) — transparent, avascular, highly refractive (accounts for approximately 2/3 of total refractive power); the junction with the sclera is the limbus (contains stem cells for corneal epithelial renewal); five layers: epithelium → Bowman's membrane → stroma → Descemet's membrane → endothelium

Clinical note: The cornea has the highest density of sensory nerve endings in the body (supplied by CN V1 — ophthalmic, via long ciliary nerves) — hence extreme sensitivity to foreign bodies and corneal abrasions.

2. Middle Vascular Coat (Tunica Vasculosa = Uvea)

Three parts from back to front:
• Choroid — posterior 2/3; highly vascular layer between sclera and retina; nourishes the outer layers of the retina; pigmented (black) to prevent internal reflection; choriocapillaris = dense capillary network
• Ciliary body — annular thickening at the junction of choroid and iris; contains the ciliary muscle (smooth muscle for accommodation — changes lens shape; innervated by CN III parasympathetic via ciliary ganglion); connected to the lens by zonular fibres (suspensory ligament of lens); secretes aqueous humour via ciliary processes
• Iris — thin, pigmented, muscular diaphragm with the pupil at its centre; contains:
- Sphincter pupillae (circular) — constricts pupil (miosis); parasympathetic (CN III via ciliary ganglion)
- Dilator pupillae (radial) — dilates pupil (mydriasis); sympathetic (long ciliary nerve from superior cervical ganglion via nasociliary nerve)

3. Inner Neural Coat (Retina)

Two layers:
• Outer pigmented layer (RPE) — single layer of pigmented cells attached to choroid; does NOT detach in retinal detachment
• Inner neural/sensory layer — contains photoreceptors (rods for scotopic/dim-light vision; cones for photopic/colour vision)

Key regions of the retina:
• Macula lutea — lateral to optic disc; the region of highest cone density; responsible for central detailed vision; has a central depression — the fovea centralis (purest cone region, no rods, no overlying cells → highest visual acuity)
• Optic disc (blind spot) — medial to macula; where optic nerve fibres exit; NO photoreceptors → physiological blind spot; the lamina cribrosa of the sclera supports it

Chambers of the Eyeball

Chamber	Location	Contents
Anterior chamber	Between cornea and iris	Aqueous humour
Posterior chamber	Between iris and lens	Aqueous humour
Vitreous chamber	Behind lens	Vitreous body (gel-like, not regenerated)

Part 2: Refractive Media, Aqueous Humour, and Glaucoma (AN41.1, AN41.2)

Refractive Media

Light passes through four refractive media before reaching the retina:
1. Cornea — greatest refractive power (~43 dioptres); fixed; avascular; nourished by aqueous humour and tear film; responsible for most refractive errors when its shape is abnormal (astigmatism)
2. Aqueous humour — fills anterior and posterior chambers; refractive index similar to water
3. Lens — biconvex crystalline structure; approximately 9–10 dioptres (variable with accommodation); suspended by zonular fibres from the ciliary body; lacks blood vessels and nerves; nourished by aqueous humour; composed of lens fibres (elongated transparent cells, no nuclei) arranged in concentric layers around the nucleus
4. Vitreous body — gel filling the vitreous chamber (99% water + hyaluronic acid + collagen fibrils); maintains eyeball shape; not regenerated if lost (as in penetrating trauma)

Aqueous Humour — Production and Drainage

Production:
• Secreted by the non-pigmented epithelium of the ciliary processes (in the ciliary body) by active secretion + ultrafiltration
• Flows from the posterior chamber → through the pupil → into the anterior chamber

Drainage (trabecular pathway — 90%):
• From the anterior chamber angle (iridocorneal angle/irido-corneal angle) → trabecular meshwork (spongy tissue at the limbus) → Canal of Schlemm (circular venous sinus at the limbus) → aqueous veins → episcleral veins → venous system

Uveoscleral drainage (10%):
• Across the ciliary body stroma into the suprachoroidal space → out through the sclera

Glaucoma — Anatomical Basis (AN41.2)

Glaucoma = raised intraocular pressure (>21 mmHg) damaging the optic nerve at the lamina cribrosa (the weakest point in the sclera where nerve fibres exit).

Types:
• Open-angle glaucoma (chronic) — the iridocorneal angle is open but the trabecular meshwork is dysfunctional; aqueous outflow is reduced; slow progressive optic nerve damage; painless and asymptomatic until late; most common type
• Closed-angle glaucoma (acute) — the iris physically occludes the iridocorneal angle, blocking drainage; causes sudden elevation of IOP → red eye, severe headache, halos around lights, nausea; ophthalmic emergency

Optic disc changes in glaucoma: increased cup-to-disc ratio (cupping of optic disc due to atrophy at the lamina cribrosa), nasal shift of blood vessels, flame-shaped haemorrhages.

