View source for Sclera

{{Short description|White part of an eyeball}}
{{Multipl
| Latin       = sclera
| Image       = Sclera.PNG
| sclera, as separated from the [[cornea]] by the [[limbus]].
|=
| Caption2    =
| Width       = 300

| Precursor   =
| System      = [[Visual system]]
| part_of     = [[Eye]]
| Artery      = [[Anterior ciliary arteries]], [[long posterior ciliary arteries]], [[short posterior ciliary arteries]]
| Vein        =
| Nerve       =
| Lymph       =
}}
The '''sclera''',{{notetag|1=The word ''sclera'' ({{IPAc-en|ˈ|s|k|l|ɛər|ə}} or {{IPAc-en|ˈ|s|k|l|ɪər|ə}}; both are common), plural ''sclerae'' ({{IPAc-en|ˈ|s|k|l|ɛər|i}} or {{IPAc-en|ˈ|s|k|l|ɪər|i}}) or ''scleras'', is from the Greek {{lang|grc|σκληρός}} (''sklērós''), meaning hard.<ref>{{cite book |title=Mosby's Medical, Nursing, and Allied Health Dictionary |date=1994 |publisher=Mosby |location=St. Louis |isbn=978-0-8151-6113-4 |page=1402 |edition=4th}}</ref>}} 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]].<ref>{{cite book |last1=Cassin |first1=Barbara |last2=Solomon |first2=Sheila A.B. |title=Dictionary of Eye Terminology |date=1990 |publisher=Triad Pub. Co |location=Gainesville, Fla. |isbn=978-0-937404-33-1 |edition=2nd}}</ref> 

In the development of the [[embryo]], the sclera is derived from the [[neural crest]].<ref>Hermann D. Schubert. ''Anatomy of the Orbit'' {{cite web |url=http://www.nyee.edu/pdf/schubert.pdf |title=New York Eye and Ear Infirmary of Mount Sinai - New York City - NYEE |access-date=2008-05-19 |archive-url=https://web.archive.org/web/20081008043132/http://www.nyee.edu/pdf/schubert.pdf |archive-date=2008-10-08 }}</ref> 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.<ref>{{cite web |last1=Mukamal |first1=Reena |title=Why Are the Whites of My Eyes Discolored? |url=https://www.aao.org/eye-health/tips-prevention/discolored-sclera-whites-of-my-eyes-turn-yellow |website=American Academy of Ophthalmology |access-date=11 December 2020 |language=en |date=30 July 2020}}</ref>

In [[human]]s, and some other [[vertebrates]], the whole sclera is white or pale, contrasting with the coloured [[iris (anatomy)|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 [[chimpanzee]]s, many [[orangutan]]s, some [[gorilla]]s, and [[bonobo]]s.<ref>{{Cite journal |last1=Clark |first1=Isabelle R. |last2=Lee |first2=Kevin C. |last3=Poux |first3=Tucker |last4=Langergraber |first4=Kevin E. |last5=Mitani |first5=John C. |last6=Watts |first6=David |last7=Reed |first7=James |last8=Sandel |first8=Aaron A. |date=2023-03-01 |title=White sclera is present in chimpanzees and other mammals |journal=Journal of Human Evolution |language=en |volume=176 |article-number=103322 |doi=10.1016/j.jhevol.2022.103322 |s2cid=256314941 |issn=0047-2484|doi-access=free |pmid=36706647 |pmc=9998187 |bibcode=2023JHumE.17603322C }}</ref>

==Structure==
{{multiple image
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 | image1 = Closeup of goat eye.jpg
 | caption1 = [[Goat]] eye with relatively dark sclera and horizontal pupil
 | image2 = A cow's eye.jpg
 | caption2 = [[Cow]] eye with dark sclera
 | image3 = Western Lowland Gorilla2010501A (cropped).jpg
 | caption3 = Rare whites in a [[western lowland gorilla]]
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The sclera forms the posterior five-sixths of the [[connective tissue]] coat of the [[globe (human eye)|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 [[choroid]]al tissue to form a plate ([[Lamina cribrosa sclerae|lamina cribrosa]]) across the optic nerve with perforations through which the optic fibers ([[Nerve fascicle|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 [[Extraocular muscles#Recti muscles|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.<ref name=eb>"eye, human."Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] 2009</ref>

In many [[vertebrate]]s, 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 fish]]es, and much higher in [[lobe-finned fish]]es, [[Crocodilia|crocodilians]], various [[reptile]]s, and [[bird]]s. The ring has disappeared in many groups, including living [[amphibian]]s, some reptiles and fish, and all [[mammal]]s.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|page= 461|isbn= 978-0-03-910284-5}}</ref>

