The synovial membrane extends beneath the quadriceps extensor as a cul-de-sac, which either communicates with the sub-crural bursa, or forms with it one continuous cavity. When the joint is distended with fluid, this upper pouch bulges above and on either side of the patella, and this bone is "floated" off the condyles of the femur. When there is only a small amount of fluid, it is most easily recognised while the patient stands with his feet together and the trunk bent forwards at the hip-joints, and the quadriceps completely relaxed; the fluid then bulges above and on each side of the patella, and its presence is readily detected, especially on comparison with the joint of the other side. On account of the great extent of the synovial membrane, a large quantity of serous effusion may accumulate in the joint in a comparatively short time, as a result either of injury or disease. The villous processes and fringes may take on an exaggerated growth, and give rise to pedunculated and other forms of loose body. The bursæ in the popliteal space, especially that between the semi-membranosus and the medial head of the gastrocnemius, as well as the sub-crural bursa, frequently communicate with the synovial cavity of the knee and may share in its diseases. As the epiphyses at the knee are mainly responsible for the growth in length of the lower extremity, and are late in uniting with their respective shafts--twenty-one to twenty-five years--serious shortening of the limb may result if their functions are interfered with, whether by disease or injury. The epiphysial cartilages lie beyond the limits of the synovial cavity, so that infective lesions at the ossifying junctions are less likely to spread to the joint than is the case at the hip or shoulder, where the upper epiphysis lies partly or wholly within the joint; disease in the lower end of the femur is more likely to implicate the knee-joint than disease in the upper end of the tibia. One of the commonest causes of prolonged disability and feeling of insecurity in the knee, is to be found in the wasting and loss of tone in the quadriceps extensor muscle; the feeling of insecurity is most marked in coming down stairs. The instability of the joint is often added to by stretching of the ligaments and lateral mobility. As a result of both of these factors the joint is liable to repeated slight strains or jars which irritate the synovial membrane and tend to keep up the effusion and excite the overgrowth of its tissue elements. TUBERCULOUS DISEASE While tuberculous disease of the knee is specially common in childhood and youth, it may occur at any period of life, and is not uncommon in patients over fifty. The disease originates in the synovial membrane and in the bones respectively with about equal frequency.