Acetabulum
The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure. There are three bones of the "os coxae" (hip bone) that come together to form the "acetabulum". Contributing a little more than two-fifths of the structure is the ischium, which provides lower and side boundaries to the acetabulum. The ilium forms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest is formed by the pubis, near the midline . It is bounded by a prominent uneven rim, which is thick and strong above, and serves for the attachment of the acetabular labrum, which reduces its opening, and deepens the surface for formation of the hip joint. At the lower part of the "acetabulum" is the acetabular notch, which is continuous with a circular depression, the acetabular fossa, at the bottom of the cavity of the "acetabulum". The rest of the "acetabulum" is formed by a curved, crescent-moon shaped surface, the lunate surface, where the joint is made with the head of the femur. Its counterpart in the pectoral  girdle is the glenoid fossa. The "acetabulum" is also home to the acetabular fossa, an attachment site for the "ligamentum teres", a triangular, somewhat flattened band implanted by its apex into the antero-superior part of the fovea capitis femoris. The notch is converted into a foramen by the transverse acetabular ligament; through the foramen nutrient vessels and nerves enter the joint. This is what holds the head of the femur securely in the acetabulum. The well-fitting surfaces of the femoral head and acetabulum, which face each other, are lined with a layer of slippery tissue called articular cartilage , which is lubricated by a thin film of synovial fluid. Friction inside a normal hip is less than one-tenth that of ice gliding on ice. Blood supply. The acetabular branch of the obturator artery supplies the "acetabulum" through the acetabular notch. The pubic branches supply the pelvic surface of the "acetabulum". Deep branches of the superior gluteal artery supply the superior region and the inferior gluteal artery supplies the postero-inferior region. Reptiles and birds. In reptiles and birds, the "acetabula" are deep sockets. Organisms in the dinosauria clade are defined by a perforate "acetabulum", which can be thought  of as a "hip-socket". The perforate "acetabulum" is a cup-shaped opening on each side of the pelvic girdle formed where the ischium, ilium, and pubis all meet, and into which the head of the femur inserts. The orientation and position of the "acetabulum" is one of the main morphological traits that caused dinosaurs to walk in an upright posture with their legs directly underneath their bodies. In a relatively small number of dinosaurs, particularly ankylosaurians (e.g. "Texasetes pleurohalio"), an imperforate "acetabulum" is present, which is not an opening, but instead resembles a shallow concave depression on each side  of the pelvic girdle. Development. In infants and children, a 'Y'-shaped epiphyseal plate called the triradiate cartilage joins the ilium, ischium, and pubis. This cartilage ossifies as the child grows. History. The word "acetabulum" literally means "little vinegar cup". It was the Latin word for a small vessel for serving vinegar. The word was later also used as a unit of volume.