150 ARTHROLOGY, OR ANATOMY OF ARTICULATIONS 



is arrested mostly by the anterior crucial ligament; the anterior 

 fibers of the posterior ligament are also stretched. At the 

 beginning of flexion both crucial ligaments become relaxed; 

 both are stretched in extension, especially the posterior short 

 fibers of the posterior crucial. In extension the lateral liga- 

 ments are tense, and do not allow any motion but flexion. 

 Flexion and extension do not occur in a purely hinge-like 

 manner; the same part of one articular surface is not always 

 applied to the same part of another; the axis of motion is 

 not a fixed one. The motion of the femur on the tibia is likened 

 to .that of a carriage wheel on the ground; it advances or recedes 

 while it rotates. 



The semilunar cartilages are loosely attached, and move 

 forward in extension and backward in flexion of the joint 

 like movable wedges; as the condyles roll and present different 

 curvatures, each cartilage contracts or expands to fit the surface 

 above. The actual contact of the femur with the tibia is hardly 

 more than linear. 



In extension the anterior capsular wall is raised by the 

 subcrural muscle; in flexion the posterior wall has two muscles 

 to prevent its bulging into the joint. The semimembranosus 

 acts through its oblique ligament when the flexors from the 

 thigh and pelvis are in operation; the popliteus, through the 

 arcuate ligament when the plantaris and those attached to 

 the os calcis act. 



As flexion increases, rotation is possible, and increases to 

 a total of 39 degrees, due to a relaxation of the lateral and 

 crucial ligaments. Rotation out (supination) is most extensive, 

 as the external lateral ligaments are more loose than the internal ; 

 this occurs on an axis through the inner condyle and inner 

 tuberosity of the tibia. This motion is checked by the internal 

 lateral ligament and the winding of the posterior crucial around 

 the spine of the tibia. Rotation in (pronation) on an axis 

 through the outer condyle and outer tuberosity of the tibia 

 is never more than 5 to 10 degrees; this motion is checked by 

 the anterior crucial ligament and by the twisting of these 

 crucial ligaments around each other. 



At the close of full extension there is a movement of adapta- 

 tion, or gliding back of the inner condyle upon the tibia; this 

 axis is through the external condyle. At the beginning of 

 flexion a reverse motion takes place. 