Cataract — Anatomical Basis (AN41.2)

Cataract = opacity of the lens or its capsule, causing visual loss.

Why the lens becomes opaque:
The lens has no blood supply — it depends entirely on aqueous humour diffusion for nutrition. When lens fibres degenerate (due to aging, diabetes, radiation, trauma, congenital infections), they lose transparency.

Types:
• Nuclear cataract — affects the central lens nucleus; causes index myopia (patients initially read better without glasses), then progressive blurring
• Cortical cataract — affects the peripheral lens cortex; spoke-like opacities on slit-lamp
• Posterior subcapsular cataract — posterior capsule; affects near vision early; common in steroid use and diabetes

Surgical anatomy: In phacoemulsification, the anterior lens capsule is opened (continuous curvilinear capsulotomy), the lens nucleus is emulsified and removed through a 2–3 mm corneal incision, and an intraocular lens is implanted within the remaining capsule bag.

Central Retinal Artery Occlusion (CRAO) — Anatomical Basis (AN41.2)

The central retinal artery (branch of ophthalmic artery, from internal carotid) enters the optic nerve 1 cm behind the eyeball, travels within the optic nerve sheath, and enters the eye at the optic disc. It is a functional end-artery — no anastomoses within the retina.

In CRAO: Sudden, painless, monocular loss of vision ("curtain falling"). The entire inner retinal layers lose blood supply (outer layers supplied by choroid via diffusion — survives).

Fundoscopic findings:
• Pale, milky retina (oedema of infarcted inner layers)
• Cherry-red spot at the macula — the fovea is thin enough that the choroidal circulation (behind it) shows through → appears red against the white surrounding oedema

Part 3: Intraocular Muscles — Position, Nerve Supply, and Actions (AN41.3)

Intraocular Muscles

There are two intraocular muscles — smooth muscles contained within the eyeball itself (distinct from the seven extraocular skeletal muscles):

1. Sphincter Pupillae
• Circular smooth muscle fibres arranged concentrically around the pupil margin
• Located in the iris
• Action: Constricts the pupil (miosis)
• Nerve supply: Parasympathetic — preganglionic fibres from the Edinger-Westphal nucleus (CN III) → ciliary ganglion → short ciliary nerves → sphincter pupillae
• Clinical: In CN III palsy, parasympathetic fibres are compressed first (they run on the outside of the nerve trunk) → fixed dilated pupil (mydriasis)

2. Dilator Pupillae
• Radially arranged smooth muscle fibres in the iris (like spokes of a wheel)
• Action: Dilates the pupil (mydriasis)
• Nerve supply: Sympathetic — postganglionic fibres from the superior cervical ganglion → internal carotid plexus → ophthalmic artery → long ciliary nerves → dilator pupillae
• Clinical: Horner's syndrome (sympathetic chain lesion) → loss of dilator pupillae + Müller's muscle in upper lid → miosis + mild ptosis (partial)

3. Ciliary Muscle
• Ring of smooth muscle in the ciliary body
• Action: Accommodation — when it contracts, the zonular fibres relax → the lens becomes more convex (increasing refractive power) → near vision
• When it relaxes, zonular fibres become taut → lens flattens → far vision
• Nerve supply: Parasympathetic — same pathway as sphincter pupillae (CN III → ciliary ganglion → short ciliary nerves)
• Clinical: Cycloplegia (paralysis of accommodation) is produced by atropine (muscarinic antagonist), which blocks both sphincter pupillae (causing dilation) and ciliary muscle (preventing accommodation) — used for fundoscopy and refraction testing

Pupillary Light Reflex Pathway (Summary)

Afferent: Retina → optic nerve → optic tract → pretectal nucleus (midbrain)
Efferent: Edinger-Westphal nucleus (CN III) → oculomotor nerve → ciliary ganglion → short ciliary nerves → sphincter pupillae

Direct response: light in one eye → that pupil constricts
Consensual response: light in one eye → both pupils constrict (decussation at pretectal nucleus to both Edinger-Westphal nuclei)

Condition	Afferent pupil defect	Efferent defect
Optic nerve lesion (left)	Left RAPD — swinging torch shows left pupil dilates	—
CN III palsy (left)	—	Left pupil fixed and dilated, doesn't respond to direct or consensual
Horner's (left sympathetic)	—	Left pupil constricted (miosis), partial ptosis