THE KNEE-JOINT. 



157 



and thence to the tibia. The crucial ligaments are invested in front by a 

 reflected portion of the membrane continued forwards from the posterior 

 wall of the joint. Between the tibia and patella the synovial membrane 

 lies upon a large pad or cushion of fat, on the surface of which it forms two 

 lateral folds (alar ligaments) which fit into the space between the tibia, 

 patella and femur, while from the middle of the pad it sends backwards a 

 tapering process, the ligamentum mucosum, through the joint to the front 

 of the intercondyloid fossa. Above the patella the synovial membrane 

 extends upwards some distance, forming a large pouch between the extensor 

 tendons and the femur. 



MOVEMENTS, &c. In order to explain the nature of the movements, it is neces- 

 sary to state some considerations with regard to the relations of the several parts of 

 the knee-joint to each other. The knee-joint may be regarded as consisting of 

 three articulations conjoined, viz., that between the patella and femur, and two others, 

 one between each condyle of the femur and the tibia. In most mammals the synovial 

 membranes of those three joints are either completely distinct or communicate with 

 each other by only small openings. In the human subject the ligamentum mucosum 

 is an indication of the original distinctness of the synovial membranes of the inner 

 and outer joints, and the crucial ligaments may be looked upon as the external and 

 internal lateral ligaments of those two joints respectively. Each portion of the 

 articular surface of the femur belongs either to one or other of the three component 

 joints of the knee, and no part is common to any two of them-. On a well-marked 



Fig. Hi. 



Fig. 144. THE SUPERFICIAL PARTS OF THE KNEE-JOINT REMOVED, AND THE EXTERNAL 

 CONDYLE OF THE FEMUR SAWN OFF OBLIQUELY, TOGETHER WITH HALF THE PATELLA, 

 SO AS TO EXPOSE BOTH THE CRUCIAL LIGAMENTS TOGETHER. J 



lu A, the parts are in the position of extension, in B, that of flexion, the figures being 

 designed to show the different state of tension of the crucial ligaments in these positions. 

 1, sawn surface of the femur ; 2, sawn surface of the patella; 3, ligamentum patellae; 

 4, anterior or external crucial ligament, tense in A, and relaxed in B ; 5, posterior or 

 internal crucial ligament, relaxed in A, tense in B ; 6, internal, and 7, external semilunar 

 cartilage ; 8, transverse ligament ; 9, articular surface of the tibia, extending behind the 

 external semilunar cartilage ; 10, on the head of the fibula, points to the anterior superior 

 tibio-peroneal ligament ; 11, upper part of the interosseous membrane. 



