The pelvic girdle is made up of the paired hip or innominate bone. The hip bone (os coxae) is a flat bone concerned with the function of movement (takes part in articulations with the sacrum and femur), protection (shields the pelvic organs), and support (transfers the weight of the whole prox­imal part of the body to the lower limbs). The latter function prevails, and this determines the complex structure of the hip bone and its formation from fusion of three separate bones, the ilium (os ilium), the pubis (os pubis) and the ischium (os ischii). These bones fuse in the region bearing the greatest weight, namely, in the region of the acetabulum, the articular cavity of the hip joint, by means of which the pelvic girdle is connected to the free lower limb. The ilium is above the acetabulum, the pubis below and to the front of it, and the ischium is below and to the back of the acetabulum. In individuals under 16 years of age these bones are separated one from another by layers of cartilage which in an adult undergo ossification, i.e. synchondrosis changes to synostosis. As a result the three bones fuse to form a single bone possessing great strength necessary for bearing the weight of the whole trunk and head. The acetabulum ("vinegar curet" from L acetum vinegar) is on the lateral surface of the hip bone and serves for articulation with the head of the femur. It is a rather deep, cup-shaped cavity with a high rim, in the medial side of which is a notch (incisura acetabuli). The smooth articular surface of the acetabulum (facies lunata) is crescent-shaped; the centre, the acetabular fossa (fossa acetabuli) and the part nearest to the notch are rough.