The =symphysis pelvis= is formed by the junction of the two ossa coxarum at the ventral median line. In the young subject the bones are united by a layer of cartilage; in the adult the latter is gradually replaced by bone, the process beginning in the pubic portion and extending backward. The union is strengthened by white fibrous tissue above and below, and a transverse band also covers the anterior border of the pubis (pecten). No appreciable movement occurs even before synostosis takes place. OBTURATOR MEMBRANE This (Membrana obturatoria) is a thin layer of fibrous tissue which covers the obturator foramen, leaving, however, a passage (Canalis obturatorius) for the obturator vessels and nerve. THE HIP JOINT This joint (Articulatio coxæ) is an enarthrosis formed by the proximal end of the femur and the acetabulum. =Articular Surfaces.=—The head of the femur presents an almost hemispherical articular surface, which is continued a short distance on the upper surface of the neck. It is more extensive than the socket which receives it. Internally it is cut into by a deep notch for the attachment of the round and pubo-femoral ligaments. The acetabulum is a typical cotyloid cavity. Its articular surface is somewhat crescentic, being deeply cut into internally by the acetabular notch and fossa. It is increased and deepened by a ring of fibro-cartilage, the =cotyloid ligament= (Labrum glenoidale), which is attached to the bony margin; that part of the ligament which crosses the notch is called the =transverse ligament= (Fig. 456). [Illustration: FIG. 154.—PELVIC LIGAMENTS AND HIP JOINT. _1_, Dorsal sacro-iliac ligament; _2_, lateral sacro-iliac ligament; _3_, sacro-sciatic ligament; _4_, greater sciatic foramen; _5_, lesser sciatic foramen; _6_, line of attachment of intermuscular septum between biceps femoris and semitendinosus; _7_, capsule of hip joint; _8_, rectus parvus or capsularis muscle; _9_, outer tendon of origin of biceps femoris; _10_, internal, _11_, external, angle of ilium; _12_, shaft of ilium; _13_, superior ischiatic spine; _14_, pubis; _15_, tuber ischii; _16_, trochanter major; _17_, semimembranosus; _18_, fifth lumbar spine; _19_, _20_, first and second coccygeal vertebræ. ] The =joint capsule= is roomy. It is attached around the margin of the acetabulum and the neck of the femur. It is thickest externally.