_Congenital dislocation_ of the hip occurs. _Separation of the epiphysis of the femur_ may occur and sometimes the neck, rarely the lower part of the shaft, is _fractured_. Either condyle may be fractured off or there may be a T-fracture, in which case the popliteal artery may be injured. In _dislocation_ the head may be behind or in front of the acetabulum. _Impacted hip_, where the neck of the femur has, in a fall, been driven into the head, is common in old people. Sometimes, especially in young children, the bone is infected, _osteomyelitis_. _Sarcoma_ occurs. Most tubercular disease of the hip originates at the upper extremity of the femur, _tuberculosis_ generally starting in the head and then attacking the capsule and the soft parts of the joint. If neglected, shortening of the leg may result, in which case the bone has to be broken and set at an angle in order to enable the child to walk. [Illustration: FIG. 78.] [Illustration: FIG. 79. FIG. 78.--Right knee-joint, posterior view. (Leidy.) FIG. 79.--Right knee-joint, showing internal ligaments: 2, anterior crucial ligament; 3, posterior crucial ligament; 4, transverse ligament; 6, 7, semilunar fibro-cartilages. (Leidy.)] Occasionally a bit of cartilage gets broken off in the knee-joint and wedged between the bones, so that the joint cannot be straightened. This is _dislocation of the semilunar cartilage_ and necessitates an operation for removal of the piece. The cartilage will eventually be replaced by fibrous tissue and in a few months the leg will be all right. _Dislocation of the knee_ is rare, though it may occur in any direction. Often the bursæ of the joint are irritated, as by kneeling to scrub floors, and _bursitis_ or _housemaid’s knee_ results. _Fracture of the patella_ may be caused by muscular traction or by direct violence, and is generally repaired by making an incision and sewing the parts of the bone together. _Tumor albus_ or _white swelling_ is _tuberculosis of the knee_ and is fairly common in children. _Specific knee_ means _syphilis of the knee_ and generally occurs in both knees.