The cornea is the transparent front part of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 that covers the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

, pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

, and anterior chamber. Together with the lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

, the cornea refracts
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 light, with the cornea accounting for approximately two-thirds of the eye's total optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

. In humans, the refractive power of the cornea is approximately 43 dioptre
A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...

s. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature
In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...

 of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance. Medical terms related to the cornea often start with the prefix "kerat-" from the Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 word κέρας, horn.


The cornea has unmyelinated nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

 endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex
Reflex action
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 to close the eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

. Because transparency is of prime importance the cornea does not have blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s; it receives nutrients via diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 from the tear fluid at the outside and the aqueous humour
Aqueous humour
The aqueous humour is a clear, gelatinous fluid similar to plasma, but containing low-protein concentrations. It is secreted from the ciliary epithelium, a structure supporting the lens. It is located in the space between the lens and the cornea...

 at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In human
Humans are the only living species in the Homo genus...

s, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege
Immune privilege
Immune privilege is a term used to describe certain sites in the body which are able to tolerate the introduction of antigen without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigen by the body and attacked by the immune system...

 makes the cornea a very special tissue. The cornea has no blood supply; it gets oxygen directly through the air. Oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy.

It borders with the sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

 by the corneal limbus
Corneal limbus
The corneal limbus is the border of the cornea and the sclera . The limbus is a common site for the occurrence of corneal epithelial neoplasm....


The most abundant soluble protein in mammalian cornea is albumin
Albumin refers generally to any protein that is water soluble, which is moderately soluble in concentrated salt solutions, and experiences heat denaturation. They are commonly found in blood plasma, and are unique to other blood proteins in that they are not glycosylated...


In lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, the cornea is solely an extension of the sclera, and is separate from the skin lying above it, but in more advanced vertebrates it is always fused with the skin to form a single structure, albeit one composed of multiple layers. In fish, and aquatic vertebrates in general, the cornea plays no role in focusing light, since it has virtually the same refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 as water.


The human cornea, like those of other primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, has five layers; the corneas of cats, dogs, wolves, and other carnivores only have four. From the anterior to posterior the five layers of the human cornea are:
  1. Corneal epithelium
    Corneal epithelium
    The corneal epithelium is made up of epithelial tissue and covers the front of the cornea. It acts as a barrier to protect the cornea, resisting the free flow of fluids from the tears, and prevents bacteria from entering the epithelium and corneal stroma.The corneal epithelium consists of several...

    : a thin epithelial
    Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

      multicellular tissue layer (non-keratinized stratified squamous epithelium) of fast-growing and easily regenerated cells, kept moist with tears
    Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of 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. It is continuous with the conjunctival epithelium, and is composed of about 6 layers of cells which are shed constantly on the exposed layer and are regenerated by multiplication in the basal layer.
  2. Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of a similar irregularly arranged collagen
    Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

     fibers, mainly type I collagen fibrils, essentially a type of stroma. These fibrils interact with and attach on to each other. This layer is eight to 14 micrometres thick and is absent or very thin in non-primates.
  3. Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly arranged collagen fibers along with sparsely distributed interconnected keratocytes
    Corneal keratocyte
    Corneal keratocytes are specialized fibroblasts residing in the stroma. This corneal layer, representing about 85-90% of corneal thickness, is built up from highly regular collagenous lamellae and extracellular matrix components. Keratocytes play the major role in keeping it transparent, healing...

    , which are the cells for general repair and maintenance. They are parallel and are superimposed like book pages The corneal stroma consists of approximately 200 layers of mainly type I collagen fibrils. Each layer is 1.5-2.5μm. Up to 90% of the corneal thickness is composed of stroma. There are 2 theories of how transparency in the cornea comes about:
    1. 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, 1957)
    2. The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)
  4. Descemet's membrane
    Descemet's membrane
    Descemet's membrane is the basement membrane that lies between the corneal proper substance, also called stroma, and the endothelial layer of the cornea. It is composed of a different kind of collagen than the stroma. The endothelial layer is located at the posterior of the cornea...

    (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived. This layer is composed mainly of collagen type IV fibrils, less rigid than collagen type I fibrils, and is around 5-20μm thick, depending on the subject's age.
  5. Corneal endothelium
    Corneal endothelium
    The corneal endothelium is a single layer of cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris....

