The HIP JOINT, like the shoulder, is a ball and socket, but does not allow such free movement; this is due to the fact that the socket or acetabulum is deeper than the glenoid cavity and that the capsule is not so lax. At the same time the loss of mobility is made up for by increased strength. The capsule has three thickened bands, of which the most important is the _ilio-femoral_ or _Y-shaped ligament of Bigelow_. The stalk of the Y is attached to the anterior inferior spine of the ilium, while the two limbs are fastened to the upper and lower parts of the spiral line of the femur. The ligament is so strong that it hardly ever ruptures in a dislocation of the hip. As a plumb-line, dropped from the centre of gravity of the body, passes behind the centre of the hip joint, this ligament, lying as it does in front of the joint, takes the strain in Man's erect position. The other two thickened parts of the capsule are known as _pubo-femoral_ and _ischio-femoral_, from their attachments. Inside the capsule, and deepening the margin of the acetabulum, is a fibrous rim known as the _cotyloid ligament_, which grips the spherical head of the femur and is continued across the cotyloid notch as the _transverse ligament_. The floor of the acetabulum has a horseshoe-shaped surface of articular cartilage, concave downward, and, occupying the "frog" of the horse's hoof, is a mass of fat called the _Haversian pad_. Attached to the inner margin of the horseshoe, and to the transverse ligament where that is deficient, is a reflexion of synovial membrane which forms a covering for the pad and is continued as a tube to the depression on the head of the femur called the _fossa capitis_. This reflexion carries blood-vessels and nerves to the femur, and also contains fibrous tissue from outside the joint. It is known as the _ligamentum teres_. _Comparative Anatomy._--Bland Sutton regards the _ilio-femoral ligament_ as an altered muscle, the scansorius, though against this is the fact that, in those cases in which a scansorius is present in Man, the ligament is as strong as usual, and indeed, if it were not there in these cases, the erect position would be difficult to maintain. He also looks upon the _ligamentum teres_ as the divorced tendon of the pectineus muscle. The subject requires much more investigation, but there is every reason to believe that it is a tendon which has sunk into the joint, though whether that of the pectineus is doubtful, since the intra-capsular tendon comes from the ischium in Reptiles. In many Mammals, and among them the Orang, there is no ligamentum teres. In others, such as the Armadillo, the structure has not sunk right into the joint, but is connected with the pubo-femoral part of the capsule.