Anatomical & physiological considerations
The cornea forms the anterior 1/6 of outer coat of globe. It is transparent and avascular. It has the following dimensions:
- Diameters: Vertical 9-11 mm & horizontal 11-12 mm.
- Thickness: 0.8 mm at limbus & 0.5 in centre.
- Radius of curvature of anterior surface is about 7.8 mm.
- Radius of curvature of posterior surface is about 6.2 mm.
- Refractive index = 1.37.
- Refractive power = 42 D.
It consists of the following 5 layers:
- It is stratified squamous non‑keratinizing epithelium. It is formed of 5 - 6 layers, the basal layer is columnar, the intermediate layers polyhedral and superficial layers are flat.
- Epithelial cells rest on basement membrane which is secreted by the basal cells. It is very important to fix the epithelial cells to the underlying layers. When the basement membrane is damaged, recurrent corneal erosion syndrome develops.
2) Bowman’s membrane (10-12 micron):
It is a homogeneous acellular condensation of the anterior stromal lamellae. When it is injured it does not regenerate and a permanent opacity is produced. It ends at limbus.
3) Stroma (substantia propria):
- It forms 90 % of the thickness of the cornea.
- It is formed of C.T. bundles arranged in about 300 layers, the lamellae in each layer are parallel to each other but perpendicular to those in the next layer.
- The collagenous fibrils are very uniform. The distance between each other is less than half the wave length of visible light. This is very important for corneal transparency.
- In-between the corneal lamellae, there are spaces known as lacunae containing cells known as keratocytes.
- The matrix is formed of special types of mucopolysaccharides that are not present in any other tissue in the human body. It can absorb large amounts of water causing corneal edema. Normal hydration of the cornea is maintained by the corneal endothelium.
- It is continuous with the sclera and limbus.
4) Descemet's membrane:
- It is elastic and resists digestion by proteolytic enzymes.
- At the limbus it becomes 2 layers. One of them continues over the trabecular meshwork. The other layer ends by a condensation forming "the anterior ring of Schwalbe".
- It is formed of one layer of flat hexagonal cells arranged in a mosaic pattern.
- It is continuous with endothelium of the trabecular meshwork and anterior surface of the iris.
- It is responsible for the deturgescence of the cornea, so that when it is injured corneal edema occurs.
- It is not capable of mitosis and regeneration. When injured, the healthy cells flatten to cover the area of lost endothelium.
Supplies of the cornea:-
* Nerve supply:
- It is derived from long & short ciliary nerves of nasociliary nerves which are branches of ophthalmic division of trigeminal nerve (see the nerve supply of eyeball). In the cornea the nerve fibers are non myelinated.
* Nutrition of cornea:
The cornea being avascular, derives its nutrition from the following sources:
- Diffusion from Tears: 85% of oxygen is taken through the tear film. During lid closure, oxygen diffuses from upper palpebral conjunctival vessels.
- Glucose and other nutrients diffuse from aqueous humour.
- Diffusion from limbal capillaries.
Cause of corneal transparency:
1. Anatomical factors:
a. Cornea does not contain opaque structures:
- No keratin in the epithelium.
- No blood vessels.
- No myelin sheath around nerve fibers.
b. Regular arrangement of collagenous bundles in the stroma.
c. Smooth anterior surface.
2. Physical factors:
- The spacing between the collagenous fibrils is uniform and less than half the wavelength of visible light (400-700 nm). So that the scattered light rays destroy each other allowing clear vision.
3. Physiological factors (deturgescence of the cornea):
- The corneal lamellae are not fully hydrated. It is an energy consuming function of the endothelium. If the cornea becomes edematous, it will be opaque because the spacing between the collagenous fibrils will be greater than half the wavelength of visible light. Also the corneal epithelium has a role in prevention of corneal edema.
Causes of corneal lustre:
- Healthy intact epithelium.
- Intact tear film.
Classification of corneal diseases
I. Inflammatory (Keratitis).
II. Degenerative diseases.
III. Ectatic corneal conditions.
IV. Miscellaneous subjects.