Ciliary body

Corpus ciliare

Also known as: ciliary muscle, ciliary processes, pars plicata, pars plana, ciliary epithelium

A ring-shaped thickening of the uvea between the choroid and iris that contains the ciliary muscle (accommodation) and the ciliary processes (aqueous humour production), and anchors the zonule of the lens.

Show on body map Eye — Horizontal Section

Overview

Width (meridional)≈6 mm

Ciliary processes≈70–80

Pars plana entry≈3.5–4 mm behind the limbus

In meridional section the ciliary body is triangular, ≈6 mm wide, extending from the ora serrata to the scleral spur and root of the iris. Its anterior third, the pars plicata, carries ≈70–80 radial ciliary processes; its posterior two-thirds, the flat pars plana, is the safe entry route for intravitreal injections and vitrectomy (≈3.5–4 mm behind the limbus), because it lacks retina and major vessels.

The ciliary muscle is smooth muscle with three fibre groups: longitudinal (meridional, Brücke) fibres inserting into the scleral spur (also opening the trabecular meshwork), radial (oblique) fibres, and circular (sphincteric, Müller) fibres near the lens equator. Parasympathetic stimulation (via M3 muscarinic receptors) contracts the muscle, which moves the ciliary body forwards and inwards, slackening the zonular fibres and allowing the elastic lens to become more convex — accommodation for near vision.

The ciliary processes are covered by a double-layered ciliary epithelium: an outer pigmented layer (continuous with the RPE) and an inner non-pigmented layer (continuous with the neural retina), joined apex-to-apex. Tight junctions between non-pigmented cells form the blood–aqueous barrier. These cells secrete aqueous humour by active transport (Na⁺/K⁺-ATPase, carbonic anhydrase), ultrafiltration and diffusion from the fenestrated capillaries in the process stroma.

Location: Ring between the ora serrata (posteriorly) and the iris root/scleral spur (anteriorly), deep to the limbus and anterior sclera.

Parts & subdivisions

Ciliary muscle
Smooth muscle (longitudinal, radial, circular fibres). Contracts in accommodation → zonules relax → lens rounds up. Parasympathetic via M3 receptors.
Ciliary processes (pars plicata)
≈70–80 vascular folds whose epithelium secretes aqueous humour into the posterior chamber.
Pars plana
Flat posterior part (≈4 mm wide); surgical access to the vitreous cavity; origin of many zonular fibres.
Ciliary epithelium
Bilayer: outer pigmented and inner non-pigmented (secretory, blood–aqueous barrier) cells.
Ciliary zonule
Fibrillin-rich suspensory fibres from the processes and pars plana to the lens capsule.

Anatomy

Innervation: Parasympathetic: Edinger–Westphal nucleus → oculomotor nerve → ciliary ganglion → short ciliary nerves (≈95% of postganglionic ciliary ganglion fibres go to the ciliary muscle). Sympathetic: minor relaxing effect (β₂) and vasomotor; β-adrenergic and α₂ receptors on the epithelium modulate aqueous secretion.
Arterial supply: Long posterior ciliary and anterior ciliary arteries, forming the major arterial circle of the iris at the root of the iris.
Histology: Smooth muscle, loose vascular stroma with fenestrated capillaries, and a two-layered cuboidal epithelium (pigmented outer, non-pigmented inner).

Function

Accommodation — focus the lens for near vision
Produce aqueous humour (≈2–3 µL/min)
Suspend the lens via the zonule
Increase trabecular outflow by pulling on the scleral spur

Physiology: Aqueous secretion is reduced by β-blockers (timolol), carbonic anhydrase inhibitors (acetazolamide, dorzolamide) and α₂-agonists (brimonidine). Muscarinic antagonists (atropine, cyclopentolate, tropicamide) paralyse the ciliary muscle (cycloplegia) and dilate the pupil (mydriasis) — used for refraction in children and to rest the eye in uveitis.

Clinical significance

Presbyopia

The ciliary muscle remains functional with age, but the lens hardens and can no longer round up when the zonules relax, so the near point recedes.

Ciliary body melanoma and cyclodestruction

Ciliary body tumours may present late (hidden behind the iris), sometimes with sentinel episcleral vessels. In refractory glaucoma, laser cyclophotocoagulation destroys part of the ciliary epithelium to reduce aqueous production.

Test yourself

Which change occurs during accommodation for near vision?

Ciliary muscle relaxes, zonules tighten, lens flattens
Ciliary muscle contracts, zonules slacken, lens becomes more convex
Ciliary muscle contracts, zonules tighten, lens becomes more convex
Dilator pupillae contracts and the eyes diverge

Answer: Ciliary muscle contracts, zonules slacken, lens becomes more convex

Parasympathetic (CN III → ciliary ganglion) contraction of the ciliary muscle reduces the diameter of the ciliary ring, relaxing zonular tension so the elastic capsule rounds the lens. This is accompanied by convergence and miosis (the near triad).