1. epithelium
2. Bowman’s (anterior limiting) membrane
3. stroma (substantia propria)
4. Descemet’s (posterior limiting) membrane
5. endothelium (posterior epithelium)

The transparency of the cornea is due to the fact that it contains hardly any cells and no blood vessels (although blood vessels can creep in from around it if it is constantly irritated or infected).  On the other hand, the cornea contains the highest concentration of nerve fibers of any body structure.  The nerve fibers enter on the margins of the cornea and radiate toward the center.  These fibers are associated with numerous pain receptors that have a very low threshold.  Cold receptors also are abundant in the cornea, although heat and touch receptors seem to be lacking.

sclera
Along its circumference, the cornea is continuous with the sclera: the white, opaque portion of the eye.  The sclera makes up the back five-sixths of the eye’s outer layer.  It provides protection and serves as an attachment for the extraocular muscles which move the eye.

tears and tear glands
Coating the outer surface of the cornea is a “pre-corneal tear film.”  People normally blink the eyelids of their eyes about every six seconds to replenish the tear film.  Tears have four main functions on the eye:

wetting the corneal epithelium, thereby preventing it from being damaged due to dryness,
creating a smooth optical surface on the front of the microscopically irregular corneal surface,
acting as the main supplier of oxygen and other nutrients to the cornea, and
containing an enzyme called “lysozyme” which destroys bacteria and prevents the growth of microcysts on the cornea.

The tear film resting on the corneal surface has three layers:

1. lipid or oil layer,
2. lacrimal or aqueous layer, and
3. mucoid or mucin layer

The most external layer of the tear film is the lipid or oil layer.  This layer prevents the lacrimal layer beneath it from evaporating, as well as preventing the tears from flowing over the edge of the lower eyelid (“epiphora”).  The lipid component of the tear film is produced by sebaceous glands known as “Meibomian” glands (located in the tarsal plates along the eyelid margins) and the glands of “Zeis” (which open into the hair follicles of the eyelashes).  An enlargement of a Meibonian gland is known as a “chalazion,” while an infection of a Zeis gland is known as a “hordeolum” or “sty(e).”

Below the lipid layer is located the lacrimal or aqueous layer of the tear film.  This middle layer is the thickest of the three tear layers, and it is formed primarily by the glands of “Krause” and “Wolfring” and secondarily by the “lacrimal” gland, all of which are located in the eyelids.  (The lacrimal gland is the major producer of tears when one is crying or due to foreign body irritation.)  The lacrimal fluid, containing salts, proteins, and lysozyme, has several functions:

taking the main nutrients (such as oxygen) to the cornea,
carrying waste products away from the cornea,
helping to prevent corneal infection, and
maintaining the tonicity of the tear film.

If the eye’s tears are “isotonic,” there will be no change in water volume in the cornea and vision will remain normal.  (Tears normally have a tonicity equal to .9% saline.)  If the tears are “hypotonic,” water will flow into the cornea (such as when crying or swimming in a pool) and it will swell, causing it to become more myopic.  If the tears are “hypertonic,” water will flow out of the cornea (such as when swimming in the ocean) and it will shrink, causing it to become more hyperopic.

The epithelial surface of the cornea is naturally “hydrophobic” (water-repelling).  Therefore, for a tear layer to be able to remain on the corneal surface without rolling off, the “hydrophilic” (water-attracting) mucoid or mucin layer of the tear film is laid down onto the surface of the cornea by “goblet cells,” which are present in the bulbar conjunctiva.  In turn, the lacrimal layer of the tear film, located above the mucoid layer, can defy gravity and remain on the front of the eye.

Dry Eye :

A deficiency of any of the three layers of the tear film can lead to a “dry eye” condition, causing anything from mild eye irritation to severe pain.  Interestingly, in some cases, excessive tearing or watering of the eyes can be a symptom of a dry eye condition.  This is because when, for whatever reason, there is an inadequate normal tear layer on the eye, irritation results; the latter causes an overproduction of the lacrimal gland and a flooding of lacrimal fluid into the eye (“reflex tearing”).

Besides excessive tearing, symptoms associated with dry eyes can include the following:

eye irritation, scratchiness, grittiness, or pain,
redness of the eye(s),
a burning sensation in the eye(s),
a feeling of something in the eye(s),
eyes that feel “glued shut” after sleeping,
blurred vision, and
eye discomfort with contact lens wear.

There can be multiple causes of a dry eye condition, and these are some of the possibilities:

lid or blinking problems (for instance, an injury or stroke affecting one of the nerves which helps us blink),
reading or working at a computer screen for long periods of time,
medications like antihistamines, oral contraceptives, beta blockers, diuretics, tranquilizers, pain relievers, or antidepressants,
a dry climate (including heating and air conditioning in a home, airplane, or motel room), wind, UV radiation, tobacco smoke, and dust,
diseases such as rheumatoid arthritis, Sjogren’s syndrome, keratoconjunctivitis sicca, xerophthalmia, lupus erythematosus, Grave’s disease, diabetes, or scleroderma
hormonal changes accompanying menopause,
chemical, radiation, or thermal burns to the eye,
vitamin A deficiency,
aging, since the tear glands produce fewer tears as we age, and
idiopathic (unknown) causes.

