Also interesting: The conjunctiva – the transparent protective layer of the eye
Read more
Pixel-Shot - stock.adobe.com
The cornea
The cornea of the human eye is responsible on the one hand for light refraction and on the other hand serves as protection. However, most people do not notice this complex system of different cell layers and fibres until inflammation or injury occurs. One good reason to take a closer look at its structure and function.
What is the Cornea?
The cornea is the slightly convex layer that covers the front surface of the eyeball. It is approximately 0.5 mm to 0.7 mm thick and has a diameter of about 11 mm. It is thicker towards its edge than in the centre. In order for our eyes to be able to see , the cornea does not contain any blood vessels. On the one hand, it is as transparent as glass, but at the same time it is very tough and solid in texture.
How is the cornea structured?
The cornea consists of five layers, which should not have any cloudy areas, otherwise vision would be impaired:
- The corneal epithelium is located on the outside and comprises several layers of cells.
- Behind it lies Bowman's layer, a cell-free membrane of collagen fibres.
- The thickest layer of the cornea is the stroma, comprising cells and collagen fibres.
- This is followed by another cell-free layer of collagen fibres, Descemet's membrane.
- On its inner side, the cornea ends in a thin layer of cells, the inner corneal epithelium.
Directly beneath the cornea is the anterior chamber of the eye, which is filled with aqueous humour. Surrounding the cornea is the white sclera of the eye. Since the cornea has no blood vessels to remove metabolic residues and supply it with nutrients, it relies on oxygen from the surrounding air. The cornea obtains its required nutrients from the tear fluid in the outer layers, while the inner corneal layers are supplied by the aqueous humour of the eye.
What is the function of the cornea?
With its special composition, the cornea performs two important functions:
- It protects the eye, e.g., from infections, foreign bodies, environmental influences and UV light.
- It refracts the light. In fact, the cornea contributes the majority of the refractive power, about 43 out of 60 dioptres.
Furthermore, the very sensitive nerves within the cornea serve as a warning system that can be vital, for example, in the event of injury or inadequate lubrication of the cornea. This is because the nerves trigger the so-called corneal reflex when the cornea is mechanically or chemically irritated. This causes the eyes to close involuntarily, stimulating tear production.
What is astigmatism?
Astigmatism is quite common and is often the reason for refractive error. With astigmatism, the vertical curvature of the cornea differs from the horizontal curvature. This makes you perceive a dot as a line, for example. Irregular corneal curvature can differ in both eyes or affect only one eye. More rarely, keratoconus, a hereditary curvature abnormality of the cornea, is present. This disease usually affects both eyes. With keratoconus, the cornea bulges into a cone shape due to thinning of the tissue. Treatment usually involves corneal transplantation.
Zarina Lukash - stock.adobe.com
What types of corneal disease are there?
The cornea may become inflamed due to infection. Causes can be bacteria, viruses or fungi, as well as chemical and mechanical irritations. Such a corneal inflammation, referred to as keratitis in medical terminology, presents as a reddened cornea and severe pain in the eye. It should always be treated by an eye specialist to prevent potential longterm damage. Usually, eye drops and medications are considered for this purpose.
Corneal opacity is also relatively common. This means that the cornea, which is actually transparent, is streaked with a light veil or, in severe cases, is even completely opaque. Causes of corneal opacity include corneal inflammation, corneal oedema with increased water retention in the cornea, or various metabolic diseases. A slight cloudiness sometimes disappears on its own or can be treated with medication. Sometimes, however, a corneal transplant is necessary if the opacity threatens blindness.
What is corneal detachment?
Corneal detachment is when one or more layers detach from the cornea. This usually happens only partially, but can cause severe pain and result in impaired vision. The cause is often a tear in the cornea due to an accident or foreign body.
The sooner the treatment, the better the chances of recovery. Often ointments and medications are all that is needed, but corneal transplantation may be necessary depending on the severity of the damage.