338 THE ARTICULATIONS, OR JOINTS 



along the inner border is in contact with the outer aspect of the internal condyle when the leg 

 is fully flexed at the knee-joint. As in the elbow, so it is in the knee the axis of rotation in 

 flexion and extension is not precisely at right angles to the axis of the bone, but during flexion 

 there is a certain amount of alteration of plane; so that, whereas in flexion the femur and tibia 

 are in the same plane, in extension the one bone forms an angle of about 10 degrees with the 

 other. There is, however, this difference between the two extremities: that in the upper, during 

 extension, the humeri are parallel and the bones of the forearm diverge; in the lower, the femora 

 converge below and the tibiae are parallel. 



In addition to the slight rotation during flexion and extension, the tibia enjoys an independent 

 rotation on the condyles of the femur in certain positions of the joint. This movement takes 

 place between the articular menisci and the tibia, whereas the movement of flexion and extension 

 takes place between the articular menisci and the femur. So that the knee may be said to consist 

 of two joints, separated by the menisci an upper, meniscof emoral, in which flexion and extension 

 take place; and a lower, meniscotibial, 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. During life flexion is checked 

 by the contact of the leg with the thigh. In the act of extending the leg upon the thigh the liga- 

 mentum 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 the 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 ligaments, as well as of 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 it is checked by the lateral ligaments, 

 especially the internal. 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 back- 

 ward 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 back- 

 ward by the Flexors. They also assist the lateral ligaments in resisting any lateral bending of the 

 joint. The semilunar cartilages are intended, evidently, to adapt to a certain extent the surface 

 of the tibia to the shape of the femur, in order to fill intervals which would otherwise occur in the 

 changing of joint position and to interrupt jars which otherwise would be so frequently trans- 

 mitted up the limb through jumping or falling on the feet. These cartilages also contribute to 

 the varieties of motion flexion, 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 when 

 it acts upon the tibia; and Mr. Ward has pointed out 1 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 struc- 

 tures takes place. 



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

 string muscles, assisted by the Gracilis and Sartorius, and, indirectly, by the Gastrocnemius, 

 Popliteus, and Plantaris; rotation outward, by the Biceps femoris; 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. Wiien the 

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

 may be fairly w r ell mapped out. 



Applied Anatomy. The bursce about the knee are frequently the seat of inflammation. 

 Enlargement of the prepatellar bursa constitutes housemaid's knee. The bursa beneath the 



1 Human Osteology, p. 405. 



