|anterior ciliary arteries, long posterior ciliary arteries, short posterior ciliary arteries|
The sclera (from the Greek skleros, meaning hard), also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In humans the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show. 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.
The human eye is relatively rare for having an iris that is small enough for its position to be plainly visible against the sclera. This makes it easier for one individual to infer where another individual is looking, and it may have evolved as a method of nonverbal communication.
The sclera forms the posterior five-sixths of the connective tissue coat of the globe. It is continuous with the dura mater and the cornea, and maintains the shape of the globe, 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 1mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. 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 scleral 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, various reptiles, and birds. The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals.
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 five layers. The middle, 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.
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 our social nature as the eye became a useful communication tool in addition to a sensory organ. It is believed that the conspicuous sclera of the human eye makes it easier for one individual to infer where another individual is looking, increasing the efficacy of this particular form of nonverbal communication. 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.
Schematic diagram of the human eye.
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|Wikimedia Commons has media related to Sclera.|
- 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