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The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber.
In the development of the embryo, the sclera is derived from the neural crest. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow. People with dark skin can have naturally darkened sclerae, the result of melanin pigmentation.
In humans, and some other vertebrates, the whole sclera is white or pale, contrasting with the coloured iris. The cooperative eye hypothesis suggests that the pale sclera evolved as a method of nonverbal communication that makes it easier for one individual to identify where another individual is looking. Other mammals with white or pale sclera include chimpanzees, many orangutans, some gorillas, and bonobos.




Structure



The sclera forms the posterior five-sixths of the connective tissue coat of the human eyeball. It is continuous with the dura mater and the cornea, and maintains the shape of the eyeball, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions. The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve. At the optic disc, the outer two-thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve. The inner third joins with some choroidal tissue to form a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi) pass. The thickness of the sclera varies from 1 mm at the posterior pole to 0.3 mm just behind the insertions of the four rectus muscles. The sclera's blood vessels are mainly on the surface. Along with the vessels of the conjunctiva (which is a thin layer covering the sclera), those in the episclera render the inflamed eye bright red.
In many vertebrates, the sclera is reinforced with plates of cartilage or bone, together forming a circular structure called the sclerotic ring. In primitive fish, this ring consists of four plates, but the number is lower in many living ray-finned fishes, and much higher in lobe-finned fishes, crocodilians, various reptiles, and birds. The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals.
The eyes of all non-human primates have been thought to be dark with small, barely visible sclera, but recent research has suggested that white sclera are not uncommon in chimpanzees, and are also present in other mammals.




= Histology

=
Histologically speaking, it is characterized as dense connective tissue made primarily of type 1 collagen fibers. The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are:

episclera
stroma
lamina fusca
endothelium
The sclera is opaque due to the irregularity of the Type I collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils.
The cornea, unlike the sclera, has six layers. The middle (third in number; provided the first layer being the anterior and outermost and the sixth layer being the posterior and the inner most), thickest layer is also called the stroma. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells.
Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.




Function


Human eyes are somewhat distinctive in the animal kingdom in that the sclera is very plainly visible whenever the eye is open. This is not just due to the white color of the human sclera, which many other species share, but also to the fact that the human iris is relatively small and comprises a significantly smaller portion of the exposed eye surface compared to other animals. It is theorized that this adaptation evolved because of humans’ social nature as the eye became a useful communication tool in addition to a sensory organ. It is believed that the exposed sclera of the human eye makes it easier for one individual to identify where another individual is looking, increasing the efficacy of this particular form of nonverbal communication, called cooperative eye hypothesis. Animal researchers have also found that, in the course of their domestication, dogs have also developed the ability to pick up visual cues from the eyes of humans. Dogs do not seem to use this form of communication with one another and only look for visual information from the eyes of humans.
Another hypothesis states that the distinctiveness of the human's sclera is due to genetic drift and sexual selection. A visibly white sclera is perceived as a sign of good health and youthfullness, this could have been a criterion of selection when looking for a mate. In this case, humans' ability to communicate with their eyes (glancing, cluing at others) would only be a consequence of a very visible sclera.




= Trauma

=
The bony area that makes up the human eye socket provides exceptional protection to the sclera. However, if the sclera is ruptured by a blunt force or is penetrated by a sharp object, the recovery of full former vision is usually rare. If pressure is applied slowly, the eye is actually very elastic. However, most ruptures involve objects moving at some velocity. The cushion of orbital fat protects the sclera from head-on blunt forces, but damage from oblique forces striking the eye from the side is not prevented by this cushion. Hemorrhaging and a dramatic drop in intraocular pressure are common, along with a reduction in visual perception to only broad hand movements and the presence or absence of light. However, a low-velocity injury which does not puncture and penetrate the sclera requires only superficial treatment and the removal of the object. Sufficiently small objects which become embedded and which are subsequently left untreated may eventually become surrounded by a benign cyst, causing no other damage or discomfort.




= Thermal trauma

=
The sclera is rarely damaged by brief exposure to heat: the eyelids provide exceptional protection, and the fact that the sclera is covered in layers of moist tissue means that these tissues are able to cause much of the offending heat to become dissipated as steam before the sclera itself is damaged. Even relatively low-temperature molten metals when splashed against an open eye have been shown to cause very little damage to the sclera, even while creating detailed casts of the surrounding eyelashes. Prolonged exposure, however—on the order of 30 seconds—at temperatures above 45 °C (113 °F) will begin to cause scarring, and above 55 °C (131 °F) will cause extreme changes in the sclera and surrounding tissue. Such long exposures even in industrial settings are virtually nonexistent.




