Ocular Anatomy In Depth: Globe, Angle, Retina, Optic Nerve and Blood Supply

Last updated: November 2026

Clinical ophthalmology is applied anatomy: the angle explains glaucoma, the blood supply explains vascular occlusions, and the layers of the retina explain every OCT. This guide goes structure by structure, with the measurements and relations that examiners ask and surgeons use.

The globe

Cornea

Sclera and limbus

• Sclera: mainly type I collagen; thickest posteriorly (about 1 mm), thinnest just behind the rectus insertions (about 0.3 mm), the usual site of rupture in blunt trauma.
• The lamina cribrosa is the sieve-like posterior scleral opening for the optic nerve axons.
• The limbus is a 1–2 mm transition zone; limbal epithelial stem cells live in the palisades of Vogt. The surgical limbus is the landmark for incisions.
• Rectus insertions follow the spiral of Tillaux: medial about 5.5 mm, inferior 6.5 mm, lateral 6.9 mm, superior 7.7 mm from the limbus.

The anterior chamber angle

Aqueous leaves through Schlemm canal, collector channels and aqueous veins into the episcleral veins (conventional route), or through the ciliary muscle into the suprachoroidal space (uveoscleral route).

Uvea

Lens

• About 9–10 mm in diameter and 4 mm thick, growing thicker throughout life; refractive power about +20 D in the eye.
• Capsule: the thickest basement membrane in the body; thickest at the anterior pre-equatorial zone (about 14 µm), thinnest at the posterior pole (about 4 µm), which is why the posterior capsule ruptures easily.
• Epithelium only on the anterior surface; equatorial cells form new fibres throughout life, compressing old fibres into the nucleus.
• Sutures: an upright Y anteriorly and an inverted Y posteriorly.
• Zonules arise from the pars plana and pars plicata and insert around the equator.

Vitreous

• About 4 mL, 99% water, with type II collagen fibrils and hyaluronan.
• Firmest attachment at the vitreous base, which straddles the ora serrata (about 2 mm in front and 3 mm behind); also at the disc, the fovea, retinal vessels and the posterior lens capsule (Wieger ligament, enclosing Berger space).
• Cloquet canal is the remnant of the hyaloid artery path; Mittendorf dot and Bergmeister papilla are its remnants.

Retina

• Macula about 5.5 mm across; fovea about 1.5 mm; foveola about 0.35 mm; foveal avascular zone about 0.5 mm. The foveola contains only cones.
• About 120 million rods and 6 million cones; cone density peaks at the foveola, rod density about 20° from fixation.
• Ora serrata: the anterior end of the retina, about 6 mm (nasal) to 7 mm (temporal) behind the limbus.

Optic nerve

• About 1.2 million ganglion cell axons; the disc is about 1.5–1.8 mm across, slightly taller than wide.
• The orbital and intracranial nerve is wrapped in dura, arachnoid and pia: raised intracranial pressure is transmitted along the sheath (papilloedema).
• Blood supply of the disc: the surface nerve fibre layer from the central retinal artery; the prelaminar and laminar parts from the short posterior ciliary arteries (circle of Zinn-Haller): the territory affected in NAION and GCA.

Blood supply

• The ophthalmic artery is the first intracranial branch of the internal carotid artery, entering the orbit through the optic canal below the nerve.
• Central retinal artery: enters the optic nerve about 10–12 mm behind the globe and supplies the inner two-thirds of the retina (an end artery: occlusion causes a cherry-red spot).
• Posterior ciliary arteries: short (about 15–20 branches) to the choroid and optic nerve head; two long ones run in the horizontal meridian to the major arterial circle of the iris.
• Anterior ciliary arteries: from the muscular branches, two per rectus muscle except the lateral rectus (one), so operating on many recti at once risks anterior segment ischaemia.
• A cilioretinal artery (from the choroidal circulation) is present in about 15–30% of eyes and can spare central vision in CRAO.
• Veins: the central retinal vein and the vortex veins (usually four, one per quadrant) drain into the superior and inferior ophthalmic veins and then the cavernous sinus.

Nerves

Lids and lacrimal system

• Lid layers: skin, orbicularis, orbital septum, preaponeurotic fat, levator aponeurosis and Müller muscle (upper lid), tarsus and conjunctiva. The grey line (muscle of Riolan) divides anterior and posterior lamellae.
• Tarsal plates: upper about 10 mm high, lower about 4–5 mm; meibomian glands (about 25 upper, 20 lower).
• Lid retractors: levator (CN III), Müller muscle (sympathetic, 2 mm of lift), and the capsulopalpebral fascia in the lower lid.
• Lacrimal gland: orbital and palpebral lobes divided by the levator aponeurosis; ducts open into the superotemporal fornix.
• Drainage: puncta, canaliculi (2 mm vertical then 8 mm horizontal), common canaliculus, sac (valve of Rosenmüller), nasolacrimal duct (about 12–18 mm) opening into the inferior meatus under the valve of Hasner.

For the orbit, see orbital anatomy made easy; for development, embryology made easy; for histological layers, layers of the eye.

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Related: Orbital Anatomy Made Easy: Walls, Fissures and Muscles, Layers in Ophthalmology: Cornea to LGN in Tables, Ocular Physiology Basics: Tear Film, Aqueous, Cornea, Lens and Retina, Revise ophthalmology checklist.

Key sources

• American Academy of Ophthalmology. BCSC Section 2: Fundamentals and Principles of Ophthalmology (current edition)
• Snell RS, Lemp MA. Clinical Anatomy of the Eye. 2nd ed. Blackwell Science; 1998
• Remington LA. Clinical Anatomy and Physiology of the Visual System. 3rd ed. Elsevier; 2012
• Dua HS, et al. Human corneal anatomy redefined: a novel pre-Descemet layer (Dua's layer). Ophthalmology. 2013;120:1778-1785