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Diagram of a human eye; note that not all eyes have the same anatomy as a human eye. The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment. [10] The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere.
The size of the eyes in mammals is relatively small; in humans, eye weight is 1% of the mass of the head, while in a starling it reaches 15%. Nocturnal animals (for example, tarsiers) and animals that live in open landscapes have larger eyes.
Almost all mammals have brown or darkly-pigmented irises. [33] In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it. [34] Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed. [35]
This type of eye is common in mammals, including humans. The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. [1] Eyes enable several photo response functions that are independent of vision.
Melanin is the protein that creates skin, eye and hair color. More melanin means darker eyes, hair or skin. The color of the melanin in the eyes is determined by three other genes, EYCL1, 2 and 3.
Hair curls were compared to hooks and chains, letters (such as fa, waw, lam, and nun), scorpions, annelids, and polo sticks. [80] An example was the poet Ibn al-Mu'tazz, who compared a lock of hair and a birthmark to a polo stick driving a ball. [80]
[50] [51] The differences in color are mainly caused by higher levels of melanin in the skin, hair and eyes in males. [52] [53] In one study, almost twice as many females as males had red or auburn hair. A higher proportion of females were also found to have blond hair, whereas males were more likely to have black or dark brown hair. [54]
The cooperative eye hypothesis is not the only one that has been proposed to explain the appearance of the human eye. Other hypotheses include the proposal that white sclerae are a sign of good health, useful in mate selection , or that eye visibility promotes altruistic behaviour by letting people know they are being watched.