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Schematic diagram of the human eye ensvg
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Schematic diagram of the human eye (Cornea labeled at center top)
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Vertical section of human cornea from near the margin (Waldeyer) Magnified <BR>1 Epithelium <BR>2 Anterior Elastic Lamina <BR>3 Substantia Propria <BR>4 Posterior Elastic Lamina <BR>5 Endothelium of the Anterior Chamber <BR>a Oblique fibers in the anterior layer of the Substantia Propria <BR>b Lamellæ the fibers of which are cut across, producing a dotted appearance <BR>c Corneal corpuscles appearing Fusiform in section <BR>d Lamellæ the fibers of which are cut longitudinally <BR>e Transition to the Sclera , with more distinct fibrillation, and surmounted by a thicker Epithelium <BR>f Small bloodvessels cut across near the margin of the cornea
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The is the transparent front part of the of the lens can be adjusted to "tune" the focus depending upon the object's distance, the curvature of the cornea is fixed.
The cornea has
Unmyelinated Nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary
Reflex to close the
Eyelid . Because transparency is of prime importance the cornea does not have
Blood Vessel s; it receives nutrients via
Diffusion from the tear fluid at the outside and the
Aqueous Humour at the inside and also from
Neurotrophins supplied by nerve fibres that innervate it. In
Human s, the cornea has a diameter of about 11.5
Mm and a thickness of 0.5 mm - 0.6 mm in the center and 0.6 mm - 0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue.
In humans, the
Refractive Power of the cornea is approximately 43
Dioptre s, roughly two-thirds of the eye's total refractive power.http://www.eyeweb.org/optics.htm
Medical terms related to the cornea often start with the prefix "''''".
The human cornea, like that of other
Primate s, has five layers. The corneas of cats, dogs, and other carnivores have only four.Merindano MD; Costa J; Canals M; Potau JM, and Ruano D.
"A comparative study of Bowman's layer in some mammals: Relationships with other constituent corneal structures." ''European Journal of Anatomy.'' Volume 6, Number 3, December 2002. From the anterior to posterior they are:
#''' multicellular layer of fast-growing and easily-regenerated
Cells , kept moist with
Tears . Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity.
#''' fibers. This layer is absent in carnivores.
#''' fibrils. There are 2 theories of how transparency in the cornea comes about:
## The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils.(Maurice)
## The spacing of the neighbouring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)
# (also '''posterior limiting membrane'''): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium.
#''' or low
Cuboid al monolayer of mitochondria-rich cells responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. (The term ''endothelium'' is a
Misnomer here. The corneal endothelium is bathed by
Aqueous Humour , not by
Blood or
Lymph , and has a very different origin, function, and appearance from
Vascular Endothelia .)
The cornea is one of the most sensitive tissues of the body, it is densely innervated with sensory nerve fibres via the
Ophthalmic Division of the
Trigeminal Nerve by way of 70 - 80
Long Ciliary Nerves and
Short Ciliary Nerves .
The nerves enter the cornea via three levels, ''scleral, episcleral and conjunctival''. Most of the bundles give rise by subdivision to a network in the stroma, from which fibres supply the different regions. The three networks are ''midstromal, subepithelial/Bowman's layer, and epithelium''. The receptive fields of each nerve ending are very large, and may overlap.
Corneal nerves have been observed to terminate in a
Logarithmic Spiral pattern.Yu CQ, Rosenblatt MI. Transgenic corneal neurofluorescence in mice: a new model for in vivo investigation of nerve structure and regeneration.
Invest Ophthalmol Vis Sci. 2007 Apr;48(4):1535-42.
See Also: List of eye diseases and disorders
Various
Refractive Eye Surgery techniques change the shape of the cornea in order to reduce the need for corrective lenses or otherwise improve the refractive state of the eye. In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer
Laser .
If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be
Transplanted . Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea.
There are also synthetic corneas (keratoprostheses) in development. Most are merely plastic inserts, but there are also composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration.
Orthokeratology is a method using specialized hard or rigid gas-permeable
Contact Lens es to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.
- Daxer A, Misof K, Grabner B, Ettl A, Fratzl P. "Collagen fibrils in the human corneal stroma: structure and aging." Invest ''Ophthalmol Vis Sci.'' 1998 Mar;39(3):644-8. PMID 9501878.
- Daxer A, Fratzl P. "Collagen fibril orientation in the human corneal stroma and its implication in keratoconus." ''Invest Ophthalmol Vis Sci.'' 1997 Jan;38(1):121-9. PMID 9008637.
- Fratzl P, Daxer A. "Structural transformation of collagen fibrils in corneal stroma during drying. An x-ray scattering study." ''Biophys J.'' 1993 Apr;64(4):1210-4. PMID 8494978.