= Chemical injury

=
The sclera is highly resistant to injury from brief exposure to toxic chemicals. The reflexive production of tears at the onset of chemical exposure tends to quickly wash away such irritants, preventing further harm. Acids with a pH below 2.5 are the source of greatest acidic burn risk, with sulfuric acid, the kind present in car batteries and therefore commonly available, being among the most dangerous in this regard. However, acid burns, even severe ones, seldom result in loss of the eye.
Alkali burns, on the other hand, such as those resulting from exposure to ammonium hydroxide or ammonium chloride or other chemicals with a pH above 11.5, will cause cellular tissue in the sclera to saponify and should be considered medical emergencies requiring immediate treatment.




Abnormal coloring



Redness of the sclera is typically caused by eye irritation causing blood vessels to expand, such as in conjunctivitis ("pink eye"). Episcleritis is a generally benign condition of the episclera causing eye redness. Scleritis is a serious inflammatory disease of the sclera causing redness of the sclera often progressing to purple.
Yellowing or a light green color of the sclera is a visual symptom of jaundice.
In cases of osteogenesis imperfecta, the sclera may appear to have a blue tint, more pronounced than the slight blue tint seen in children. The blue tint is caused by the showing of the underlying uveal tract (choroid and retinal pigment epithelium).
In those with Ehlers-Danlos syndrome, the sclera may be tinted blue due to the lack of proper connective tissue.
In very rare but severe cases of kidney failure and liver failure, the sclera may turn black.




See also


Extraocular implant
Extraocular muscles
Rectus muscles:
Inferior rectus muscle
Lateral rectus muscle
Medial rectus muscle
Superior rectus muscle
Scleral tattooing
Sclerotomy




Notes






References






External links



Histology image: 08008loa – Histology Learning System at Boston University
Atlas image: eye_1 at the University of Michigan Health System—"Sagittal Section Through the Eyeball"
MedlinePlus Encyclopedia: 002295

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• Sclera - Wikipedia
• Sclera (White of the Eye): Definition, Anatomy & Function
• Sclera of the Eye: Anatomy, Function & Problems - Vision Center
• Scleritis: What It Is, Causes, Symptoms & Treatment - Cleveland Clinic
• Scleritis: Symptoms, Causes, & Treatment - WebMD
• Sclera | White of the Eye - Definition and Detailed Illustration
• The Function of the Sclera in a Human Eye - Verywell Health
• Sclera: MedlinePlus Medical Encyclopedia
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• Sclera: Anatomy, Function, and Related Eye Problems - Oscar …

Sclera - Wikipedia

The sclera, [note 1] also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber.

Sclera (White of the Eye): Definition, Anatomy & Function

The sclera, or white of the eye, is strong tissue that wraps around your eyeball. It helps maintain your eye’s shape and protects it from injury. Many irritants or health conditions can make the entire sclera change color or cause spots of color.

Sclera of the Eye: Anatomy, Function & Problems - Vision Center

Oct 10, 2024 · The sclera is commonly known as the white of the eye. It’s the opaque tissue that surrounds your entire eyeball, except for the clear cornea (the covering in the front of your eye). The sclera is strong, fibrous tissue made mostly of collagen and elastic fibers.

Scleritis: What It Is, Causes, Symptoms & Treatment - Cleveland Clinic

May 10, 2023 · Scleritis often involves piercing pain in your eye that gets worse with eye movement. It can cause permanent damage and vision loss. Treatment options include nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroids. It may be caused by an underlying inflammatory disease, fungus or injury. Scleritis should be treated.

Scleritis: Symptoms, Causes, & Treatment - WebMD

Dec 30, 2024 · The white part of your eye (called the sclera) is a layer of tissue that protects the rest of your eye. When this area is inflamed and hurts, doctors call that condition scleritis.

Sclera | White of the Eye - Definition and Detailed Illustration

Feb 26, 2019 · The sclera is the dense connective tissue of the eyeball that forms the "white" of the eye. It is continuous with the stroma layer of the cornea. The junction between the white sclera and the clear cornea is called the limbus. The sclera ranges in thickness from about 0.3 millimeter (mm) to 1.0 mm.

The Function of the Sclera in a Human Eye - Verywell Health

Jun 9, 2023 · The sclera is tough and fibrous, protecting the interior components of the eye from injury, and makes up the exterior coating of the eye. 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.

Sclera: MedlinePlus Medical Encyclopedia

The sclera is the white outer coating of the eye. It is tough, fibrous tissue that extends from the cornea (the clear front section of the eye) to the optic nerve at the back of the eye. The sclera gives the eyeball its white color. The cornea and sclera are made of …

Sclera - Structure, Location, Function, Diagram, Anatomy

Jan 22, 2025 · The sclera is the white, opaque outer layer of the eye that provides structural support and protection. Composed mainly of collagen and elastic fibers, the sclera is tough and durable, forming the majority of the outer coating of the eye.

Sclera: Anatomy, Function, and Related Eye Problems - Oscar …

Mar 8, 2024 · The sclera is the white part of the eye and its main function is to maintain the shape of the eye. The sclera is a fibrous tissue that extends from the cornea to the optic nerve. Sclera-related eye problems include jaundice, scleritis, blue sclera and ocular melanosis.