triangular bone at base of spine that forms by fusing of sacral vertebrae (s1-s5) between ages 18 and 30, situated at upper, back part of pelvic cavity, between two wings of pelvis
Joe Williams, John Carlson, Damon Hernandez, Don Brutzman
usage examples https://www.web3d.org/x3d/content/examples/HumanoidAnimation/Medical
X3D-Edit 4.0, https://www.web3d.org/x3d/tools/X3D-Edit
X3D Tidy, https://www.web3d.org/x3d/stylesheets/X3dTidy.html