348 THE ARTICULATIONS OR JOINTS. 



conducted to the femur. Again, the convex or peripheral margins of each meniscus 

 possess certain attachments to the deep surface of the fibrous stratum of the capsule 

 on its medial and posterior aspects, as has already been explained, but, in addition, 

 they are attached to the non-articular circumference of the tibial head by short 

 fibrous bands known as the ligamenta coronaria. Lastly, a rounded band which 

 varies in strength, the lig. transversum genu (transverse ligament) (Figs. 317 and 320), 

 stretches between the anterior convex margins of the two menisci, crossing the front 

 part of the non-articular area on the tibial head in its course. 



The stratum synoviale of the knee-joint is not only the largest, but the most 

 elaborately arranged of its kind in the body. It not only lines the fibrous stratum 

 of the capsule articularis, but it forms a more or less extensive covering for the intra- 

 capsular -ligaments and the free surface of the infra-patellar pad of fat. This pad 

 acts as a wedge which fits into the interval between the patella, tibia, and femoral 

 condyles, and the synovial stratum upon its surface forms a band or fold which 

 extends from the region distal to the level of the patellar articular surface to the 

 anterior part of the intercondyloid notch. It is named the plica synovialis patellaris. 

 At its femoral end it is narrow and attenuated, but at its patellar end it expands 

 on each side to form wing-like fringes or membranes the plicae alares medial 

 and lateral. These folds are more or less loaded with fat. 



Apart from these special foldings, the synovial stratum lines the deep surface 

 of the common extensor tendon, and extends for a variable distance proximal to the 

 patella. This extension of the articular cavity almost always communicates with a 

 large bursa situated still more proximally on the front of the femur. Tracing the 

 synovial stratum distally, it will be found to cover both surfaces of the two menisci. 

 The peripheral or convex margins of these menisci are only covered by this membrane 

 where they are unattached to the capsule. A prolongation invests the intracapsular 

 portion of the tendon of the popliteus muscle, and separates this tendon from the 

 posterior part of the tibial head, besides intervening between the lateral meniscus 

 and the head of the tibia. 



From the posterior part of the articular cavity the synovial stratum extends 

 forwards, and provides a partial covering for the cruciate ligaments between which a 

 bursa may be found. 



This somewhat complicated arrangement of the synovial stratum may be 

 readily comprehended if it is borne in mind that it really represents the fusion of 

 three separate synovial cavities, which in some animals are permanently distinct. 

 These are indicated in the two femoro-tibial and the single femoro-patellar parts of 

 the articulation. 



The articular cavity may communicate with bursse situated in relation to the 

 medial head of the gastrocnemius muscle and the tendon of the semi-membranosus 

 muscle, besides the large supra-patellar bursa already described. Lastly, there 

 may be intercommunication between this articular cavity and that of the proximal 

 tibio-fibular articulation. 



Movements at the Knee-Joint. In studying the movements which may occur at the 

 human knee-joint, it is necessary to bear in mind that the lower limb of man is primarily required 

 for purposes of support and locomotion. The principal requirement of the former function is 

 stability accompanied by rigidity, whereas in the latter function the special desideratum is regu- 

 lated and controlled mobility. Thus, in the same joint, two entirely opposite conditions have 

 to be provided. The stable conditions of support are chiefly concerned in the maintenance 

 of the erect attitude, and the mechanism associated therewith does not call for the exertion of a 

 large degree of sustained muscular effort. 



In standing erect the attitude of the limb is that of extension, which mainly concerns the 

 femoro-tibial parts of the joint. In this position the force of gravity acts along a vertical line 

 which falls in front of the transverse axis of the joint, and therefore any tendency to flexion, i.e. 

 bending backwards, is mechanically counteracted by the application of a force which tends t< 

 produce bending forwards (so-called over-extension). This, however, is absolutely prohibited ii 

 normal states of the joint, by the tension of the posterior and collateral ligaments aided by the 

 anterior cruciate ligament. The value of this fact may be seen by observing the effect proaucec 

 by giving the joint a sudden push from behind, which causes an immediate reversal of the 

 positions of the transverse and vertical axes, whereby the body weight at once produces flexior 

 of the joint. 



The menisci and the infra-patellar pad of fat also assist in maintaining extensioi 

 by reason of their close adaptation to, and packing round the condyles as these rest upon the tibij 



