The patella, in extension of the knee, is nearly all above the condyles; in flexion, it lies in the inter-condyloid fossa (more on the external condyle), and thus protects the joint in kneeling. Its inner border is thicker and more prominent than the outer, which slopes down towards its condyle. 108. =Ligamentum patellæ.=--The line of the ligamentum patellæ is vertical. Hence any deviation from this line, one way or the other, indicates more or less dislocation of the tibia. There is a pellet of fat under the ligament, which answers a ‘packing’ purpose--sinking in when the knee is bent; rising when the knee is extended, and bulging on either side of the tendon, almost enough to give the feel of fluctuation. In a well-formed leg the ligamentum patellæ, the tubercle of the tibia, and the middle of the ankle should be in the same straight line. A useful point in the adjustment of fractures. Behind the upper half of the ligamentum patellæ is the synovial membrane of the knee-joint; behind the lower half is a synovial bursa and a pad of fat. It is well to remember this in cases of injury to the ligamentum patellæ. 109. =Patellar bursa.=--The patellar or housemaids’ bursa is situated not only over the patella, but over the upper part of the ligament. This is plain enough when the bursa becomes enlarged. There is another subcutaneous bursa over the insertion of the ligament into the tubercle of the tibia. This is quite independent of the deep bursa between the tendon and the bone. 110. =Synovial membrane of knee.=--The synovial membrane of the knee, when the joint is extended, rises like a cul-de-sac above the upper border of the patella about two inches. It ascends, too, a little higher under the vastus internus than the vastus externus--a fact very manifest when the joint is distended. When the knee is bent this cul-de-sac is drawn down--hence the rule of bending the knee in operations near the lower end of the femur. The lower level of the synovial membrane of the knee is just above the level of the upper part of the head of the fibula. The tibio-fibular synovial membrane is, with rare exceptions, independent of that of the knee. 111. =Popliteal tendons.=--The tendons forming the boundaries of the popliteal space can be distinctly felt when the muscles which bend the knee are acting. On the outer side, we have the biceps running down to the head of the fibula. On the inner side we feel three tendons, disposed as follows:--nearest to the middle of the popliteal space is the semitendinosus, very salient and traceable high up the thigh; next comes the thick round tendon of the semimembranosus; still more internally is the gracilis. The sartorius, which forms a graceful muscular prominence on the inner side of the knee, does not become tendinous until it gets below it.