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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. [4] In humans, and some other vertebrates, the whole sclera is white or pale, contrasting with the coloured iris.
Unlike other primates, all human beings have eyes with a distinct colour contrast between the white sclera, the coloured iris, and the black pupil. This is due to a lack of pigment in the sclera. Other primates mostly have pigmented sclerae that are brown or dark in colour. There is also a higher contrast between human skin, sclera, and irises.
Eye color outside of the iris may also be symptomatic of disease. Yellowing of the sclera (the "whites of the eyes") is associated with jaundice, [82] and may be symptomatic of liver diseases such as cirrhosis or hepatitis. [83] A blue coloration of the sclera may also be symptomatic of disease. [82]
The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera. [4] The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape. [5]
The cornea is transparent and more curved and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.45 in) in diameter, and 0.5 mm (500 μm) in thickness near its centre.
According to traditional Chinese and Japanese face reading, the eye is composed of two parts, the yin (black, iris and pupil) and the yang (white, sclera).The visibility of the sclera beneath the iris is said to represent physical imbalance in the body, and is claimed to be present in alcoholics, drug addicts, and people who over-consume sugar or grain.
But wearing white wasn't just a way to identify the wealthy from the working class. Society people (particularly women) made it a point to also use the color white to draw lines between "old money ...
The extracellular mesenchyme forms the sclera, the corneal endothelium and stroma, blood vessels, muscles, and vitreous. The eye begins to develop as a pair of optic vesicles on each side of the forebrain at the end of the fourth week of pregnancy.