    : a simple squamous or low cuboid
    In geometry, a cuboid is a solid figure bounded by six faces, forming a convex polyhedron. There are two competing definitions of a cuboid in mathematical literature...

    al monolayer, approx 5μm thick, of mitochondria-rich cells. These cells are responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. (The term endothelium is a misnomer
    A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...

     here. The corneal endothelium is bathed by aqueous humor, not by blood
    Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

     or lymph
    Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...

    , and has a very different origin, function, and appearance from vascular endothelia
    The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...

    .) Unlike the corneal epithelium the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells which reduces the overall cell density of the endothelium and has an impact on fluid regulation. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur and this may cause corneal edema and interference with the transparency of the cornea and thus impairing the image formed.

The mnemonic "EBSDEin", read as "Ebstein" can be used to remember the layers in sequence.

Another Mnemonic is "ABCDE": Anterior epithelium, Bowman's layer, Corneal stroma, Descemet's membrane and Endothelium

Keeping the cornea transparent

Upon death or removal of an eye the cornea absorbs the aqueous humor, thickens, and becomes hazy. Transparency can be restored by putting it in a warm, well-ventilated chamber at 31 °C (88 °F, the normal temperature), allowing the fluid to leave the cornea and become transparent. The cornea takes in fluid from the aqueous humor and the small blood vessels of the limbus, but a pump ejects the fluid immediately upon entry. When energy is deficient the pump may fail, or works too slowly to compensate, causing swelling. This could arise at death, but a dead eye can be placed in a warm chamber and the reservoirs of sugar and glycogen can keep the cornea transparent for at least 24 hours. The endothelium controls this pumping action, and as discussed above, damage thereof is more serious, and is a cause of opaqueness and swelling. When damage to the cornea occurs, such as in a viral infection, the collagen used to repair the process is not regularly arranged, leading to an opaque patch (leukoma). When a cornea is needed for transplant, as from an eye bank, the best procedure is to remove the cornea from the eyeball, preventing the cornea from absorbing the aqueous humor.


The cornea is one of the most sensitive tissues of the body, as it is densely innervated with sensory nerve fibres via the ophthalmic division
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 of the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 by way of 70–80 long ciliary nerves
Long ciliary nerves
The long ciliary nerves, two or three in number, are given off from the nasociliary, as it crosses the optic nerve.They accompany the short ciliary nerves from the ciliary ganglion, pierce the posterior part of the sclera, and running forward between it and the choroid, are distributed to the iris...

 and short ciliary nerves
Short ciliary nerves
The branches of the ciliary ganglion are the short ciliary nerves.These are delicate filaments, from six to ten in number, which arise from the forepart of the ganglion in two bundles connected with its superior and inferior angles; the lower bundle is the larger.They run forward with the ciliary...

. The ciliary nerves run under the endothelium and exit the eye through holes in the sclera apart from the optic nerve (which transmits only optic signals).

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 of the subepithelial layer terminate near superficial epithelial layer of the cornea in a logarithmic spiral
Logarithmic spiral
A logarithmic spiral, equiangular spiral or growth spiral is a special kind of spiral curve which often appears in nature. The logarithmic spiral was first described by Descartes and later extensively investigated by Jacob Bernoulli, who called it Spira mirabilis, "the marvelous...

 pattern. The density of epithelial nerves decreases with age, especially after the seventh decade.

Refractive nature

The optical component is concerned with producing a reduced inverted image on the retina. The eye's optical system consists of not only two but four surfaces—two on the cornea, two on the lens. Rays are refracted toward the midline. Distant rays, due to their parallel nature, converge to a point on the retina. The cornea admits light at the greatest angle. The aqueous and vitreous humors both have a refractive index of 1.336, whereas the cornea has a refractive index of 1.376. Because the change in refractive index between cornea and aqueous humor is relatively small compared to the change at the air–cornea interface, it has a negligible refractive effect, typically -6 diopters.

Treatment and management

Surgical procedures

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
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...


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 those composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration.

Non-surgical procedures

Orthokeratology , marketed under brand names like "", "MiracLens", "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea...

 is a method using specialized hard or rigid gas-permeable contact lens
Contact lens
A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

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.

In 2009, researchers at the University of Pittsburgh Medical center demonstrated that stem cells collected from human corneas can restore transparency without provoking a rejection response in mice with corneal damage.

General references

External links

- "Sagittal Section Through the Eyeball"
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