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"In humans, the widely exposed white sclera (white of the eye) surrounding the darker colored iris makes it easy for others to discern the direction of gaze and has been said to be a characteristic of humans not found in other primate species. ... The results clearly showed exceptional features of the human eye: (1) the exposed white sclera is void of any pigmentation, (2) humans possess the largest ratio of exposed sclera in the eye-outline, and (3) the eye-outline is extraordinarily elongated in the horizontal direction. The close correlation of the parameters reflecting (2) and (3) with habitat type or body size of the species examined suggested that these two features are adaptations for extending the visual field by eyeball movement, especially in the horizontal direction. Comparison of eye coloration and facial coloration around eye suggested that the dark coloration of exposed sclera of non-human primates is adaptation to camouflage the gaze direction against other individuals and/or predators, and that the white sclera of human is adaptation to enhance the gaze signal. The uniqueness of human eye morphology among primates illustrates the remarkable difference between human and other primates in the ability to communicate using gaze signals." (from: http://www.saga-jp.org/coe_abst/ kobayashi.htm)