The eyes of all non-human [[primate]]s 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.<ref name="JHE">{{cite journal|last=Clark|first=Isabella R.|author2=Lee, Kevin C. |author3=Poux, Tucker |author4=Langergraber, Kevin E. |author5=Mitani, John C.|author6=Watts, David|author7=Reed, James|author8=Sandel, Aaron A.|title= White sclera is present in chimpanzees and other mammals|journal=Journal of Human Evolution|date=March 2023|volume=176|doi=10.1016/j.jhevol.2022.103322|pmid=36706647 |pmc=9998187 |bibcode=2023JHumE.17603322C }}</ref>

===Histology===
Histologically speaking, it is characterized as [[dense connective tissue]] made primarily of [[Type I collagen|type 1 collagen fibers]].<ref>{{Citation |last1=Pradeep |first1=Tejus |title=Histology, Eye |date=2023 |url=https://www.ncbi.nlm.nih.gov/books/NBK544343/ |work=StatPearls |access-date=2023-11-11 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31335063 |last2=Mehra |first2=Divy |last3=Le |first3=Patrick H.}}</ref> The [[collagen]] of the sclera is continuous with the [[cornea]]. From outer to innermost, the four layers of the sclera are:
* [[episclera]]
* [[stroma of cornea|stroma]]
* [[Suprachoroid lamina|lamina fusca]]
* [[Corneal endothelium|endothelium]]

The sclera is opaque due to the irregularity of the Type I<ref>{{cite journal|last=Keeley|first=FW|author2=Morin, JD |author3=Vesely, S |title=Characterization of collagen from normal human sclera.|journal=Experimental Eye Research|date=November 1984|volume=39|issue=5|pages=533–42|pmid=6519194|doi=10.1016/0014-4835(84)90053-8}}</ref>  collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more [[Glycosaminoglycan|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.<ref name="eb"/>

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 (anatomy)|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 animal|social]] nature as the eye became a useful [[animal communication|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]].<ref>Michael Tomasello, Brian Hare, Hagen Lehmann, Josep Call. "Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis" http://www.chrisknight.co.uk/wp-content/uploads/2008/06/eyes-cooperation.pdf</ref> 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.<ref name="secret life">{{cite episode|title=The Secret Life of the Dog|url=https://www.bbc.co.uk/programmes/b00pssgh|series-link=Horizon (BBC TV series)|series=Horizon|credits=Director and Producer: Dan Child, Executive Producer: Andrew Kohen|network=[[BBC]]|station=[[BBC2]]|airdate=2010-01-06}}</ref>

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,<ref>Russell, R., Sweda, J. R., Porcheron, A., & Mauger, E. (2014). Sclera color changes with age and is a cue for perceiving age, health, and beauty. ''Psychology and Aging, 29''(3), 626–635. https://doi.org/10.1037/a0036142</ref> 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.<ref>Caspar, K.R., Biggemann, M., Geissmann, T. ''et al.'' Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions. ''Sci Rep'' 11, 12994 (2021). https://doi.org/10.1038/s41598-021-92348-z</ref>
===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.<ref name="Watson2012">{{cite book|author=Peter G Watson|title=The Sclera and Systemic Disorders|chapter-url=https://books.google.com/books?id=sHlqhV7jUmQC&pg=PA293|date=11 April 2012|publisher=JP Medical Ltd|chapter=Chapter 9|isbn=978-1-907816-07-9|page=293}}</ref>

===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 {{convert|45|C|F}} will begin to cause scarring, and above {{convert|55|C|F}} will cause extreme changes in the sclera and surrounding tissue.  Such long exposures even in industrial settings are virtually nonexistent.<ref name="Watson2012"/>

===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.<ref name="Watson2012"/>

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 [[saponification|saponify]] and should be considered medical emergencies requiring immediate treatment.<ref name="Watson2012"/>

==Abnormal coloring==
{{Redirect|Red sclera|more information|Red eye (medicine)}}
{{Anchor|Red sclera|Pink sclera|Purple sclera|Yellow sclera|Green sclera|Blue sclera|Black sclera}}

{{More citations needed section|date=May 2024}}

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.<ref>Asanad, S., Bayomi, M., Brown, D., Buzzard, J., Lai, E., Ling, C., Miglani, T., Mohammed, T., Tsai, J., Uddin, O., & Singman, E. (2022). Ehlers-Danlos syndromes and their manifestations in the visual system. Frontiers in Medicine, 9. https://doi.org/10.3389/fmed.2022.996458</ref>

In very rare but severe cases of [[kidney failure]] and [[liver failure]], the sclera may turn black.{{cn|date=September 2021}}

==See also==

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

==Notes==
{{NoteFoot}}

==References==
{{Reflist}}

==External links==
{{Commons}}
* {{BUHistology|08008loa}}
* {{UMichAtlas|eye_1}}—"Sagittal Section Through the Eyeball"
* {{MedlinePlusEncyclopedia|002295}}

{{Eye}}
{{Visual_system}}
{{Muscles of orbit}}
{{Authority control}}

[[Category:Eye]]
[[Category:Human eye anatomy]]