cornea anatomy and physiology

1. CORNEA

2.  A transparent, avascular, watch-glass like
structure
 Forms anterior one-sixth of the outer fibrous
coat of the eyeball.
 Dimensions
 Average horizontal diameter of 11.7 mm
 vertical diameter of 11 mm.
 Thickness of cornea in the centre is about
0.52 mm while at the periphery it is 0.7 mm.

3. • The central 5 mm area of the cornea forms the powerful refracting surface of
the eye.
• Refractive power of cornea -45 dioptres,
• total refractive power of the eye (60 dioptres).
• Refractive index of cornea 1.376.

5. HISTOLOGY
 1.Epithelium.
• Stratified squamous type.
• 5-6 layers of cells.
• Deepest layer is made up of columnar cells
• Next 2-3 layers of wing or umbrella cells
• Most superficial two layers are of flattened cells.
 2. Bowman's membrane.
• Acellular mass of condensed collagen fibrils.
• 12μm thick
• Binds the corneal stroma anteriorly with basement membrane of
the epithelium.
• A condensed superficial part of the stroma
• Once destroyed, it does not regenerate.

6.  3. Stroma (substantia propria) (90% of corneal thickness)
• 0.5 mm thick.
• Collagen fibrils (lamellae) embedded in hydrated matrix of
proteoglycans.
• The lamellae are arranged in many layers parallel to each other
and to corneal plane
• 4. Pre Descemet’s membrane(Dua’s Layer)
• It is about 15 micrometer thick
• Acellular
• Strong and impervious to air
 5. Descemet's membrane (posterior elastic lamina).
• binds the stroma posteriorly.
• consists of collagen and glycoprotein.
• it can regenerate

7.  5. Endothelium.
• single layer of flat polygonal (mainly hexagonal) cells
• Cell density is around 3000 cells/sq mm in young adults
• Have no ability to divide

9.  Blood supply
• Cornea is an avascular structure.
 Nerve supply
• Cornea is supplied by anterior ciliary nerves - branches of ophthalmic division
of the 5th cranial nerve.
• After going about 2 mm in cornea the nerves lose their myelin sheath and
divide dichotomously and form three plexuses — the stromal, subepithelial
and intraepithelial.

10. Factors maintaining corneal
transparency
1. Peculiar arrangement of corneal lamellae (Lattice theory of Maurice)
2. Avascularity
3. Relative state of dehydration (78% water content) maintained by the barrier
effects of epithelium and endothelium, and active Na+ K+ ATPase pump of
endothelium
4. Swelling pressure of stroma which counters the imbibition effect of intra
ocular pressure.