280 THE ARTICULATIONS. 



the tibia, whereas the movement of flexion and extension takes place between the 

 interarticular fibro-cartilages and the femur. So that the knee may be said to 

 consist of two joints, separated by the fibro-cartilages : an upper (menisco-femoral), 

 in which flexion and extension take place ; and a lower (menisco-tibial), allowing 

 of a certain amount of rotation. This latter movement can only take place in the 

 semiflexed position of the limb, when all the ligaments are relaxed. 



During flexion the ligamentum patellae is put upon the stretch, as is also 

 the posterior crucial ligament 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. Flexion is only checked during life by the contact of the leg with 

 the thigh. In the act of extending the leg upon the thigh the ligamentum patellae is 

 tightened by the Quadriceps extensor; but when the leg is fully extended, as in the 

 erect posture, the ligament becomes relaxed, so as to allow free lateral movement to 

 the patella, which then rests on the front of the lower end of the femur. The other 

 ligaments, with the exception of the posterior crucial, which is partly relaxed, are all 

 on the stretch. When the limb has been brought into a straight line, extension is 

 checked mainly by the tension of all the ligaments except the posterior crucial and 

 ligamentum patellae. The movements of rotation of which the knee is capable are 

 permitted in the semiflexed condition by the partial relaxation of both crucial liga- 

 ments, as well as the lateral ligaments. Rotation inward appears to be limited by 

 the tension of the anterior crucial ligament, and by the interlocking of the two liga- 

 ments ; but rotation outward does not appear to be checked by either crucial 

 ligament, since they uncross during the execution of this movement, but by the 

 lateral ligaments, especially the internal. The main function of the crucial liga- 

 ments is to act as a direct bond of union between the tibia and femur, preventing 

 the former bone from being carried too far backward or forward. Thus the 

 anterior crucial ligament prevents the tibia being carried too far forward by the 

 extensor tendons, and the posterior crucial checks too great movement backward 

 by the flexors. 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; also to permit of the 

 two varieties of motion, flexion and extension, and rotation, as explained above. 

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

 joint, so that the action of the muscles produces velocity at the expense of force in 

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

 tends to diminish velocity, and therefore the shock to the ligaments at the moment 

 tension of the structures takes place. 



Extension of the leg on the thigh is performed by the Quadriceps extensor ; 

 flexion by the hamstring muscles, assisted by the Gracilis and Sartorius, and, 

 indirectly, by the Gastrocnemius, Popliteus, and Plantaris ; rotation outward, by 

 the Biceps ; and rotation inward by the Popliteus, Semitendinosus, and, to a 

 slight extent, the Semimembranosus, the Sartorius, and the Gracilis. 



Surface Form. The interval between the two bones entering into the formation of the 

 knee-joint can always easily be felt. If the limb is extended, it is situated on a slightly higher level 

 than the apex of the patella; but if the limb is slightly flexed, a knife carried horizontally back- 

 ward immediately below the apex of the patella would pass directly into the joint. When the 

 knee-joint is distended with fluid, the outline of the synovial membrane at the front of the knee 

 may be fairly well mapped out. 



Surgical Anatomy. From a consideration of the construction of the knee-joint it would 

 at first sight appear to be one of the least secure of any of the joints in the body. It is formed 



1 Human Osteology, p. 405. 



