Thesclera,[note 1] also known as thewhite of the eye or, in older literature, as thetunica albuginea oculi, is the opaque, fibrous, protective outer layer of theeye containing mainlycollagen and some crucialelastic fiber.[2]
In the development of theembryo, the sclera is derived from theneural crest.[3] 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 ofmelanin pigmentation.[4]
The sclera forms the posterior five-sixths of theconnective tissue coat of thehuman eyeball. It is continuous with thedura mater and thecornea, and maintains the shape of the eyeball, offering resistance to internal and external forces, and provides an attachment for theextraocular muscle insertions. The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by theoptic nerve. At theoptic 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 somechoroidal 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 theposterior pole to 0.3 mm just behind the insertions of thefour rectus muscles. The sclera's blood vessels are mainly on the surface. Along with the vessels of theconjunctiva (which is a thin layer covering the sclera), those in theepisclera render the inflamed eye bright red.[6]
The eyes of all non-humanprimates 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.[8]
The sclera is opaque due to the irregularity of the Type I[10] collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears moremucopolysaccharide (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.[6]
Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.
Human eyes are somewhat distinctive in theanimal 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 humaniris 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 usefulcommunication tool in addition to asensory 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 ofnonverbal communication, calledcooperative eye hypothesis.[11] 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.[12]
Another hypothesis states that the distinctiveness of the human's sclera is due togenetic drift andsexual selection. A visibly white sclera is perceived as a sign of good health and youthfullness,[13] 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.[14]
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 benigncyst, causing no other damage or discomfort.[15]
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.[15]
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, withsulfuric 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.[15]
Alkali burns, on the other hand, such as those resulting from exposure toammonium hydroxide orammonium chloride or other chemicals with a pH above 11.5, will cause cellular tissue in the sclera tosaponify and should be considered medical emergencies requiring immediate treatment.[15]
Redness of the sclera is typically caused byeye irritation causing blood vessels to expand, such as inconjunctivitis ("pink eye").Episcleritis is a generally benign condition of theepisclera 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 ofjaundice.
^Pradeep, Tejus; Mehra, Divy; Le, Patrick H. (2023),"Histology, Eye",StatPearls, Treasure Island (FL): StatPearls Publishing,PMID31335063, retrieved2023-11-11
^Keeley, FW; Morin, JD; Vesely, S (November 1984). "Characterization of collagen from normal human sclera".Experimental Eye Research.39 (5):533–42.doi:10.1016/0014-4835(84)90053-8.PMID6519194.
^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
^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
^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
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