136 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 221. — The condyles of the tibia with the two menisci and the origins of the crucial ligaments (J). 

 Fig. 222. — Sagittal section of the right knee-joint in extension. The section passes through the external 

 condyle of the tibia (J). 



Fig. 223. — The right tibia and fibula with their ligaments (£). 



muscle of the knee (subcrureus) (see page 215). The suprapatellar bursa does not lie directly 

 upon the anterior surface of the femur, but is separated from it by a cushion of fat. 



There are two or three other considerably smaller diverticula of the synovial membrane at 

 the posterior portion of the articulation. These are the popliteal bursa, beneath the tendon of 

 the popliteus, the semimembranous bursa, beneath the tendon of the semimembranosus, and the 

 internal gastrocnemial bursa (Fig. 304), beneath the tendon of the inner head of the gastrocnemius. 

 The last two bursae may communicate. 



There are other bursae in the neighborhood of the knee-joint which have no direct relation to the articulation. In 

 addition to the previously mentioned deep infrapatellar bursa, these are: the subcutaneous prepatellar bursa (Fig. 222), 

 a subcutaneous bursa which is constantly found in front of the patella; the subfascial prepatellar bursa, between the fascia 

 and the tendon of the quadriceps; the subtendinous prepatellar bursa, between the quadriceps tendon and the periosteum 

 of the patella; and the subcutaneous infrapatellar bursa, which is situated in front of the patellar ligament (see also 

 page 234). 



As might be supposed from the shape of the articulating surfaces, there are two kinds of motion possible in the knee- 

 joint, a hinge motion (flexion of the leg and the return to the extended position) and a movement of rotation which is 

 possible only when the knee is flexed. Rotation is impossible when the knee is extended, not only from the shape of 

 the articulating surfaces (see page 133), but especially on account of the tension of the lateral ligaments, which are 

 relaxed only during flexion of the joint. The lateral ligaments also prevent a lateral displacement of the bones during 

 flexion of the articulation. The crucial ligaments serve mainly to hold the femur and tibia together; they are so situated 

 that one of them is always tense in any position of the joint, the posterior ligament being tense during part of the move- 

 ment of flexion and part of the movement of extension, and the anterior one during the whole of flexion. The crucial 

 ligaments also check the movement of rotation. 



The patella glides upon the surface of the femur, and has no influence upon the mechanism of the articulation. 

 During extension of the joint it is pulled upward by muscular action and during flexion it descends toward the tibia. 



The function of the menisci is rather to form an articular cushion than to supplement and deepen the articular 

 socket. In some positions of the joint they act both as cushions and as portions of the articular socket, in other positions 

 they act only as cushions, and in still others they exert no influence whatever upon the mechanism of the joint. During 

 some of the movements of the articulation they are markedly displaced or strongly compressed. 



THE ARTICULATIONS OF THE TIBIA AND FIBULA. 



The tibia and fibula are connected with each other in three ways: their upper extremities 

 articulate by means of a small joint, the tibiofibular articulation; the bodies of the bones are 

 connected by the interosseous membrane; and the lower extremities arc united by tense ligaments, 

 forming the tibiofibular syndesmosis. 



The tibiofibular articulation (Figs. 217, 218, 220, and 223) is the joint between the fibular 

 articular surface of the tibia and the capitular articular surface of the fibula. It is an arthrodium 

 with almost plane articular surfaces and possesses strong accessory ligaments, which reinforce the 

 capsule anteriorly and posteriorly and arc known as the anterior and posterior capitular liga- 

 ments (Figs. 218, 220, and 223). The tense capsular ligament snugly embraces the cartilaginous 

 surfaces. This articulation may occasionally communicate with the knee-joint (through the 

 popliteal bursa). 



