Eye (eyeball)
Bulbus oculi
Also known as: eyeball, globe, oculus
The paired, roughly spherical organ of vision (≈24 mm in diameter) housed in the orbit, which focuses light onto the retina and converts it into neural signals carried by the optic nerve.
Overview
The eyeball is composed of three concentric tunics: an outer fibrous tunic (opaque sclera posteriorly, transparent cornea anteriorly), a middle vascular tunic or uvea (choroid, ciliary body and iris) and an inner neural tunic (retina). It is not a perfect sphere: the cornea forms a more steeply curved segment of a smaller sphere set into the front of the larger scleral sphere.
Internally the eye is divided by the lens and iris into three spaces: the anterior chamber (between cornea and iris) and posterior chamber (between iris and lens), both filled with aqueous humour, and the much larger vitreous chamber behind the lens, filled with the gel-like vitreous body. The refractive media that light traverses are the tear film, cornea, aqueous humour, lens and vitreous body.
The optic nerve leaves the globe ≈3–4 mm nasal to the posterior pole, where the fovea lies. Six extraocular muscles move the globe, and it is protected by the bony orbit, the eyelids, the conjunctiva and the tear film. The anatomical (optic) axis and the visual axis (object → fovea) differ by ≈5°.
- Location
Anterior part of the bony orbit, cushioned by orbital fat and suspended by the fascial sheath of the eyeball (Tenon capsule) and its suspensory ligament (of Lockwood).
Parts & subdivisions
- Sclera (posterior 5/6) and cornea (anterior 1/6), meeting at the corneoscleral limbus. Protects the eye, maintains its shape and gives attachment to extraocular muscles.
- Choroid, ciliary body and iris: pigmented, highly vascular layer that nourishes the outer retina, produces aqueous humour, controls accommodation and regulates pupil size.
- Optic (photosensitive) part lining the posterior eye to the ora serrata, plus non-visual pigmented extensions over the ciliary body and iris.
- Between the cornea and iris; ≈3 mm deep, ≈0.25 mL of aqueous humour.
- Narrow space between iris, ciliary processes and lens; ≈0.06 mL of aqueous humour.
- Behind the lens; contains the vitreous body (≈4 mL, ≈80% of the eye's volume).
- Biconvex transparent crystalline lens suspended by the ciliary zonule; variable focus.
Anatomy
- Innervation
Sensory: ophthalmic nerve (V1) via the nasociliary nerve — long ciliary nerves (cornea, iris, ciliary body) and sensory root through the ciliary ganglion. Parasympathetic: oculomotor nerve (III) → ciliary ganglion → short ciliary nerves (sphincter pupillae, ciliary muscle). Sympathetic: superior cervical ganglion → internal carotid plexus → long and short ciliary nerves (dilator pupillae, blood vessels). Special sense: optic nerve (II).
- Arterial supply
Ophthalmic artery (first branch of the internal carotid artery): central retinal artery (inner retina), short posterior ciliary arteries (choroid, optic nerve head), long posterior ciliary and anterior ciliary arteries (ciliary body, iris — forming the major arterial circle of the iris).
- Venous drainage
Central retinal vein and the 4 (4–6) vortex veins draining the choroid → superior and inferior ophthalmic veins → cavernous sinus (and pterygoid plexus).
- Relations
Surrounded by orbital fat and the extraocular muscles; separated from fat by the fascial sheath (Tenon capsule). Anteriorly covered by the eyelids and conjunctiva. Optic nerve exits posteriorly within its dural sheath.
- Development
Optic vesicle (diencephalic neuroectoderm) → optic cup (retina, RPE, iris and ciliary epithelium); surface ectoderm → lens and corneal epithelium; neural crest/mesoderm → sclera, corneal stroma and endothelium, choroid, ciliary and iris stroma.
Function
- Refract light to form a focused, inverted image on the retina
- Adjust focus for near objects (accommodation) and control light entry (pupil)
- Transduce light into neural signals (rods and cones) and begin visual processing
- Provide binocular input for depth perception (stereopsis)
- Physiology
Total refractive power ≈60 D in the relaxed (distance) state, of which the cornea contributes ≈43 D and the lens ≈15–20 D; accommodation adds up to ≈12–14 D in childhood. The eye is kept inflated and its optics stable by an intraocular pressure of 10–21 mmHg.
Clinical significance
Red reflex and fundoscopy
Shining an ophthalmoscope into the pupil gives an orange-red reflex from the choroidal circulation through clear media. An absent or white reflex (leukocoria) in an infant suggests congenital cataract or retinoblastoma and needs urgent referral.
Penetrating eye injury
Any history of high-velocity foreign body (hammering metal) requires imaging; an irregular (peaked) pupil or a shallow anterior chamber suggests globe rupture. Protect with a rigid shield, avoid pressure, and refer.
Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.