Radius (bone) ulna (center of the circle) to the edge of the radius (the circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move. In four-legged animals, the radius is the main load-bearing bone of the lower forelimb. Its structure is similar in most terrestrial tetrapods, but it may be fused with the ulna in some mammals (such as horses) and reduced or modified in animals with flippers or vestigial forelimbs. Radius (bone) The radius or radial bone is one of the two large bones of the forearm, the other
Hip bone muscles. Monotremes and marsupials also possess a fourth pair of bones, the prepubes or "marsupial bones", which extend forward from the pubes, and help to support the abdominal muscles and, in marsupials, the pouch. In placental mammals, the pelvis as a whole is generally wider in females than in males, to allow for the birth of the young. The pelvic bones of cetaceans were formerly considered to be vestigial, but they are now known to play a role in sexual selection. Hip bone The hip bone (os coxa, innominate bone, pelvic bone or coxal bone) is a large flat bone,