HIP-JOINT is a ball-and-socket joint formed by the reception of the globular head of the thigh-bone (or femur) into the .deep pit or cup in the os innominatum. which is known as the acetabulum (so called from its resemblance to the vinegar cups used by the Romans. Ifthe variety of the movements of this joint—viz., flexion, extension, abduc tion, adduction, and rotation inwards and outwards, and at the same time its great strength are considered, it may well claim to be regarded as the most perfect joint in the whole body.
The ligaments are usually described as five iu number—viz.; 1. The capsular; 2. The 1110-femoral; 3. Theieres or round; 4. The cotyloid; and 5. The transverse. Of these, the capsular ligament is the most important, and extends from the edge of the cup to the circumference of the neck upon which the ball is carried, inclosing the bony parts in a strong sheath. The ilio-femoral is merely an accessory band of fibers which give increased thickness to the capsular ligament in front, where strength 'Ts specially required. The great use of 'the capsular ligament is to limit the extension of the hip-joint, and thus to give steadiness to time erect posture. The only other ligament requiring notice is the L. tens, or round ligament, which is in reality triangular rather than round, and has its apex attached to the head of the thigh-bone, while its base is connected with the cavity of the acetalmlum. Its use is not very clearly known,
but probably is to limit movement in one direction, It is sometimes absent in cases in which no special weakness of the joint was observed during life, and is of by no means constant occurrence iu mammals. Time joint is much strengthened by a huge number of surrounding muscles. sonic of which arc of considerable power.
In such a joint as this, although the ligaments materially assist in preventing dislo cation, it is obvious that the articular surfaces cannot, under ordinary circumstances, lie kept in appositiou by them, inasmuch as they must be loose in their whole circum ference, to permit of the general movements of the joint. The experiments of Weber show that atmospheric pressure is the real power by which the head of the femur is retained in the acetabulum when time muscles are at rest. "One convincing, experiment is easily repeated—that, namely, of holding up a side of the pelvis, with its appended lower the joint not having been opened, and then boring a hole through the seetalmlum, so as to admit air into the joint, when the weight of the limb will cause it to drop from half an inch to an inch, the,head of the thigh-bone being pulled out of the amtabulnm as soon as the air is permitted to pass between the articular surfaces."— HumpMy On the Human Skeleton, p. 74.