A dry eye problem often can be relieved with the use of over-the-counter eyedrops which behave as “artificial tears” on the eyes.  These types of drops can soothe the eyes, moisturize dry spots, supplement tears, and protect eyes from further irritation.  Some drops are formulated to match the pH of human tears for added comfort.  Special ocular lubricant ointments, applied to the eyes for overnight use, also are available.

Artificial tears may be preserved or unpreserved.  Bottle contamination is less likely with preserved drops; however, an allergic reaction to the preservatives can occur.  If unpreserved eyedrops are used, care must be taken not to contaminate the bottle by touching the tip to any surface, including the eyeball.

Some eyedrops contain “vasoconstrictors” (chemicals such as tetrahydrozaline or naphazoline), which constrict the conjunctival blood vessels, thereby reducing the amount of redness on the surface of the eyes.  These drops may or may not contain a tear substitute component for red eyes, and overuse can cause eyes to become even more red (“rebound hyperemia”) due to a weakening of the muscles persistently constricting the blood vessels.

In certain cases, artificial tear drops do not relieve the discomfort due to dry eyes.  In such cases, if the discomfort is severe enough, other options are available.  The most common of these involves closing the tear ducts (drains).  Using either a silicone plug or scarring the tear duct closed by cauterization (with a “hot poker”) decreases or stops the passage of the tears into the tear ducts.  That way, any tears naturally produced or artificially placed into eyes will remain longer (until they evaporate).  It can be a very successful way to make irritated eyes with a chronic dry eye syndrome feel more comfortable.

Keratoconus :

“Keratoconus”—which is a combination of two Greek words: karato, meaning cornea, and konos, meaning cone—is a non-inflammatory condition in which there is progressive central thinning of the cornea, changing it from dome-shaped to cone-shaped.  A distorting, cone-like bulge develops, eventually resulting in significant visual impairment.

As keratoconus progresses, the corneal tissue continues to thin and to bulge forward and downward, becoming very irregular.  In advanced stages, there can be a precipitous drop in vision due to sudden clouding of the cornea, referred to as “acute hydrops,” due to a sudden infusion of fluid into the stretched cornea.  This usually resolves over weeks to months but often is followed by central corneal scarring, further impairing vision.

An uncommon disorder, keratoconus affects about 1 out of every 2000 people.  It almost always is bilateral (affecting both eyes) and typically takes years or decades to progress, usually beginning at puberty or later in the teens.  Its progression can halt at any stage, from mild to severe.  Although keratoconus does not cause complete blindness, it often causes a dramatic increase in myopia (nearsightedness) and irregular astigmatism, significantly blurring and distorting vision, as well causing significant photophobia (light sensitivity) and glare.

The cause of keratoconus most likely is genetic; about 7-8% of people with the condition also have other family members with it.  Severe rubbing of the eyes also may exacerbate the problem.  In its early stages, keratoconus can be diagnosed only by using detailed computer maps of the corneal surface, detecting subtle changes in corneal shape.

Initially, stronger eyeglasses are successful in correcting the progressive myopia and astigmatism; however, as the disease advances, rigid gas permeable (RGP) contact lenses are necessary to flatten the anterior corneal surfaces and obtain optimal visual acuity.  Contact lens fitting can be difficult in patients with keratoconus, requiring frequent doctor visits and lens changes.

Ultimately, if good vision no longer can be attained with contact lenses, or if intolerance to the contact lenses develops, corneal transplantation is recommended.  This procedure is necessary in only about 10% of people with keratoconus and is successful in greater than 90% of cases, one of the highest success rates for corneal transplantation.  Although this procedure replaces the thinned central portion of the cornea (with a section of donor cornea), contact lenses and/or eyeglasses often continue to be required for maximal visual acuity.

IRIS:

The iris, visible through the clear cornea as the colored disc inside the eye, is a thin diaphragm composed mostly of connective tissue and smooth muscle fibers.  It lies between the cornea and the crystalline lens.  The iris divides the anterior compartment, the space separating the cornea and the lens, into the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris and the lens).

The iris is composed of 3 layers, from the front to the back:

1. endothelium

2. stroma

3. epithelium

Eye Color :
The color of the iris, which is established genetically, is determined by the amount of pigment present in this eye structure.  No pigment at all (in the case of an albino) results in a pink iris.  Some pigment causes the iris to appear blue.  Increasing amounts of iris pigment produce green, hazel, and brown irises (or irides).  There actually are two pigments, melanin and lipochrome, which determine eye color.  Melanin (brown) deposition is controlled by a gene on chromosome 15.  Lipochrome (yellowish-brown) deposition is controlled by a gene on chromosome 19.

Rarely, one iris can be a different color than the other iris.  This is known as “heterochromia irides” and is determined genetically.  Also, a section of one iris may be a different color from the rest of that iris; this is known as “heterochromia iridum” or “sectoral heterochromia iridis.”  Usually, if these conditions are present, they are noticeable at birth, although they can be acquired due to various ocular pathologies.

Unlike what is commonly believed, the iris does not change colors in an adult (except in the case of certain pathologies, such as pigment dispersion syndrome), although it may appear to do so depending upon the color of clothing a person is wearing.  Moreover, the color(s), texture, and patterns of each person’s iris are as unique as a fingerprint.

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