A tissue in front of the iris built of five layers, with a basement membrane named after a French anatomist at its back
The cornea — Descemet's membrane is the basement membrane of the endothelium
Anatomy · Eye, ear & senses
Cornea
The transparent dome at the front of the eye
Eye · outer coat
The clear, dome-shaped window that forms the front sixth of the eye's outer coat, continuous with the white sclera at a rim called the limbus. It has no blood vessels, yet is one of the most densely nerve-supplied tissues in the body, and it is built of five layers, the thickest of which, the stroma, is a precisely stacked lattice of collagen that keeps it transparent.
How to spot them
- ▸A clear dome sitting in front of the iris, meeting the white of the eye at a rim called the limbus
- ▸Five layers from front to back: epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium
- ▸Avascular — nourished by the tear film in front, the aqueous humour behind and oxygen from the air
- ▸Touching it triggers the blink reflex, carried by the ophthalmic division of the trigeminal nerve
What to know
A living tissue with no blood vessels at all, fed by tears in front and watery fluid behind
The cornea is avascular and takes oxygen largely from the air
Wisp of cotton wool: both eyelids snap shut, the sensory limb running in the ophthalmic nerve and the motor limb in the facial nerve
The corneal reflex, tested to check cranial nerves V and VII
The ring of tissue where a clear dome joins the white coat, home to the stem cells that renew the surface
The corneal limbus
Tissue transplanted between strangers without blood-group matching, first successfully done in Olomouc in 1905 (1905)
Corneal transplant (keratoplasty) — Eduard Zirm's full-thickness graft; the site is 'immune privileged'
A dome that thins and bulges into a cone in young adults, sometimes treated by cross-linking with riboflavin and ultraviolet light
Keratoconus, a disease of the cornea
The surface reshaped by an excimer laser in the operation known as LASIK
The cornea, thinned in the centre to change its focusing power
Key ideas
- Transparency depends on order: the stromal collagen fibrils are all of one small diameter and spaced at less than the wavelength of light, so scattered light cancels out. Swelling disturbs the spacing and the cornea goes cloudy.
- The endothelium pumps fluid out of the stroma to keep it about 78 per cent water. Those cells barely divide in adults, which is why loss of them is a common reason for a graft.
Quick-fire facts
- •About 0.5 mm thick in the centre and about 0.7 mm at the edge, with a horizontal diameter of roughly 11.5 mm
- •Provides roughly 43 of the eye's approximately 60 dioptres of focusing power, because the step from air to cornea is the steepest change in refractive index the light meets
- •The five classic layers are epithelium, Bowman's layer, stroma (about 90 per cent of thickness), Descemet's membrane and endothelium
- •In 2013 Harminder Dua and colleagues in Nottingham proposed a sixth layer, 'Dua's layer'; whether it is a distinct layer is disputed
- •Its nerves come chiefly from the ophthalmic division of the trigeminal nerve via the long ciliary nerves, so abrasions are exquisitely painful
- •Eduard Zirm performed the first successful human corneal transplant in Olomouc, now in the Czech Republic, in 1905
- •Because no blood vessels reach it, grafts are rarely rejected; corneal grafts are among the commonest and most successful transplants
- •Contact-lens wearers can starve it of oxygen if lenses are worn too long, prompting the growth of vessels into the tissue
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Sources
- TeachMeAnatomy — The Eyeball — Structure, Vasculature
- National Eye Institute (NIH) — How the Eyes Work