THE KNEE-JOINT 339 



The hip-joint presents a very striking contrast to the shoulder-joint in the much more com- 

 plete mechanical arrangements for its security and for the limitation of its movements. In the 

 shoulder, as has been seen, the head of the humerus is not adapted at all in size to the glenoid 

 cavity, and is hardly restrained in any of its ordinary movements by the capsule. In the hip- 

 joint, on the contrary, the head of the femur is closely fitted to the acetabulum for an area extend- 

 ing over nearly half a sphere, and at the margin of the bony cup it is still more closely embraced 

 by the glenoidal labrum, so that the head of the femur is held in its place by that ligament even 

 when the fibers of the capsule have been quite divided. The iliofemoral ligament is the strongest 

 of all the ligaments in the body, and is put on the stretch by any attempt to extend the femur 

 beyond a straight line with the trunk. That is to say, this ligament is the chief agent in main- 

 taining the erect position without muscular fatigue; for a vertical line passing through the center 

 of gravity of the trunk falls behind the centers of rotation in the hip-joints, and therefore the 

 pelvis tends to fall backward, but is prevented by the tension of the iliofemoral ligaments. The 

 security of the joint may be provided for also by the two bones being directly united through the 

 ligamentum teres; but it is doubtful whether this ligament has much influence upon the mechanism 

 of the joint. When the knee is flexed, flexion of the hip-joint is arrested by the soft parts of the 

 thigh and abdomen being brought into contact, and when the knee is extended, by the action of 

 the hamstring muscles; extension is checked by the tension of the iliofemoral ligament; adduc- 

 tion by the thighs coming into contact; adduction with flexion by the lateral band of the ilio- 

 femoral ligament and the lateral part of the capsule; abduction by the medial band of the 

 iliofemoral ligament and the pubocapsular ligament ; rotation outward by the lateral band of the 

 iliofemoral ligament; and rotation inward by the ischiocapsular ligament and the hinder part of 

 the capsule. The muscles which flex the femur on the pelvis are the Psoas major, Iliacus, Rectus 

 femoris, Sartorius, Pectineus, Adductores longus and brevis, and the anterior fibers of the Glutsei 

 medius and minimus. Extension is mainly performed by the Glutseus maximus, assisted by the 

 hamstring muscles and the ischial head of the Adductor magnus. The thigh is adducted by the 

 Adductores magnus, longus, and brevis, the Pectineus, the Gracilis, and lower part of the Glutseus 

 maximus, and abducted by the Glutsei medius and minimus, and the upper part of the Glutseus 

 maximus. The muscles which rotate the thigh inward are the Glutseus minimus and the anterior 

 fibers of the Glutseus medius, the Tensor fasciae latse and the Iliacus and Psoas major; while 

 those which rotate it outward are the posterior fibers of the Glutseus medius, the Piriformis, 

 Obturatores externus and internus, Gemelli superior and inferior, Quadratus femoris, Glutseus 

 maximus, the Adductores longus, brevis, and magnus, the Pectineus, and the Sartorius. 



n. The Knee-joint (Articulatio Genu). 



The knee-joint was formerly described as a ginglymus or hinge-joint, but is 

 really of a much more complicated character. It must be regarded as consisting 

 of three articulations in one: two condyloid joints, one between each condyle 

 of the femur and the corresponding meniscus and condyle of the tibia; and a third 

 between the patella and the femur, partly arthrodial, but not completely so, 

 since the articular surfaces are not mutually adapted to each other, so that the 

 movement is not a simple gliding one. This view of the construction of the knee- 

 joint receives confirmation from the study of the articulation in some of the lower 

 mammals, where, corresponding to these three subdivisions, three synovial cavities 

 are sometimes found, either entirely distinct or only connected together by small 

 communications. This view is further rendered probable by the existence in the 

 middle of the joint of the two cruciate ligaments, w r hich must be regarded as 

 the collateral ligaments of the medial and lateral joints. The existence of the 

 patellar fold of synovial membrane would further indicate a tendency to separa- 

 tion of the synovial cavity into two minor sacs, one corresponding to the lateral 

 and the other to the medial joint. 



The bones are connected together by the following ligaments: 



The Articular Capsule. The Anterior Cruciate. 



The Ligamentum Patellae. The Posterior Cruciate. 



The Oblique Popliteal. The Medial and Lateral Menisci. 



The Tibial Collateral. The Transverse. 



The Fibular Collateral. The Coronary. 



