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orbital index[′ȯr·bəd·əl ′in‚deks]
the percentage ratio of the height of the orbit—the eye opening in the human skull—to the width. Chamaeconchy is characterized by an orbital index under 75.9; the orbital index in mesoconchy ranges from 76.0 to 84.9; and indexes of 85.0 or higher result in hypsiconchy. High orbits are typical of the majority of Mongoloid peoples, while Tasmanians and Melanesians usually have very low orbits. In women the width of the orbits is considerably less than in men, whereas the sex difference in orbital is less dramatic. In children the orbital index is higher and the relative size of the orbits is greater than in adults.