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Structure

Eye

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The eye is the sensory organ of vision, a fluid-filled globe about 24 mm in diameter lying in the orbit. It focuses light onto the retina, where photoreceptors convert it into neural signals carried by the optic nerve.

Terminologia Anatomica (Latin): Oculus; Bulbus oculi · Open in the 3D atlas

Location

The globe sits in the bony orbit, cushioned by orbital fat and moved by six extrinsic muscles; the eyelids, conjunctiva and lacrimal apparatus protect its anterior surface.

Structure

Three tunics: fibrous (sclera and cornea), vascular uvea (choroid, ciliary body, iris) and neural retina with ten layers, macula and fovea; the optic disc is the blind spot. The anterior chamber (between cornea and iris) and posterior chamber contain aqueous humour, made by the ciliary body and drained through the trabecular meshwork to the canal of Schlemm; the vitreous fills the posterior segment, and the lens is held by zonular fibres.

Blood supply

The ophthalmic artery, a branch of the internal carotid, gives the central retinal artery, an end-artery supplying the inner retina, and the ciliary arteries supplying the choroid, outer retina, iris and ciliary body. Venous drainage is through the central retinal and ophthalmic veins to the cavernous sinus.

Innervation

Vision: optic nerve (II). Extraocular muscles: oculomotor (III) for superior, inferior and medial rectus and inferior oblique; trochlear (IV) for superior oblique; abducens (VI) for lateral rectus (LR6 SO4). Sensation: ophthalmic division of the trigeminal nerve (V1). Parasympathetic fibres via the ciliary ganglion supply the sphincter pupillae and ciliary muscle; sympathetic fibres from the superior cervical ganglion supply the dilator pupillae.

Function

The cornea and lens refract light onto the retina; accommodation occurs when the ciliary muscle contracts, the zonular fibres relax and the lens becomes more convex for near vision. The iris adjusts pupil size, and rods (dim light) and cones (colour, acuity) initiate phototransduction.

Clinical correlations

Central retinal artery occlusion causes sudden painless monocular vision loss with a cherry-red macula. Acute angle-closure glaucoma is a painful emergency with a fixed mid-dilated pupil. Other conditions include cataract, retinal detachment, diabetic retinopathy, papilloedema and cranial nerve III, IV or VI palsies with diplopia. Management follows local ophthalmic guidelines.

Common exam points

  • Layers: fibrous (sclera, cornea), vascular (uvea) and neural (retina).
  • LR6 SO4: lateral rectus by the abducens, superior oblique by the trochlear, all others by the oculomotor.
  • Central retinal artery is an end-artery from the ophthalmic artery; occlusion gives a cherry-red spot.
  • Aqueous humour is produced by the ciliary body and drains through the trabecular meshwork to the canal of Schlemm.
  • Accommodation: ciliary muscle contracts, zonules relax, lens thickens (parasympathetic, via CN III).

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. McGraw-Hill, 6th edition (2021) — Tier 3 (textbook)

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