The lens and zonules through life
- Anatomy and physiology
The lens grows all life and slowly hardens and clouds. That is why near vision fails after 40 and why cataract is the leading cause of blindness.
See it in 3D: Lens · Ciliary body
What it is
The crystalline lens is a clear, flexible, biconvex structure held behind the iris by the zonules, fine fibres attached to the ciliary body. It has no vessels or nerves and is enclosed in a capsule. It adds new fibres throughout life, so it becomes larger, harder and less clear with age. Loss of flexibility causes presbyopia; loss of clarity is cataract.
Clinical anatomy
- Capsule: an elastic basement membrane around the whole lens; the posterior capsule is left in place in cataract surgery to hold the implant.
- Epithelium under the anterior capsule, which makes new fibres at the equator.
- Cortex (newer fibres) and nucleus (oldest fibres, in the centre).
- Zonules running from the ciliary processes to the lens equator.
Physiology
- The lens adds focusing power that can change: in accommodation the ciliary muscle contracts, the zonules slacken and the lens becomes rounder.
- Its proteins (crystallins) are packed in a regular array that keeps it transparent.
- It filters ultraviolet light and protects the retina.
Clinical correlation
- Presbyopia starts in the early forties: near vision needs reading glasses.
- Nuclear cataract causes a myopic shift; posterior subcapsular cataract (steroids, diabetes) causes glare and early trouble reading.
- Lens dislocation: upwards and outwards in Marfan syndrome, downwards and inwards in homocystinuria, and after trauma.
- Cataract surgery removes the lens contents by phacoemulsification and places an artificial lens in the capsular bag.
Next step / Initial management
- Cataract that limits daily life: routine ophthalmology for surgery; there is no medical treatment.
- Cataract in a child: urgent paediatric ophthalmology (amblyopia).
- Dislocated lens: ophthalmology, and workup for Marfan syndrome or homocystinuria if there was no trauma.
- Patients on long-term corticosteroids: ask about glare and blurred vision and refer if present.
Test yourself
Think of the answer, then tap the question to see it.
What happens to the zonules and lens during accommodation?
The ciliary muscle contracts, the zonules slacken and the lens becomes rounder, increasing its power.
Which type of cataract is associated with corticosteroids?
Posterior subcapsular cataract.
In which direction does the lens usually dislocate in Marfan syndrome?
Upwards and outwards (superotemporally).
Elsewhere in DurgiPedia
Connections
See it in 3D2
The structures of Durgical's 3D anatomy where it is found.
Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Anatomy, Head and Neck, Eye Ciliary Muscles (StatPearls Publishing (NCBI Bookshelf), 2025); World report on vision (World Health Organization, 2019).
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Representative model, for educational purposes only.