Optometry: CORNEA :- Definition, Function, Microscopic structure

*Cornea is the one the most important part of the eye ,it allow to enter the light in the eye.
*It is a transparent, avascular, watchglass like structure.
*It forms anterior 1/6th part of the fibreous layer of the eyeball.

DIMENSIONS -

Horizontal diameter(Anterior) :- 11.75 mm
Vertical diameter(anterior) :- 11 mm
Thickness (center ) :- 0.52 mm
Thickness (periphery) :- 0.67 mm
Anterior Radius of curvature :- 7.8 mm
Posterior radius of curvature :- 6.5 mm
Refractive power (anterior) :- +48 D
Refractive Power (posterior) :- -5 D
Net refractive power of cornea :- +43 D
Refractive index :- 1.37

HISTOLOGY :-

The cornea was considered to consists of five distinct layer, from anterior to posterior are :-

* Epithelium
* Bowman's membrane
* Stroma
* Descemet's membrane
* Endothelium
* Pre-descemet's membrane (dua's layer) {recently found , it lies between Stroma and descemet's membrane}

EPITHELIUM :-

*It is about 50-90 µm thick (10% of corneal thickness)
*It consists of 5-7 layers of cell.
*It is composed by Stratified squamous non-stop keratinized.
*It becomes continue with epithelium of Bulbar Conjunctiva at the Limbus.

BOWMAN'S MEMBRANE :-

*It is about 8-14 µm in thickness .
*It attach to the corneal Stroma.
*It is consists of Acellular mass of condensed collagen fibres( type I and IV)
*It acts as a smooth base for epithelium uniformity , helps in refraction.

STROMA ( substantia propia) :-

*It consists of collagen fibrils .
*Cell embedded in hydrated matrix of proteoglycans.
*It is about 0.5 mm in thickness (90% of total thicknesses of cornea).

DESCEMET'S MEMBRANE :-

*It represents the Basement membrane of the corneal epithelium.
*It composed by Collagen and glycoproteins ( no elastic fibres)
*It's thickness varies with age , 3 µm at birth and 10-12 µm in young adult.
*It is very resistant to Chemical agent , Trauma , InfevInfe, and Pathological processes.
*It is a strong Homogeneous layer.
*It is separated from the Stroma by pre- descemet's membrane.

Endothelium :-

*Endothelial cells are attached to the descemet's membrane.
*It contains an active pump mechanism.
*Involved in active secretion and protein synthesis.
*It consists of a single layer of Flat polygonal cells .

BLOOD SUPPLY OF THE CORNEA -

*The cornea is an avascular structure.
*Small loops derived from the Anterior ciliary vessels invade it's periphery for about 1 mm and provide nourishment.( loops are not in the cornea but in the subconjunctival tissue which overlap the Cornea.)

NERVE SUPPLY OF THE CORNEA -

*The cornea has rich supply of sensory nerve endings derived mainly from the long ciliary nerves which are branches of the nasociliary nerve.