HIP-JOINT. A hall-and-socket joint, formed by the reception of the globular head of the thigh ) one (or femur) into the deep pit or cup in the s innominateent. which is known as the aeetabee lain (so called from its resemblanee to the vine gar cups used by the Romans). If the variety of movements of this joint—viz., flexion. exten sion, abduction, adduction, and rotation inward and outward—and at the same time its great strength are considered, it may well claim to he regarded as the most perfect joint in the whole Ludy.
The ligaments are usually described as five in number: (1) The capsular: (2) the ilio-femoral; (3) the teres or round; (4) the cotyloid ; and 151 the transverse. The capsular ligament ex tends from the edge of the cup to the cireum ference of the neek upon which the ball is ear tied. inclosing the bony parts in a strong sheath. The ilio-femoral, or Y-ligament, is merely an ne cessary band of fibres which give increased thickness to the capsular ligament in front, where strength is specially required. The great use of the capsular ligament is to limit the extension of the hip-joint, and thus to give steadiness to the nrect posture. The only other ligament re quiring notice is the teres. or round ligament,
which is in reality triangular rather than round, and has its apex attached to the head of the thigh-hone. while its base is eonnected with the cavity of the aeetabulum. Its use is not very clearly known. but probably is to support and protect vessels giving blood-supply to the head of the bone. it is sometimes absent in cases in which no special weakness of the joint was ob served during life. and is of by no means constant oceurrenee in mammals. The joint is much by a large number of surrounding museles, some of which are of considerable power.
In such a joint as this, although the ligaments materially assist in preventing dislocation, it is obvious that the articular surfaces cannot. under ordinary circumstances. he kept in apposition by them, inasmuch as they must he loose in their whole circumference, to permit of the general movements of the joint. The experiments of Weber have shown that atmospheric pressure is the lea] power by which the head of the femur is retained in the acetabulum when the muscles are at. rest.