What is the cornea (eye)?
The cornea of the eye is the translucent, anterior part of the outer skin of the eye. The much larger part of this eye skin is the sclera, which can be seen as the white part of the eye.
The cornea is a flat protrusion at the front of the eyeball. Like a window, it allows light to enter the eye. Due to its natural curvature, it – together with the crystalline lens – takes over most of the refraction of light in the eye.
As the cornea reflects part of the light that hits it like a convex mirror, the eye shines. After death, the cornea clouds over and becomes dull and opaque.
The name cornea (eye) comes from the fact that the cornea is as hard as corneal substance, but extremely thin, so that you can see through it: In the center, the cornea is only half a millimeter thick, in the peripheral area about one millimeter. The parts of the eye behind it, such as the iris (iris), are visible through it.
The aqueous humor (inside) and the lacrimal fluid (outside), both of which have a high salt content, keep the cornea in a constant state of deswelling – it has a water content of only 76 percent.
The five layers of the cornea (eye)
The cornea (eye) consists of five layers. From the outside in, these are
Anterior corneal epithelium
The outer layer is the anterior corneal epithelium, which merges into the conjunctiva without a clear boundary. It prevents germs from entering the eye. The corneal nerves also end in this corneal layer – injuries to the cornea such as small scratches (e.g. from fingernails) are therefore very painful.
Bowman’s membrane
The anterior corneal epithelium is adjoined on the inside by a cell-free glass membrane, the so-called Bowman membrane. Its surface is smooth and forms the basement membrane as a transition to the anterior corneal epithelium. In the event of injury, it only heals with scarring – it is not capable of regeneration.
Stroma
The parallel arrangement of the lamellae made of collagen fiber bundles makes the stroma transparent. However, if this arrangement is disturbed (e.g. by inflammation or injury), the transparency is lost. A scar forms and vision becomes blurred. A corneal transplant can then help.
Descemet’s membrane
The stroma is followed (inwards) by a second Glaush membrane, also known as Descemet’s membrane or Demours membrane. It consists of a simple cell layer, but is particularly important for the structure of the cornea. It is very resistant and increases in thickness in the course of life. So even if the cornea of the eye is injured or dies due to illness, the Descemet’s membrane usually remains intact and prevents the aqueous humor from flowing out of the anterior chamber of the eye.
However, if the Descemet’s membrane is injured, aqueous humor flows out and the cornea of the eye swells – with the result that it loses its transparency. Such an injury to the Descemet’s membrane would heal with scarring, as is the case with Bowman’s membrane.
Endothelium
As the last, innermost layer, a single-layered endothelium separates the cornea of the eye from the anterior chamber filled with aqueous humor: the anterior side of the cells borders flat against the Descemet’s membrane, while the posterior side adjoins the anterior chamber of the eye. The endothelial cells are closely connected to each other by complex junctions and are actively involved in the eye’s metabolism.
Corneal function
The cornea of the eye is embedded in the sclera (sclera) like a watch glass and is more curved than its surroundings. It has a high refractive power of 43 diopters – the entire visual system has 60 diopters. This very high refractive power is due to the aqueous humor behind it, which is also a highly refractive fluid.
The cornea is therefore responsible for most of the refraction of light in the eye, which leads to the light rays being focused on the retina.
What problems can the cornea (eye) cause?
Various medical problems can occur in the cornea of the eye and impair vision. These include, for example
- Keratoconus: The cornea (eye) is progressively deformed into a cone shape in the center and thinned out at the edges.
- Corneal opacity: This can be the result of injuries (e.g. foreign bodies entering the eye, burns or chemical burns). A corneal ulcer (ulcus corneae) as a result of corneal inflammation can also cloud the cornea.
- Sicca syndrome (Sjörgen syndrome): In this autoimmune disease, the immune system damages the lacrimal glands, among other things, causing the cornea of the eye to dry out.
- Infections: Bacteria, viruses or fungi can infect the cornea of the eye.
