The sclera forms the entire visible white exterior of the eye, while the iris is the colored portion inside the anterior chamber of the eye. Although we can only see the visible portion of the sclera, it actually surrounds the entire eye and provides structure for the internal contents of the eye, which are mostly made up of a thick liquid called the vitreous humor.
The sclera is composed of four layers. The inside layer is called the endothelium, followed by the stroma, the lamina fusca, and the episclera on the outside.
The sclera begins to turn yellow in individuals experiencing liver failure. This condition is called jaundice and indicates that the liver is no longer filtering the blood well. In rare cases, the sclera can turn blue. This can be caused by long-term use of some medications, a rare condition called osteogenesis imperfecta brittle bone disease , ingesting silver, and conditions that cause the sclera to be thin, which allows the blue color of veins to show through.
If you notice a color change in the sclera, you should seek advice from a healthcare professional. Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. About Jaundice. Cleveland Clinic. Ocular melanosis is the pigmentation of the conjunctiva overlying the sclera.
They typically look like flat, brown spots on the white part of the eye, similar to a freckle on the skin. Melanosis can be congenital or acquired. The primary form is more common in light-skinned individuals and may grow in size. In rare cases, primary acquired melanosis may develop into malignant cancerous melanoma.
The secondary form is associated with hormonal changes, radiation exposure, chemical exposure, or metabolic disorders.
People with darker skin and hair are more likely to have secondary acquired melanosis. This form rarely develops into melanomas. Nevus of Ota is a congenital condition.
It causes hyperpigmentation of the eye, eyelids, and other surrounding areas. The pigment often appears blue or brown. Typically, the nevus does not require treatment.
Some people desire laser therapy to lighten the spots for cosmetic reasons. Nevus of Ota is associated with a higher risk of glaucoma and, in rare cases, melanoma of the eye. Ectasia is a congenital thinning and bulging of the sclera. The thinning usually occurs near the limbus and also affects the cornea. The staphyloma often presents with a blue appearance.
Staphylomas can also develop on the back of the eye, particularly in people with high myopia nearsightedness. Colobomas occur when there is some tissue missing in the eye at birth.
This results in the incomplete formation of a particular structure of the eye. Colobomas can affect the eyelid, iris, lens, optic nerve, choroid, and retina. Optic nerve, choroidal, or retinal colobomas can cause scleral thinning and staphylomas.
Scleral expansion is a somewhat controversial procedure designed to treat presbyopia. Presbyopia is the loss of the ability to focus up close with age. The surgery involves inserting four plastic pieces into the sclera, between the eye muscles. This technology is still improving, and some researchers feel that this procedure can become the gold standard in presbyopia treatment. A scleral buckle procedure is performed to repair a retinal detachment.
During the surgery, the doctor uses cryopexy freezing therapy to seal your retina to the eye. Then, the surgeon places a silicone or plastic band around your eyeball this is the scleral buckle. Lamina Fusca: Lamina fusca is the innermost layer of sclera.
It is characterised by abundance of pigmented cells or melanocytes, mostly migrated from choroid. The connective tissue of this layer is loosely arranged than rest of the sclera.
Lamina fusca is separated from choroid by a thin potential space known as suprachoroidal or perichoroidal space. Sclera is penetrated by various arteries and nerves in many places.
We will call such penetrations as aperture and can be discussed as :. Posterior scleral apertures: Posteriorly sclera is pierced by many structures and can be sub divided into Posterior scleral foramen: Sclera is perforated by optic nerve posteriorly,3 mm medial and 1 mm superior to the posterior pole.
At this point of optic nerve exit sclera blends with dural and arachnoid coverings of optic nerve and becomes a sieve like membrane internally Lamina cribrosa is described below. Sclera is also penetrated by central retinal artery and veins. Other posterior apertures : sclera is pierced by 8 to 20 short posterior ciliary arteries in a ring circle of Zinn. They are often accompanied by short ciliary nerves.
Little anterior to these, long posterior ciliary arteries two in numbers and nerves also enter the eyeball by piercing sclera. Read amazing facts. It is a circular anastomosis between short ciliary arteries while piercing sclera and supplies choroid, optic nerves and pia maters.
Middle scleral aperture: These apertures are situated approximately 4 mm behind or posterior to the equator and represent the exit of the four occasionally 5 vortex veins. Vortex veins drain the veins of choroid, ciliary body, and iris. Anterior scleral apertures: these are located near the insertions of the recti muscles and allow entry of anterior ciliary arteries.
Each rectus muscle has two anterior ciliary arteries with exception of lateral rectus muscle which has only one anterior ciliary artery. Scleral sulcus and Scleral spur : Discussed in anatomy of angle of anterior chamber. Sclerocorneal junction or limbus : Discussed in anatomy of cornea Canal of Schlemm: Discussed in anatomy of anterior chamber Lamina cribrosa: Sclera is thinned with a sieve like appearance where the optic nerve fibres pierce and this region of sclera is called lamina cribrosa.
However through one of these sieves, which is comparatively larger than the others, the central retinal artery and vein enter the eye.
As sclera is very weak at lamina cribrosa, it bulges outward in response to longstanding high intraocular pressure producing optic disc cupping.
Blood supply: The episclera recieves its blood supply from the anterior ciliary arteries, anterior to the insertions of the rectus muscles and the long and short posterior ciliary arteries. Scleral stroma is relatively avascular and receives its nutrition mainly from episcleral vascular bed and, to some extent from the underlying choroidal vasculatures.
The sclera contains numerous channels or passages through which the arteries,veins and nerves pass. These channels or passages are known as emissary canals. Lies deep to the Tenon's capsule and over sclera. Vessels are arranged in criss cross pattern.
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