The knee is an example of a joint which depends for its strength chiefly on its ligaments. Not only are the tibial and fibular collateral (external and internal lateral) ligaments and the posterior part of the capsular ligament particularly strong, but the cruciate ligaments and the menisci (semilunar cartilages) inside the cavity of the joint further add to its stability. The powerful tendon of the quadriceps extensor muscle, in which the patella is developed as a sesamoid bone, protects and strengthens the front of the joint and functionates as the anterior ligament of the joint. In the attitude of complete extension in which the joint is locked, no demand is made on the quadriceps apparatus; with the commencement of flexion, the stability of the joint, and the weight-bearing capacity of the limb as a whole, depend largely on the controlling influence of the quadriceps muscle; this becomes evident on going down an incline and more markedly on going down stairs. Hence it is, that in recurrent sprains of the knee, including under this term the various forms of internal derangement of the joint, the wasting with loss of tone of the quadriceps is an important factor in aggravating the disability of the limb and in retarding and preventing recovery. In the treatment of recurrent sprains of the knee, therefore, special attention must be directed towards the wasting of the quadriceps by means of massage and appropriate exercises. The synovial cavity extends from the level of the head of the tibia to an inch or more above the trochlear surface of the femur, passing slightly higher on the medial aspect of the joint than on the lateral (Fig. 80). The large bursa between the quadriceps muscle and the femur (_sub-crural bursa_) generally communicates with the cavity of the joint. The synovial cavity of the superior tibio-fibular articulation is usually distinct from that of the knee-joint, but may communicate with it through the popliteal bursa. [Illustration: FIG. 80.--Section of Knee-joint showing extent of Synovial Cavity. _a_, Pre-patellar bursa. _b_, Infra-patellar bursa. _c_, Ligamentum mucosum. _d_, Ligamentum patellæ. _e_, Posterior cruciate ligament. _f_, Medial semilunar meniscus. (After Braune.)] A large bursa (_pre-patellar_) lies over the lower part of the patella and upper part of the ligamentum patellæ; and a smaller one separates the ligamentum patellæ from the tuberosity of the tibia. Several important bursæ are found in the popliteal space, one of which--the semi-membranosus bursa--sometimes communicates with the knee-joint. FRACTURE OF THE LOWER END OF THE FEMUR Fractures involving the lower end of the femur, especially the supra-condylar and T-shaped fractures, are to be looked upon as serious injuries, on account of the difficulties attending their treatment, and the risk of damage to the popliteal vessels and of impairment of the usefulness of the knee-joint.