1Department of Anatomy, The University of Michigan Medical School, Ann Arbor, Mich. 48104, U.S.A.
2Biosciences Section and Department of Anthropology, The Highway Safety Research Institute, The University of Michigan, Ann Arbor, Mich. 48108, U.S.A.
3Department of Pediatrics, The University of Michigan Medical School, Ann Arbor, Mich. 48104, U.S.A.