Learning Objective: To understand the anatomical layers, internal structures, and vascular supply of the eyeball, and recognise key clinical conditions such as glaucoma and papilledema.
The eyeball is a bilateral spherical organ responsible for vision, housed within the bony orbit. Anatomically, it consists of three layers: the fibrous, vascular, and inner layers.
Layers of the Eyeball
Fibrous Layer
The outermost layer, composed of the sclera and cornea.
- Sclera: The white, opaque portion (≈85% of the fibrous layer). Provides shape and attachment to the extraocular muscles.
- Cornea: Transparent, central structure responsible for refracting incoming light.
Vascular Layer
Located beneath the fibrous layer, composed of the choroid, ciliary body, and iris.
- Choroid: A highly vascular layer supplying the outer retina.
- Ciliary body: Contains smooth muscle (controls lens shape) and ciliary processes (produce aqueous humor).
- Iris: Pigmented diaphragm surrounding the pupil; controls pupil diameter via autonomic innervation.
Inner Layer (Retina)
The retina has:
- Pigmented layer: Absorbs light, prevents scattering.
- Neural layer: Contains photoreceptors. Present only posteriorly and laterally.
Key structures:
- Macula: Central region for high acuity vision.
- Fovea: Depression within macula; dense cone concentration.
- Optic disc: Entry point of optic nerve; lacks photoreceptors (“blind spot”).
ACTIVITY
Structures of the Eyeball
Vitreous Body
Gel-like structure fills the posterior segment. Contains the hyaloid canal (fetal remnant). Functions:
- Supports lens
- Stabilizes retina
- Contributes to optical properties
Lens
Transparent biconvex structure located between the vitreous body and the pupil.
- The shape alters due to ciliary muscle activity (accommodation).
- Aging causes opacification → cataracts.
Anterior & Posterior Chambers
Filled with aqueous humor, produced by the ciliary processes.
- Anterior chamber: Between the cornea and iris
- Posterior chamber: Between the iris and the ciliary body
Aqueous humor drains via the trabecular meshwork. Obstruction → glaucoma.
Vasculature
- Ophthalmic artery supplies the eye; key branch = central retinal artery (occlusion → sudden blindness).
- Superior and inferior ophthalmic veins drain into the cavernous sinus.
ACTIVITY
Clinical Relevance
Glaucoma
Characterized by optic nerve damage from increased intraocular pressure.
- Open-angle: Gradual peripheral vision loss.
- Closed-angle: Rapid, painful rise in pressure → emergency.
Papilloedema
Swelling of the optic disc due to raised intracranial pressure.
Causes:
- Mass lesions
- Haemorrhage
- Meningitis
- Hydrocephalus
Venous congestion → retina swelling → blurred margins of optic disc.
