ANATOMY OF CORNEA
The membrane could be a clear, avascular, watch-glass like structure. It forms anterior one sixth of the outer fibrous coat of the eyeball.
Dimensions
The anterior surface of membrane is elliptical with an average horizontal diameter of 11.7 mm and vertical diameter of 11 mm .
The posterior surface of membrane is circular with an average diameter of 11.5 mm. Thickness of membrane within the centre is regarding 0.52mm whereas at the outer boundary it's 0.7 mm.
Radius of curvature. The central five millimeter space of the cornea forms the powerful refracting surface of the eye. The anterior and posterior radii of curvature of this central an of membrane are 7.8 mm and 6.5 mm, severally
Refractive power of the membrane is regarding 45 dioptres, that is roughly three-fourth of the total refractive power of the eye (60 dioptres).
Histology
Histologically, the membrane consists of 5 distinct layers. From anterior to posterior these are: Epithelium, Bowman’s membrane, substantia propria (corneal stroma), Descemet’s membrane and endothelium tissue.
1. Epithelium.
it's of stratified squamous sort and becomes continuous with the animal tissue of neural structure conjunctiva at the body structure.
It consists of 5-6 layers of cells. The deepest (basal) layer is made from columnar cells, next 2-3 layers of wing or umbrella cells and thus the most superficial 2 layers square measure of two-dimensional cells.
2. Bowman's membrane.
This layer consists of acellular mass of condensed albuminoid fibrils. It is about 12µm in thickness and binds the tissue layer stroma anteriorly with basement membrane of the animal tissue.
It is not a true elastic membrane however merely a condensed superficial a part of the stroma. It shows substantial resistance to infection. But once destroyed, it doesn't regenerate.
3. Stroma (substantia propria).
This layer is regarding 0.5 millimeter in thickness and constitutes most of the cornea (90% of total thickness). It consists of albuminoid fibrils (lamellae) embedded in hydrous matrix of proteoglycans.
The lamellae square measure organized in several layers. In every layer they're not solely parallel to every other however additionally to the tissue layer plane and become continuous with scleral lamellae at the body structure.
The alternating layers of lamellae square measure at right angle to each other. Among the lamellae square measure gift keratocytes, wandering macrophages, histiocytes and some leucocytes.
4. Descemet's membrane (posterior elastic lamina).
The Descemet's membrane could be a sturdy homogeneous layer that bounds the stroma posteriorly. It is very resistant to chemical agents, trauma and pathological processes.
Therefore, 'Descemetocele' will maintain the integrity of eyeball for long. Descemet's membrane consists of albuminoid and glycoproteins. Unlike Bowman's membrane it will regenerate.
Normally it remains throughout a state of tension and once torn it curls inwards on itself. within the outer boundary it seems to finish at the anterior limit of fibrous tissue network as Schwalbe's line (ring).
5. epithelial tissue.
It consists of one layer of flat polygonal (mainly hexagonal) cells that on slit lamp biomicroscopy seem as a mosaic. The cell density of epithelial tissue is around 3000 cells/mm2 in young adults, that decreases with the advancing age.
There is a substantial practical reserve for the endothelium. Therefore, tissue layer decompensation occurs solely once quite seventy five % of the cells square measure lost. The epithelial tissue cells contain 'active-pump' mechanism.
Blood supply
Cornea is associate avascular structure. little loops derived from the anterior ciliary vessels invade its outer boundary for regarding one millimeter. really these loops aren't within the cornea however within the subconjunctival tissue that overlaps the membrane.
Nerve supply
Cornea is equipped by anterior ciliary nerves that are branches of ophthalmic division of the fifth bone nerve. once going regarding two millimeter in membrane the nerves lose their case and divide dichotomously and kind 3 plexuses — the stromal, subepithelial and intraepithelial
OPTOMETRY-SHARP VISION
Optometrist