modern variation and evolutionary change in the hominin eye orbit

a whole, is due in part to the larger absolute volume of the human eyeball, as orbitalvolume is approximately the same in both chimpanzees and humans (Schultz, 1940).Because the eyeball does not dictate growth of the orbit, it is important tounderstand how each develops independently throughout life, and particularly in thecontext of neighboring structural and functional features of the skull. The eyeball hasbeen shown to grow most rapidly during the first years of life, with a majority of thisgrowth occurring in the anterior segment (Todd et al. 1940; Weale, 1982). It thenexpands more slowly through later life with the exception of a short spurt between 10-12,and another increased rate of growth from the age of fourteen until the early twenties(Salzmann, 1912; Weiss, 1897). In contrast to the early growth phase that takes placeprimarily in the anterior segment of the eye, during this later stage of development amajority of enlargement occurs in the posterior segment of the eyeball (Salzmann, 1912;Weiss, 1897; Weale, 1982).The orbits also complete most of their total growth relatively early in life,reaching 80% of adult size at age 3, and 94% of adult size at age 7 in humans (Scott,1953). The remaining 6 percent of growth occurs during childhood and is primarilyrestricted to the transverse plane, or in an equatorial orientation relative to the eyeball(Waitzman et al. 1992). Later growth of this region demonstrates the importance ofinvestigating each orbital area separately, as different segments develop somewhatindependently of each other during ontogeny.The lateral margin of the orbit is primarily made up of the greater wing of thesphenoid and part of the zygomatic bone, which together increase in area during growthspurts around age two and then again during separate spurts between ages 8 and 11 in the11