chap, xxi.] THE KNEE JOINT. 435 



The knee-joint. This articulation is the largest 

 in the body. The joint owes its great strength to the 

 powerful ligaments that unite the two component bones, 

 and to the muscles and fasciso that surround it. It 

 derives no strength from the shape of the articular 

 surfaces, since they are merely placed in contact with 

 one another. In spite of its frequent exposure to in- 

 jury, dislocations "at the knee are extremely rare. The 

 lateral ligaments are comparatively feeble, are tense in 

 extension and relaxed in flexion. The laxity of these 

 ligaments is such that partial luxations of the tibia 

 are possible without rupture of these bands, especially 

 in cases where the joint is found slightly flexed after 

 the accident. The crucial ligaments are very power- 

 ful, and are more or less tense in all positions of the 

 joint. The anterior of these ligaments especially 

 resists extension, forward displacement- of the tibia, 

 and rotation inwards of the leg. The posterior band 

 resists flexion and displacement backwards of the 

 tibia. In the movement of extension the tibia slides 

 a little forwards and is rotated a little outwards. In 

 flexion that bone glides backwards and rolls a little 

 inwards. Extension generally is limited by the 

 crucial and posterior ligaments ; flexion by the liga- 

 mentum patellae and anterior part of the capsule in 

 addition to the crucial ligaments. The thinnest 

 part of the posterior ligament is the portion below 

 the oblique fibres derived from the semimembra- 

 nosus. If pus finds its way from the joint into the 

 ham, it will probably escape through this part of the 

 ligament. 



In the contracted knee associated with fibrous 

 anchylosis, the chief contraction, so far as the joint 

 tissues are concerned, is in the posterior ligament, in 

 the lateral ligaments, and in the fibrous and fatty 

 tissue between the former ligament and the posterior 

 crucial band. 



