Held by a ring of fine fibres from the ciliary body; when the ciliary muscle contracts the fibres slacken and the disc rounds up
The lens and accommodation, as explained by Helmholtz
Anatomy · Eye, ear & senses
Lens
The eye's adjustable internal focusing element
Eye · behind the iris
A clear, flexible, biconvex disc about a centimetre across that sits directly behind the iris and pupil and in front of the vitreous body. It is slung from the ciliary body by a ring of fine zonular fibres and enclosed in an elastic capsule; changing its curvature lets the eye focus on near and far objects, a process called accommodation.
How to spot them
- ▸A transparent biconvex disc just behind the pupil, the back surface more curved than the front
- ▸Suspended by zonular fibres (the suspensory ligament) from the ciliary body
- ▸Wrapped in a capsule that is the thickest basement membrane in the body
- ▸Has no blood vessels or nerves; grows throughout life; stiffens with age
What to know
Enclosed in an elastic bag that is the thickest basement membrane in the body
The lens capsule
Adds new fibre cells like the layers of an onion throughout life, packed with proteins called crystallins
The lens; the oldest fibres sit in the central nucleus
By the mid-forties it stiffens, so that print must be held at arm's length
Presbyopia, loss of accommodation in the lens
Fragments of Perspex from shattered fighter-plane canopies, found harmlessly in pilots' eyes, inspired Harold Ridley's 1949 operation at St Thomas' Hospital (1949)
The first intraocular lens implant, an artificial replacement for the natural lens after cataract removal
Sits in a shallow dimple at the front of the vitreous body, the hyaloid fossa, immediately behind the iris
The lens, whose position is a favourite anatomy question
Develops in the embryo from a thickening of surface ectoderm called a placode that pinches off into a vesicle
The lens, a classic example of embryonic induction (by the optic vesicle)
Key ideas
- For near vision the ciliary muscle contracts, the ring it forms shrinks, the zonular fibres go slack and the elastic capsule moulds the lens into a rounder shape with more focusing power.
- The lens is avascular and lives off the aqueous humour; its clarity depends on ordered crystallin proteins, and cataract is what happens when they clump.
Quick-fire facts
- •About 9–10 mm across and 3.5–5 mm thick in the adult, growing thicker with age
- •Provides roughly 15–20 of the eye's approximately 60 dioptres, and is the adjustable part of the system
- •The ciliary muscle is supplied by the parasympathetic fibres of the oculomotor nerve; the lens itself has no nerve supply
- •Anterior surface has a single layer of epithelial cells; there is no epithelium on the posterior surface
- •Hermann von Helmholtz proposed in 1855 that the lens rounds up for near vision when tension on it is released
- •The absence of the natural lens is called aphakia; a lens lying out of position is ectopia lentis, a feature of Marfan syndrome
- •Sir Harold Ridley (1906–2001) implanted the first artificial lens on 29 November 1949
- •Cataract, clouding of the lens, is the leading cause of blindness worldwide, and cataract surgery is one of the commonest operations in the NHS
Drill it — 7 cards
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Sources
- TeachMeAnatomy — The Eyeball — Structure, Vasculature
- National Eye Institute (NIH) — Cataracts