KNEE-JOINT. 305 



joint renders it necessary to study each of these movements separately, pointing 

 out incidentally the functions of each of the principal components of the joint. 

 The tibia executes a rotatory movement during flexion around an imaginary 

 axis drawn transversely through its upper end. This causes a change in the 

 apposition of the tibia and femur. Thus, in extreme extension, it is the anterior 

 portion of the tibia which is in contact with the femur ; in the semiflexed posi- 

 tion, its middle; in complete flexion, its posterior edge. 1 Also, during flexion 

 the articular surface of the tibia, covered by the interarticular cartilages, glides 

 backwards on the femur. The patella is attached by the inextensible ligamentum 

 patellae to the tubercle of the tibia, and as the tibia glides backwards, the patella 

 falls more and more into the intercondyloid notch of the femur. The ligamentum 

 patellae is put on the stretch during flexion, as is also the posterior crucial liga- 

 ment in extreme flexion. The other ligaments are all relaxed by flexion of the 

 joint, though the relaxation of the anterior crucial ligament is very trifling. In 

 partial flexion of the knee before the ligamentum patellae comes upon the 

 stretch, and while both crucial ligaments are somewhat relaxed, some rotation 

 of the joint is. permitted. Flexion is only checked during life by the contact 

 of the leg with the thigh. In extension^ the ligamentum patellae becomes relaxed, 

 and, in extreme extension, completely so, so as to allow free lateral movement 

 to the patella, which then rests on the front of the condyles of the femur. The 

 other ligaments are all on the stretch. When the limb has been brought into a 

 straight line extension is checked, mainly by the tension of the posterior crucial 

 ligament. The movements of rotation of which the knee is susceptible are per- 

 mitted in the semiflexed condition by the partial relaxation of both crucial 

 ligaments, as well as the lateral ligaments. Rotation inwards (or pronation of 

 the leg) is checked by the anterior crucial ligament. The chief agent in effecting 

 this movement is the Popliteus muscle. Rotation outwards (or supination) is 

 checked by the posterior crucial ligament. It is effected mainly by the Biceps. 

 The main function of the crucial ligaments is to act as a direct bond of union 

 between the tibia and femur, preventing the former bone from being carried too 

 far backwards or forwards. They also assist the lateral ligaments in resisting 

 any lateral bending of the joint. The interarticular cartilages are intended, as 

 it seems, to adapt the surface of the tibia to the shape of the femur to a certain 

 extent, so as to fill up the intervals which would otherwise be left in the varying 

 positions of the joint, and to interrupt the jars which would be so frequently 

 transmitted up the limb in jumping or falls on the feet. The patella is a great 

 defence to the knee-joint from any injury inflicted in front, and it distributes 

 upon a large and tolerably even surface during kneeling the pressure which 

 would otherwise fall upon the prominent ridges of the condyles : it also affords 

 leverage to the Quadriceps Extensor muscle to act upon the tibia, and Mr. Ward 2 

 has pointed out how this leverage varies in the various positions of the joint, 

 so that the action of the muscle produces velocity at the expense of force in the 

 commencement of extension, and, on the contrary, at the close of extension 

 tends to diminish the velocity, and therefore the shock to the ligaments ; whilst 

 in the standing position it draws the tibia powerfully forwards, and thus main- 

 tains it in its place. 



The folds of sy no vial membrane and the fatty processes contained in them 

 act, as it seems, mainly as padding to fill up interspaces and obviate concussions. 



The bursae in connection with the synovial membrane will be found described 

 in connection with the regional anatomy of the popliteal space. 



III. ARTICULATIONS BETWEEN THE TIBIA AND FIBULA. 



The articulations between the tibia and fibula are effected by ligaments which 

 connect both extremities, as well as the shafts of the bone. They may, conse- 



1 See Plate XLVII. in Humphry, on the "Skeleton." 8 "Human Osteology," p. 405. 



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